Sustainable and Recyclable Composites for Electric Aviation and UAVs: Component-Specific Evidence, Qualification Pathways, and Circular Design

Abstract

Electric aviation and unmanned aerial vehicles (UAVs) depend on lightweight composites to preserve payload and range, yet mass reduction, recycled content, or bio-based content alone does not establish component suitability. Candidate systems must also satisfy coupled structural, thermal, fire, electrical, manufacturing, durability, repair, and circularity requirements. This structured critical narrative review evaluates thermoplastic carbon-fibre-reinforced polymer (CFRP) systems, recycled-carbon-fibre composites, natural-fibre systems, bio-based and recyclable matrices, hybrid architectures, and multifunctional composites using a component-specific framework based on source role, evidence maturity, test comparability, and failure consequence. The framework links processing and chemistry to defects, retained performance, repair and recovery, and the evidence required for defined aircraft and UAV components. Thermoplastic CFRP provides the strongest near-term pathway for secondary and semi-structural components, although weld durability, impact tolerance, fire response, and process conformity remain system specific. Recycled-carbon-fibre and natural-fibre systems are most defensible for lower-consequence covers, fairings, housings, interiors, and UAV parts when feedstock variability, moisture, porosity, and fire performance are controlled. Battery enclosures, primary structures, rotor-support members, and structural-battery systems require representative coupled-hazard and component-scale evidence. The resulting adoption pathways are bounded by component and operating conditions, with manufacturing, durability, repair, recovery, and qualification evidence specified for each application.

1. Introduction

Electric aviation intensifies the mass and safety constraints governing airframe material selection. Battery-electric and hybrid-electric aircraft carry their full battery mass essentially throughout the mission. Feasibility therefore depends strongly on battery specific energy, pack mass, range, and propulsion–airframe coupling [1,2,3,4]. Battery integration introduces thermal-runaway and loss-of-power hazards [5], together with demanding power, fast-charge, cycle-life, and continued-operation requirements [6]. Material selection must account for structural contribution, thermal protection, electrical function, inspection, repair, and the mass of additional safety layers.

Material requirements vary sharply with platform and component. Advanced air mobility (AAM) and electric vertical take-off and landing (eVTOL) concepts differ in payload, mission, propulsion, and aircraft configuration [7,8], while airspace and infrastructure shape their operating context [9]. The evidence threshold differs between a replaceable unmanned aerial vehicle (UAV) cover and an occupied eVTOL rotor-support structure, battery enclosure, radome, or primary airframe panel. Aerospace composites provide high specific stiffness and strength [10], while sandwich construction provides efficient bending stiffness at low mass [11]. Renewable or recycled content alone is insufficient for assessment. Suitability after manufacture and service exposure depends on process variability, porosity, fibre architecture, interfaces, environmental durability, fire and electrical performance, joining, inspection, repair, and statistically defensible properties.

Existing reviews treat this problem from several distinct perspectives. Aerospace-composite reviews provide broad accounts of material classes, manufacturing routes, and component applications [10]. Environmental performance, processing constraints, and material-property limitations are examined more closely in studies of sustainable fibre composites and bio-based matrices [12,13]. Other work considers material selection in relation to UAV components and life cycle effects [14]. Structural-battery reviews address a narrower multifunctional case and identify qualification gaps associated with aeronautical use [15]. Reviews of sustainable aeronautic materials and composite additive manufacturing cover recyclability, industrialisation, process quality, and certification [16,17]. Other reviews cover aviation electrification at the propulsion-system level [18], broad aerospace material selection [19], environmental degradation and failure [20], or carbon-fibre-reinforced polymer (CFRP) circularity [21].

Table 1. Comparison of related reviews and the present review across predefined analytical dimensions.

Review	Primary Scope	Materials and Processing	Electric Aviation/UAV Relevance	Coupled Safety and Qualification Coverage	Component Adoption Assessment	Analytical Emphasis and Scope Boundary
Parveez et al. [10]	Broad aerospace composites	Broad material classes and manufacturing routes	Partial	Partial treatment of safety and qualification	Partial component examples	No integrated sustainability, evidence-maturity, or component-adoption framework for electric aviation
Maiti et al. [12]	Sustainable fibre-reinforced composites	Central coverage of sustainable fibres, matrices, processing, and life cycle considerations	Not central	Partial fire and environmental discussion; qualification not central	Not central	No aerospace qualification pathway or component-consequence assessment
Ramon et al. [13]	Bio-based epoxies for engineering and aviation	One matrix family; chemistry, processing, and properties	Partial	Partial thermal and durability coverage; qualification not central	Not central	Restricted to bio-based epoxy chemistry; no cross-family or component-specific evidence assessment
Vedrtnam et al. [14]	UAV materials and energy-centred life cycle assessment	Partial sustainable-material coverage	Central UAV focus	Safety and qualification not central	Central UAV component categories	No occupied-aircraft qualification, coupled-hazard, or evidence-maturity assessment
Kühnelt et al. [15]	Structural batteries for aeronautics	Multifunctional structural-battery systems	Central electric-aircraft focus	Central battery integration and certification-gap analysis	Central for structural batteries	Restricted to structural batteries rather than multiple sustainable composite families
Carvalho et al. [16]	Durable and sustainable aeronautic materials	Central coverage of thermoplastics, biocomposites, recycled materials, and vitrimers	Partial	Partial fire, durability, and certification coverage	Partial	No integrated eVTOL battery/EMI, comparability, or evidence-maturity framework
Shen et al. [17]	Additive manufacturing of aerospace composites	Central material–process–design integration and quality control	Not central	Central manufacturing-quality and certification discussion	Central for additively manufactured components	Restricted to additive manufacturing; no cross-process component-adoption framework
Zhang et al. [18]	Aviation electrification architectures and control	Materials and processing not central	Central electric-aviation focus	Partial system-safety coverage	Not central for materials	System-level electrification review rather than a material–process–component assessment
Akter et al. [19]	Broad aerospace composite selection	Central coverage of material classes, processing, sustainability, repair, and recycling	Partial	Partial fire and certification coverage	Partial component examples	No sustained electric-aircraft coupled-function, comparability, or evidence-maturity analysis
Hamzat et al. [20]	Fibre-composite failure and environmental degradation	Central treatment of thermal, mechanical, chemical, and oxidative degradation	Partial aircraft/UAV context	Partial durability and safety implications; qualification not central	Not central	Focuses on degradation and failure rather than sustainability, circularity, and component adoption
Zabihi et al. [21]	CFRP sustainability and circularity	Central coverage of the CFRP life cycle, recyclable matrices, and recycled-fibre routes	Partial aerospace context	Coupled safety and qualification not central	Not central	Restricted to CFRP circularity; no coupled electric-aircraft safety or qualification assessment
Present review	Sustainable and recyclable composites for electric aviation and UAVs	Central coverage of thermoplastic CFRP, recycled carbon-fibre composites, natural-fibre systems, bio-based and recyclable matrices, hybrid systems, and multifunctional composites	Central electric aviation and UAV focus	Central assessment of structural, fire, thermal, electrical, durability, and qualification requirements	Central assessment of adoption for defined component classes	Integrates source role, evidence maturity, test comparability, operating conditions, failure consequence, and circular recovery

applies four predefined comparison dimensions to the related reviews and the present review: materials and processing, relevance to electric aviation or UAVs, coverage of coupled safety and qualification, and assessment of adoption for defined components.

The present review addresses all four dimensions through a framework organised around defined components. Claims used in the assessment are classified by source role, evidence maturity, and test comparability; operating conditions and failure consequence define the limits of each component conclusion. The underlying studies differ in fibre architecture, specimen geometry, thickness, conditioning history, test standard, and functional metric. Numerical comparison is limited to studies with sufficiently aligned conditions.

This review applies a structured critical narrative synthesis to thermoplastic CFRP, recycled-carbon-fibre composites, natural-fibre composites, bio-based matrices, vitrimers and other recyclable thermosets, hybrid systems, and multifunctional polymer-fibre composites. Electric aircraft and eVTOL systems define the high-assurance application context, whereas UAVs provide a lower-risk route to component validation and earlier adoption. The analysis distinguishes between direct experimental evidence, validated analytical or computational evidence, review-level synthesis, regulatory requirements, and author synthesis. Evidence maturity is recorded from material and coupon studies through joints, repairs, components, qualification, and service experience. Each material family is evaluated within a defined material–process–component combination, operating environment, and failure consequence.

Four questions guide the review: (1) how processing route, constituent variability, architecture, interfaces, and defects govern structural and functional performance; (2) how durability, fire, thermal, electrical, and electromagnetic interference (EMI) behaviour constrain component use; (3) what evidence supports classification as near term, conditional, research stage, or not assessable; and (4) which tests and process controls remain necessary for qualification. The synthesis distinguishes demonstrated capability from the evidence still required for each defined component, operating environment, and failure consequence.

The stated aims of each review and the extent of analysis devoted to each theme formed the basis for assigning coverage. Central applies when a theme governs the scope of the review and is examined throughout the main discussion. Partial applies to a theme that receives substantive treatment but remains secondary to the review’s main scope. Themes absent from the review, or mentioned only incidentally, fall under not central. Component adoption assessment requires an evaluation of suitability for defined components; isolated application examples do not satisfy this criterion. The comparison evaluates review scope and analytical coverage, excluding article quality and citation impact. Abbreviations: CFRP, carbon-fibre-reinforced polymer; EMI, electromagnetic interference; eVTOL, electric vertical take-off and landing; UAV, unmanned aerial vehicle.

2. Review Approach and Evidence Assessment

2.1. Review Design and Questions

A structured critical narrative design was used because the evidence differed in material chemistry, reinforcement architecture, processing route, specimen scale, test standard, environment, certification documentation, and life cycle model. Statistical pooling would suggest comparability not supported by the underlying conditions. Consistent with narrative-review guidance, the scope, source-identification route, analytical categories, and limitations were stated explicitly [22]. The four questions in the Introduction directed source selection and synthesis. The analysis is qualitative and at the claim level, with coverage limited to the verified source set.

2.2. Source Identification

Literature identification followed an iterative, topic-led process across several multidisciplinary scholarly platforms. Searches combined platform terms such as “electric aircraft”, “hybrid-electric aircraft”, “eVTOL”, “advanced air mobility”, “UAV”, and “drone” with terms for thermoplastic CFRP, recycled carbon fibre, natural fibres, bio-based matrices, vitrimers, recyclable thermosets, hybrid systems, and multifunctional composites. Further searches paired the platform and material terms with processing and defects, fatigue, impact, environmental ageing, fire, thermal runaway, EMI, dielectric behaviour, joining, repair, recycling, life cycle assessment, qualification, and certification. This strategy located broad reviews and primary studies supporting specific claims.

Search terms were expanded when relevant sources introduced an unanticipated material designation, manufacturing route, component, or failure mode. Backward and forward citation tracking linked field-level reviews to original studies. Full texts and bibliographic details were verified through publisher websites and DOI records; official regulator and government repositories provided airworthiness documents, certification guidance, and technical reports. Duplicate records and earlier versions were resolved in favour of the most complete final publication. Sources were retained under the eligibility and claim-support rules in Section 2.3.

The search was last updated in June 2026. No publication-year cutoff was imposed because established aerospace-composite methods and regulatory sources remain relevant; recent sources were preferred for active material, process, and aircraft-technology developments. The critical narrative design used iterative source tracing, evidence verification, and claim-level classification; its methodological protocol did not include search-result counts, formal risk-of-bias scoring, or a PRISMA flow diagram. The resulting risk of selection bias bounds the conclusions to the verified evidence.

2.3. Eligibility and Source Roles

Sources were eligible when the full text and bibliographic identity could be verified, the evidence was transferable to lightweight aerospace or electric-propulsion components, and at least one of the following was addressed: material chemistry or architecture; manufacturing route and defects; mechanical or multifunctional performance; environmental durability; fire, thermal, electrical, or EMI behaviour; joining, repair, or inspection; recycling and life cycle effects; component application; or qualification requirements. The main synthesis covered thermoplastic CFRP, recycled-carbon-fibre composites, natural-fibre composites, bio-based matrices, vitrimers and recyclable thermosets, hybrid polymer composites, and multifunctional polymer- or fibre-composite systems. Metal- and ceramic-matrix composites served only as benchmarks; metallic and ceramic constituents were included when they provided a functional coating, conductive mesh, filler, or protective barrier. Original peer-reviewed studies formed the primary basis for performance claims. Reviews supplied field-level context and unresolved questions. Standards, regulator reports, and certification guidance defined evidence requirements but were not treated as material-performance evidence. Conference papers were retained only for distinctive emerging evidence unavailable in a more complete journal article. A source was excluded when its complete text, bibliographic identity, or relevant test conditions could not be verified, when it duplicated a more complete final publication, or when its subject did not inform a material-selection boundary, component exposure, or failure consequence.

The verified evidence base contained 105 publications and technical documents. Of these, 17 were experimental studies, 20 analytical or computational studies, 57 reviews or syntheses, 10 regulatory or technical guidance sources, and one source addressing narrative review methodology. The principal source role and thematic coverage were recorded separately. Each source received one principal role based on its dominant contribution, while it was also counted in every thematic category for which it supported a claim.

Table 2. Thematic coverage of the verified source set by principal source role.

Thematic Category	EXP	COMP	REV	REG	Total
Electric aircraft, eVTOL, and AAM system context	0	12	9	2	23
UAV material, component, and life cycle assessment context	1	0	2	0	3
Structural performance, fatigue, impact, and crashworthiness	9	9	17	6	41
Thermoplastic CFRP materials, processing, welding, and repair	3	0	15	0	18
Recycled-carbon-fibre and composite recycling	2	2	12	0	16
Natural-fibre and hybrid biocomposites	2	1	9	0	12
Bio-based matrices and recyclable thermosets	8	1	5	0	14
Fire behaviour, smoke, toxicity, and residual integrity	5	1	9	3	18
Battery thermal management and enclosure containment	2	3	8	1	14
EMI shielding and aircraft EMC	0	1	2	0	3
Radomes and dielectric transmission	0	0	1	0	1
Structural batteries	0	1	1	0	2
Manufacturing variability and process control	9	2	29	3	43
Joining, repair, inspection, and continued airworthiness	3	0	11	6	20
Life cycle assessment and circularity	8	4	18	0	30
Qualification, certification, and staged validation	4	5	24	10	43

Notes: EXP denotes an original experimental study; COMP denotes an original analytical or computational study; REV denotes a review or synthesis; REG denotes regulatory or technical guidance. The narrative review methodology source is excluded from the thematic rows. A source containing more than one type of evidence is counted once according to its principal role. Sources can appear in more than one thematic category, so the row totals must not be summed. Abbreviations: AAM, advanced air mobility; CFRP, carbon-fibre-reinforced polymer; EMC, electromagnetic compatibility; EMI, electromagnetic interference; eVTOL, electric vertical take-off and landing; UAV, unmanned aerial vehicle.

presents the resulting distribution across thematic categories and principal roles. Because a source can contribute to several themes, the thematic totals overlap. They describe coverage within the verified source set, not database search yields. The counts identify categories in which the component assessment depends mainly on reviews, analytical work, or a small number of original studies.

Propulsion optimisation, aircraft configuration, airport and vertiport operations, and general electromagnetic or battery-system design were incorporated when they defined a material-selection boundary, component exposure, or failure consequence.

Selection of studies for detailed discussion was guided by four considerations: direct relevance to the material, process, function, component, or failure consequence under assessment; sufficient reporting to classify source role, evidence maturity, and comparability; the highest evidence maturity identified for the claim; and information not provided by a more complete source. Experimental studies supported measured material and component performance, while analytical or computational studies defined system boundaries, coupled behaviour, and design evidence. Reviews established field coverage, mechanisms, and unresolved questions. Regulatory and technical sources defined qualification expectations and were not treated as material performance evidence. Evidence maturity levels and adoption classifications were assigned according to the reported level of demonstration and the qualification relevance of the evidence, rather than publication count. Component conclusions were formulated within the material, process, configuration, and test conditions reported by the selected evidence.

2.4. Data Extraction and Evidence Classification

Claim-level extraction recorded the material family, reinforcement and matrix, fibre architecture, manufacturing method, density, mechanical response, impact and fatigue evidence, environmental conditioning, fire and thermal behaviour, electrical or EMI function, joining and repair, recycling route, component application, test conditions, and reported qualification limitations. Evidence used in the comparative tables and component conclusions was then classified by source role, maturity, and comparability. Source roles comprised direct experimental evidence (EXP), validated analytical or computational evidence (COMP), review-level synthesis (REV), regulatory or standards-based requirements (REG), and author synthesis based on several sources (SYN). Evidence maturity ranged from E0 to E5:E0, concept or unvalidated proposal; E1, initial material or coupon result; E2, repeated or environmentally conditioned coupon evidence; E3, joint, repair, element, or coupled-function evidence; E4, representative subcomponent or component evidence; and E5, design allowables, accepted qualification, certified use, or service evidence.

Comparability was classified as C0 when numerical comparison was indefensible because conditions were incompatible or inadequately reported. C1 denoted comparison requiring explicit qualification. C2 required sufficiently matched material description, method, units, geometry, and conditioning, whether within a single study or across studies. Each claim was given the highest maturity level supported by all defining criteria; if any criterion was unmet, the lower level was retained. Where a source contained evidence at several levels, classification followed the specific claim under evaluation.

The synthesis tables apply these rules to material systems, processes, coupled functions, component applications, and research priorities. The entries identify the supporting source role and maturity, the applicable comparability limit, and the boundary on broader component-level inference.

2.5. Comparative Synthesis and Scope of Inference

Material families were evaluated through component-specific evidence. Direct quantitative comparisons were restricted to C2 evidence. C1 results were retained as condition-specific values or ranges with the relevant material and test differences stated; C0 evidence was synthesised qualitatively. Component conclusions combined function, failure consequence, evidence maturity, and unresolved requirements for manufacturing, durability, repair, fire, thermal behaviour, electrical performance, and inspection. Four adoption classifications were used. A near-term candidate has evidence supporting a bounded application under stated conditions. A conditional candidate has a credible application route but still lacks specified validation or process controls. A research-stage system lacks representative evidence for the proposed component or consequence level. A system is not assessable when verified evidence is insufficient for a component-specific judgement. Each classification applies only to the stated material–process–component combination and operating context. Claim-level classification and explicit comparability limits were used to manage variation in formulation, processing, conditioning, and test practice. Figure 1 summarises the workflow from source identification to component-specific synthesis.

3. Coupled Material Requirements and Component Risk

Material selection for electric aircraft and UAVs is governed by component function and failure consequence as well as mass. Candidate systems must meet the structural, thermal, electrical, environmental, manufacturing, inspection, repair, and continued-airworthiness requirements assigned to the component. Battery mass remains essentially constant throughout the mission [1,2,3,4], while battery integration introduces thermal, electrical, and loss-of-power hazards [5,6]. The required evidence therefore differs by platform and component. Substantiating an avionics housing, battery barrier, or occupied-aircraft primary structure requires component-relevant evidence beyond coupon screening.

3.1. Structural Efficiency, Fatigue, and Impact

Specific stiffness and strength remain primary selection criteria because structural and battery mass constrain payload and range [1,2,3,4,23]. Deformation can affect aerodynamic shape, rotor or propeller clearance, control-surface effectiveness, vibration transmission, and distributed-propulsion alignment. Hybrid-electric aircraft studies couple propulsion, aerodynamics, mission energy management, electrical-system mass, and thermal management [24,25]. Conceptual-design studies also show that component positioning, stability, trim, and structural sizing interact [26]. Sandwich construction can provide efficient bending stiffness at low mass, although its component assessment must include facesheet damage, core crushing, disbonding, inspection, and repair [11].

Durability and damage tolerance require evidence beyond coupon strength. Aerodynamic and vibratory loads may initiate matrix cracking, delamination, joint degradation, and local damage around fasteners or bonded interfaces; impact may reduce residual strength despite limited external deformation [10,20]. Some natural-fibre and hybrid biocomposites combine low density with useful damping [27,28,29]. Bourchak et al. [30] reported mechanical, fatigue, and low-velocity-impact results for one pineapple/Kevlar hybrid, but the evidence remains material-level and provides neither structural allowables nor component validation. An AAM lattice study further shows that crashworthiness depends on energy absorption, force–displacement response, acceleration limits, and validated structural models rather than static properties alone [31].

3.2. Battery, Fire, Thermal, and Electrical Functions

Battery-adjacent structures must retain their assigned functions after mechanical abuse and exposure to the consequences of an internal short circuit, including rapid heat generation, vented gases, flame, smoke, and possible loss of electrical isolation. Lithium-ion battery safety literature distinguishes mechanical, electrical, and thermal abuse; thermal runaway; fire-resistant materials; and suppression strategies [32,33,34]. Aircraft batteries also face stringent mass, power, duty-cycle, and continued-operation requirements [5,6]. A composite enclosure or barrier must therefore be assessed for the relevant combination of heat-flux reduction, flame resistance, smoke and toxicity, venting compatibility, post-impact integrity, and residual load capacity. A single flammability result does not demonstrate resistance to a battery thermal-runaway event.

Electrical requirements are component-specific. Avionics and power-electronics enclosures may require electromagnetic interference shielding, whereas radomes and antenna windows require radio-frequency transparency supported by dielectric and transmission testing. CFRP can provide effective shielding because its conductive fibre network attenuates incident electromagnetic fields. The response is anisotropic and depends on fibre content and length for discontinuous reinforcement, laminate lay-up and orientation for continuous reinforcement, specimen thickness, frequency, processing route, and conductive additions [35]. Shielding effectiveness depends on conductivity, matched measurement methods, and component configuration. Radio-frequency-transparent components require direct dielectric and transmission evidence for the intended geometry and operating band.

3.3. Durability, Manufacturability, Repair, and Circularity

Long-term suitability depends on retained performance and process repeatability. Reviews of natural-fibre systems identify moisture uptake, swelling, interfacial degradation, thermal limitations, and biological or batch variability as recurring constraints [27,28,29]. Evidence for bio-based matrices ranges from formulation-specific resin and composite tests to reviews covering different chemistries [13,36,37,38]. Component use therefore requires hot-wet retention, thermal cycling, fire behaviour, and compatibility with the selected reinforcement and cure process to be established for the defined system. Recycled-carbon-fibre properties depend on recovery route, retained fibre length, surface condition, alignment, contamination, and the architecture achieved in the new composite [39,40,41]. These factors determine whether the recovered material can support a structural element, secondary panel, or non-structural compound.

A high production rate is useful only within a controlled material and process specification. Thermoplastic composites support forming, automated placement, welding, repair, and reprocessing, but crystallinity, void content, residual stress, interfacial bonding, and weld geometry remain process dependent [42,43,44]. Recovery quality also depends on the selected route [45]. Component-level environmental performance depends on structural mass, service life, inspectability, and recovery energy in addition to renewable content or recyclability. Meng et al. [39] demonstrated environmental and cost reductions for specified recycled-carbon-fibre conversion and aircraft-interior applications relative to virgin glass-fibre alternatives. The demonstrated benefit applies to that conversion route and application. Component-level assessment must therefore include manufacturing impacts, retained performance after repair, service life, the end-of-life route, and actual displacement of the reference material [14,16,39].

3.4. Component Consequence and Evidence Threshold

The component recommendations in this review are the result of author synthesis tied to failure consequence and evidence maturity. Conditioned coupon or element evidence may support a near-term or conditional assessment for low- to moderate-consequence covers, interiors, and non-structural UAV parts when the required functions and operating conditions are bounded. Moderate-consequence fairings, access panels, and secondary structures require representative joints, manufacturing-defect limits, environmental ageing, and subcomponent validation. Battery barriers, avionics enclosures, and passenger-adjacent interior panels require coupled safety evidence under the relevant combination of impact, thermal exposure, fire, smoke and toxicity, and electrical conditions. Primary, rotor-support, and crashworthy structures require component-scale validation, controlled manufacturing, validated inspection and repair, and statistically defensible allowables or equivalent qualification evidence. Figure 2 relates material processing, functional performance, and component consequence to an indicative threshold within the E0–E5 framework. These thresholds organise the comparative synthesis as analytical evidence categories distinct from regulatory certification criteria.

4. Sustainable Composite Families: Processing, Structure, Properties, and Limitations

The composite families considered here differ in chemistry, reinforcement form, processing route, density, defect sensitivity, recovery route, and evidence maturity. Table 3 provides a consistent basis for comparing these families. The comparison considers structural performance, environmental response, circularity, manufacturing cost, and evidence maturity. Its interpretation is limited by the compatibility of material definitions and test conditions among the supporting studies. Structural efficiency is represented by specific stiffness, E/ρ, and specific strength, σu/ρ. Comparison of these ratios across studies requires sufficient alignment in density data, fibre architecture, processing state, conditioning history, and test method. Impact tolerance is evaluated from absorbed energy, damage development, and residual strength. Fire performance is assessed from ignition behaviour, heat release, flame spread, smoke, gas toxicity, and residual integrity after fire exposure. Moisture sensitivity is evaluated from property retention following controlled conditioning. Recyclability requires a defined recovery route, recovery yield, and retained material performance. Manufacturing cost includes feedstock, processing time, production yield, tooling, inspection, and repair rather than material price alone. Evidence maturity follows the E0 to E5 scale defined in Section 2.4. Specific property and cost values were not compared across studies when the evidence did not satisfy the C2 comparability requirements. Quantitative results are retained only within the reported material and test boundaries.

4.1. Thermoplastic Carbon-Fibre Composites

Published evidence for thermoplastic CFRP covers high-performance matrices, aerospace manufacturing, forming, welding, demonstrator structures, and recycling [42,43,44,45,46,47]. The category includes markedly different material systems. Semi-crystalline matrices such as PEEK, PEKK, and PPS require control of processing temperature, melt viscosity, crystallisation, cooling rate, and consolidation pressure; lower-temperature polyamides and acrylic thermoplastics have different moisture, thermal, and durability limits [42,43,46]. Fibre sizing and surface chemistry must be compatible with the selected matrix. Transfer of properties between carbon-fibre/thermoplastic systems requires matched matrix chemistry, fibre architecture, void content, and thermal history.

Thermoplastic matrices can be formed and joined without a separate irreversible cure reaction. Ultrasonic and resistance welding [44], together with laser joining of CFRTP-to-metal assemblies [47], localise heating at the interface and can shorten assembly time. Joint quality depends on the process window, intimate contact, pressure, cooling history, energy-director or heating-element geometry, fibre orientation, and surface condition. Excessive heat can degrade the matrix or alter crystallinity, whereas inadequate heat leaves incomplete fusion. Porosity, fibre movement, residual stress, and heat-affected regions may then govern fatigue and impact response. Welding defines a joining route, not component readiness; substantiation still requires welded or repaired elements tested after representative ageing and cyclic loading.

Reprocessing and scrap recovery provide further options, but recovered material may not retain its original structural role. Repeated thermal and mechanical processing can change molecular weight, crystallinity, contamination, fibre length, and fibre/matrix adhesion, while mixed assemblies may remain difficult to separate [42,45]. Published demonstrations support component-specific assessment of formed skins, clips, brackets, access panels, fairings, and selected secondary structures when consolidation and joint quality are controlled [42,43,44]. Primary airframe and rotor-load-path applications require E4–E5 evidence for impact, fatigue, hot-wet durability, weld-defect tolerance, inspection, and repair. Qualification must refer to a defined material–process specification; any recyclability claim must also specify the recovery route.

4.2. Recycled-Carbon-Fibre Composites

Recycled-carbon-fibre performance depends on recovery route and recovered architecture. Mechanical recycling often leaves part of the original matrix and yields flakes, strips, or milled reinforcement. Thermal and chemical routes can recover cleaner fibres but may change length, sizing, and surface condition [40,41,48], affecting wetting, load transfer, dispersion, orientation, and property scatter. Environmental and economic outcomes also depend on the virgin material displaced, recovered-fibre architecture, processing route, and equivalent component performance [39].

Butenegro et al. [49] provide a traceable coupon example. Expired aerospace prepreg was cut into fragments 40 ± 2 mm long and 1.2 ± 0.2 mm wide, then consolidated with bio-based PA11 at 200 °C and 0.5 MPa. At a calculated reinforcement volume fraction of 51.5 ± 0.7 vol.%, the quasi-aligned composite reached a tensile modulus of 80 ± 14 GPa and an ultimate tensile strength of 460 ± 60 MPa under . Three-point bending gave a flexural modulus of 91 ± 3 GPa and a maximum flexural stress of 650 ± 80 MPa. These values describe one long-fragment, quasi-aligned architecture from a defined waste stream and process. Transfer to other feedstocks or production routes must address the reported scatter, feedstock variability, and scale-up.

Published routes range from moulded compounds containing milled or short fibres to compression-moulded materials based on longer, traceable prepreg fragments [48,49,50]. Aerospace assessment requires controlled feedstock identity, contamination limits, retained fibre length, orientation distribution, and surface condition, together with process monitoring, environmental conditioning, quantified batch variability, and statistically defensible allowables. Direct substitution for virgin continuous CFRP requires these controls in material and process specifications, followed by element- or component-scale verification.

4.3. Natural-Fibre Composites

Flax, hemp, jute, kenaf, sisal, pineapple-leaf fibre, and related lignocellulosic reinforcements offer low density, renewable feedstock, useful damping, and lower tool abrasion than glass-fibre reinforcement [12,27,28,29]. Their behaviour reflects a hierarchical and naturally variable microstructure. Fibre diameter, lumen content, microfibril angle, harvest and extraction conditions, yarn twist, moisture history, and surface treatment influence stiffness and strength. Hydrophilic constituents promote moisture uptake and swelling, while limited compatibility with non-polar or densely cross-linked matrices can weaken the interface. Thermal degradation and flammability further constrain processing and service temperature [27,28,29,51].

Ciardiello et al. [52] compared four-ply woven-flax laminates containing 30–31% fibre and made by VARTM with a conventional epoxy, a 31% bio-content epoxy, and a recyclable acrylic thermoplastic matrix. At 30–40% relative humidity, the flax fabric contained about 6.1 wt.% moisture and regained up to 98% of its original mass within 25 min after oven drying. Drying reduced measured porosity from 2.3% to 0.4% in the conventional epoxy laminate and from 4.6% to 0.4% in the bio-epoxy laminate; no statistically significant improvement in tensile or flexural response was observed under the tested conditions. The average tensile modulus was approximately 8.5 GPa. The recyclable-matrix laminates had approximately 8% lower tensile strength than the two epoxy systems and absorbed 50% more energy than the conventional epoxy during 25 J perforation tests. They also showed greater resistance to repeated 3.5 J impacts. For this material set, the matrix choice produced a measured trade-off between tensile strength and impact response.

The results constitute E2 evidence for one woven-flax architecture and three commercial matrices. Further evidence is required for higher humidity, freeze–thaw cycling, UV exposure, fatigue, biological variability, and fire. Flame retardants, fibre treatments, and protective coatings can modify fire response as well as mechanical and processing behaviour [29,51]. The available evidence can support screening for interiors, acoustic panels, fairings, payload housings, and non-primary UAV structures where inspection or replacement is practical and moisture, fire, and process variability are controlled. Primary structures and battery-adjacent parts require component evidence at substantially higher maturity.

4.4. Bio-Based Matrices and Recyclable Thermosets

Bio-based content and recyclability are separate material attributes. Replacing part of the petroleum-derived feedstock may reduce fossil resource demand, but a bio-based thermoset can remain permanently cross-linked and difficult to recover. Conversely, a recyclable matrix may contain limited renewable content. Resin chemistry must therefore be reported together with bio-content, curing route, glass-transition temperature, toughness, moisture resistance, fire behaviour, and the recovery mechanism [13,36,37]. Aerospace suitability depends on formulation-specific glass-transition temperature, moisture sensitivity, fire behaviour, and other service properties in addition to renewable fraction.

Mezeix et al. [38] compared one conventional epoxy with three commercial bio-based systems using standardised neat-resin tests. The recyclable formulation contained 27% bio-content and had a glass-transition temperature of 117 °C, a tensile modulus of 3.07 GPa, tensile strength of 74 MPa, and plane-strain fracture toughness of 4.50 $\mathrm{M}\mathrm{P}\mathrm{a}·\sqrt{\mathrm{m}}$. Two non-recyclable bio-epoxies with 33% and 48.9% bio-content gave glass-transition temperatures of 156 °C and 128 °C, tensile strengths near 62 MPa, and fracture-toughness values near 3.1 $\mathrm{M}\mathrm{P}\mathrm{a}·\sqrt{\mathrm{m}}$. These results show formulation-dependent mechanical response; bio-content and recyclability do not determine it independently. The evidence remains E1 because the tests concerned neat-resin coupons and did not assess fibre-reinforced laminates, joints, aged specimens, or fire-exposed components.

Covalent adaptable networks, including vitrimers, permit bond exchange under defined thermal or chemical conditions and have been investigated for repair, reshaping, reprocessing, matrix degradation, and fibre recovery [53,54]. Dinu et al. [55] demonstrated chemical recyclability, reprocessability, repair, and intrinsic flame resistance in a bio-based CFRP; Gu et al. [56] and Tian et al. [57] reported recyclable fire-safe carbon-fibre systems based on other network chemistries. Activation temperature, network chemistry, and catalyst choice influence creep, stress relaxation, and property retention. Heterogeneous test temperatures, solvents, cycle counts, and recovery metrics preclude numerical pooling. Aerospace assessment requires retained performance after repeated repair or recycling, hot-wet ageing, fatigue, impact, chemical exposure, and fire/smoke/toxicity tests. Current evidence is mainly at material and coupon scale. Conditional component use would require defined functional tests for each repairable panel, interior, barrier, or secondary structure.

4.5. Hybrid and Multifunctional Systems

Hybridisation is an architectural strategy. Natural, recycled, synthetic, conductive, insulating, thermal, and fire-resistant constituents can be positioned according to load path and component function. Carbon-, glass-, aramid-, or basalt-fibre outer plies may carry load or protect a natural-fibre layer. Local thermoplastic layers can provide weld interfaces or alter damage tolerance. Fire-retardant fillers, modified fibres, and multilayer construction can change thermal stability and flame response [51,55,56,57,58]. The resulting performance depends on constituent placement, interface design, structural loading, and exposure. Aerospace and UAV reviews provide the component context for this assessment [59,60].

Hybrid designs also add interfaces and trade-offs. Conductive additions may improve EMI shielding but increase density, radio-frequency loss and, when metallic, galvanic-corrosion risk. Fire-retardant fillers may increase viscosity or reduce toughness. Thermal barriers can impede heat rejection during normal battery operation, while phase-change materials add mass and have finite heat-storage capacity. Manufacturability, inspectability, repairability, and end-of-life separation require evidence beyond an initial coupon result. Fire and thermal-management studies differ in heat flux, enclosure geometry, boundary conditions, and performance measures [51,58,61,62,63]. Haris et al. [64] provide preliminary radome-material screening; enclosure shielding requires separate evidence. EMI studies differ in frequency, specimen thickness, architecture, and measurement method [35,65]. Across the material-family evidence summarised in Table 3, most support remains at E0–E2 with C0–C1 comparability.

Each functional addition should correspond to a defined component requirement and be tested after impact, ageing, repair, and manufacturing variation. Battery-enclosure evidence separates ground-impact resistance, flame response, normal thermal management, and thermal-runaway propagation, then evaluates their interaction [61,62]. A composite enclosure with embedded copper thermal bridges has been evaluated as a passive heat-spreading concept for one EV battery-module configuration [63]. Radome evidence uses dielectric constant, mass, toughness, and rain-erosion resistance for initial screening; [64] treats natural-fibre systems only as candidates for further study. Avionics housings are assessed for EMI shielding [35,65], while radomes are assessed for radio-frequency transmission [64]. Hybrid and multifunctional systems require defined constituent roles, inspectable interfaces, retained function after relevant exposure, and component-level accounting of end-of-life burdens.

Table 3 compares the candidate families using the common structural, impact, fire, moisture, recyclability, cost, and maturity indicators defined above. The entries report quantitative values only when the underlying study supports C1 or C2 use and otherwise identify the direction and source of uncertainty. The matrix supports component screening and does not constitute a universal material ranking.

Table 3. Common performance indicator matrix for sustainable composite families.

Candidate Family	Specific Stiffness, Strength, and Impact	Fire and Moisture Sensitivity	Recyclability and Manufacturing Cost	Evidence Maturity and Component Implication
Thermoplastic CFRP [42,43,44,45,46,47]	High structural efficiency is feasible, but E/ρ and σu/ρ are not pooled because matrix, architecture, voids, thermal history, and tests differ. Impact tolerance and weld fatigue remain system specific.	Fire, smoke, toxicity, and post-fire integrity are matrix and laminate specific. Moisture retention must be verified for each matrix, interface, and weld.	Remelting, forming, welding, and reprocessing routes are available. High resin and processing costs can be offset by rapid forming, integrated joining, lower scrap, and production volume; no common cost value is transferable.	EXP/REV; E2 to E4; C0 to C1. Strongest near-term evidence, but component use remains conditional on process conformity, impact, ageing, inspection, and repair durability.
Recycled-carbon-fibre systems [39,40,41,48,49,50]	Performance depends on recovered fibre length, alignment, surface state, and matrix. For one aligned prepreg fragment and PA11 system, E = 80 ± 14 GPa and σu = 460 ± 60 MPa [49]. Laminate density was not reported, so specific values were not calculated. Impact evidence is limited.	Fire and moisture responses depend on the new matrix, contamination, and recovered architecture. Transferable conditioned and post-fire datasets are unavailable.	Mechanical, thermal, chemical, and thermoplastic recovery preserve different fibre forms and values. Sorting, recovery yield, alignment, traceability, and quality control determine cost; no common economic basis is reported.	EXP/REV; E1 to E3; C0 to C1. Defensible for bounded lower consequence components when feedstock classes, batch variation, durability, and component repeatability are controlled.
Natural-fibre and hybrid biocomposites [27,28,29,30,51,52]	Low density can benefit structural efficiency, but cross-study E/ρ and σu/ρ values are unavailable. Woven flax laminates gave E ≈ 8.5 GPa; the recyclable acrylic system had about 8% lower tensile strength and 50% greater perforation energy absorption than the conventional epoxy system within one study [52].	Combustible fibres require a defined fire protection strategy. Reference [52] measured about 6.1 wt.% moisture in flax fabric and rapid moisture regain, showing the need for controlled storage, drying, conditioning, and retained property tests.	Renewable fibres and lower raw material cost are possible advantages, but circularity depends on the matrix and assembly. Biological variability, drying, storage, fire treatment, process yield, and inspection add manufacturing cost.	EXP/REV; E1 to E2; C0 to C2 within matched studies. Use remains conditional for UAV covers, interiors, and other lower consequence parts; occupied aircraft use needs fire, ageing, and production evidence.
Bio-based matrices and recyclable thermosets [13,36,37,38,53,54,55,56,57]	Neat resin tests report modulus and strength ranges, while several CFRP studies show promising laminate performance. Cross-formulation-specific properties are not pooled. Impact, fatigue, creep, and repeated repair data remain sparse.	Several laboratory systems combine recycling or repair with improved small-scale fire response [55,56,57]. Results are protocol specific, and comparative smoke, toxicity, post-fire strength, and hot-wet retention are unavailable.	Repair, reprocessing, matrix degradation, and fibre recovery have been demonstrated for selected chemistries. Activation conditions, solvents, recovery yield, repeated cycle retention, scale, and quality control prevent a transferable cost comparison.	EXP/REV; E1 to E2; C0 to C2 within matched studies. Evidence supports material screening, not a general component readiness claim.
Hybrid and multifunctional systems [35,51,55,56,57,58,61,62,63,64,65]	Structural benefit depends on constituent placement and the mass and defects introduced by added layers, fillers, barriers, or conductors. Impact and residual strength metrics are not comparable across architectures.	Fire, thermal, shielding, or dielectric functions can improve for a defined architecture, but metrics differ in heat flux, frequency, thickness, and geometry. Moisture and ageing can alter interfaces and functional retention.	Mixed constituents and permanent interfaces can hinder separation and recovery. Added processing, inspection, repair, and functional testing increase cost; benefit must be evaluated at component level.	EXP/REV; E0 to E2; C0 to C1. Component adoption remains research stage until structural and functional requirements are demonstrated together after ageing, impact, and repair.

Notes: E is elastic modulus, ρ is density, and σu is ultimate strength. Reported modulus and strength values are not specific properties unless the density of the tested laminate is also available. Cost statements identify manufacturing drivers because the sources do not provide a common production volume, yield, quality control scope, or economic functional unit. Every interpretation remains bounded by the reported material, process, component, and evidence maturity.

Table 3. Common performance indicator matrix for sustainable composite families.

Candidate Family	Specific Stiffness, Strength, and Impact	Fire and Moisture Sensitivity	Recyclability and Manufacturing Cost	Evidence Maturity and Component Implication
Thermoplastic CFRP [42,43,44,45,46,47]	High structural efficiency is feasible, but E/ρ and σu/ρ are not pooled because matrix, architecture, voids, thermal history, and tests differ. Impact tolerance and weld fatigue remain system specific.	Fire, smoke, toxicity, and post-fire integrity are matrix and laminate specific. Moisture retention must be verified for each matrix, interface, and weld.	Remelting, forming, welding, and reprocessing routes are available. High resin and processing costs can be offset by rapid forming, integrated joining, lower scrap, and production volume; no common cost value is transferable.	EXP/REV; E2 to E4; C0 to C1. Strongest near-term evidence, but component use remains conditional on process conformity, impact, ageing, inspection, and repair durability.
Recycled-carbon-fibre systems [39,40,41,48,49,50]	Performance depends on recovered fibre length, alignment, surface state, and matrix. For one aligned prepreg fragment and PA11 system, E = 80 ± 14 GPa and σu = 460 ± 60 MPa [49]. Laminate density was not reported, so specific values were not calculated. Impact evidence is limited.	Fire and moisture responses depend on the new matrix, contamination, and recovered architecture. Transferable conditioned and post-fire datasets are unavailable.	Mechanical, thermal, chemical, and thermoplastic recovery preserve different fibre forms and values. Sorting, recovery yield, alignment, traceability, and quality control determine cost; no common economic basis is reported.	EXP/REV; E1 to E3; C0 to C1. Defensible for bounded lower consequence components when feedstock classes, batch variation, durability, and component repeatability are controlled.
Natural-fibre and hybrid biocomposites [27,28,29,30,51,52]	Low density can benefit structural efficiency, but cross-study E/ρ and σu/ρ values are unavailable. Woven flax laminates gave E ≈ 8.5 GPa; the recyclable acrylic system had about 8% lower tensile strength and 50% greater perforation energy absorption than the conventional epoxy system within one study [52].	Combustible fibres require a defined fire protection strategy. Reference [52] measured about 6.1 wt.% moisture in flax fabric and rapid moisture regain, showing the need for controlled storage, drying, conditioning, and retained property tests.	Renewable fibres and lower raw material cost are possible advantages, but circularity depends on the matrix and assembly. Biological variability, drying, storage, fire treatment, process yield, and inspection add manufacturing cost.	EXP/REV; E1 to E2; C0 to C2 within matched studies. Use remains conditional for UAV covers, interiors, and other lower consequence parts; occupied aircraft use needs fire, ageing, and production evidence.
Bio-based matrices and recyclable thermosets [13,36,37,38,53,54,55,56,57]	Neat resin tests report modulus and strength ranges, while several CFRP studies show promising laminate performance. Cross-formulation-specific properties are not pooled. Impact, fatigue, creep, and repeated repair data remain sparse.	Several laboratory systems combine recycling or repair with improved small-scale fire response [55,56,57]. Results are protocol specific, and comparative smoke, toxicity, post-fire strength, and hot-wet retention are unavailable.	Repair, reprocessing, matrix degradation, and fibre recovery have been demonstrated for selected chemistries. Activation conditions, solvents, recovery yield, repeated cycle retention, scale, and quality control prevent a transferable cost comparison.	EXP/REV; E1 to E2; C0 to C2 within matched studies. Evidence supports material screening, not a general component readiness claim.
Hybrid and multifunctional systems [35,51,55,56,57,58,61,62,63,64,65]	Structural benefit depends on constituent placement and the mass and defects introduced by added layers, fillers, barriers, or conductors. Impact and residual strength metrics are not comparable across architectures.	Fire, thermal, shielding, or dielectric functions can improve for a defined architecture, but metrics differ in heat flux, frequency, thickness, and geometry. Moisture and ageing can alter interfaces and functional retention.	Mixed constituents and permanent interfaces can hinder separation and recovery. Added processing, inspection, repair, and functional testing increase cost; benefit must be evaluated at component level.	EXP/REV; E0 to E2; C0 to C1. Component adoption remains research stage until structural and functional requirements are demonstrated together after ageing, impact, and repair.

Notes: E is elastic modulus, ρ is density, and σu is ultimate strength. Reported modulus and strength values are not specific properties unless the density of the tested laminate is also available. Cost statements identify manufacturing drivers because the sources do not provide a common production volume, yield, quality control scope, or economic functional unit. Every interpretation remains bounded by the reported material, process, component, and evidence maturity.

5. Manufacturing, Joining, Repair, Inspection, and Circular Routes

Moving from a candidate material to an aircraft component requires coordinated control of manufacture, joining, inspection, repair, and recovery. Component relevance requires quantified production defects, inspectable joints, recovery of assigned functions after repair, and a recovery route that preserves the required architecture. For electric aircraft and UAVs, process value is determined by repeatable component quality and traceable records extending from feedstock and manufacture through inspection, repair, return to service, and recovery. Production rate and recyclability are reported as separate attributes alongside component conformity.

5.1. Process Repeatability, Defects, and Traceability

Automated fibre placement (AFP), filament winding, press consolidation, thermoforming, and in situ consolidation can raise production rate and geometric repeatability, but variability remains. In AFP, lay-up strategy, tow steering, part curvature, heat input, compaction, cutting, and machine motion interact to produce gaps, overlaps, thickness variation, wrinkles, bridging, twisted, missing, or loose tows, position errors, and foreign-object inclusions [66,67]. For thermoplastic in situ consolidation, heat-source distribution, nip-point temperature, placement speed, compaction pressure, intimate contact, interdiffusion, cooling rate, crystallinity, and void closure define the laminate state [66,68]. A qualified process window must connect machine settings with measured through-thickness thermal history, fibre placement, porosity, crystallinity, and interlaminar bonding. Surface temperature and placement speed are inputs, not independent measures of consolidation quality.

In-process monitoring and post-process inspection address different manufacturing stages. Thermography, laser profilometry, machine vision, and force or temperature records reveal placement and process anomalies; ultrasonic inspection and computed tomography target selected internal defects [66]. An indication becomes an acceptance variable only when its geometry is related to fatigue, impact, joint behaviour, or functional performance. Qualification records should connect material batch and storage history, path and process parameters, detected anomalies, disposition or repair, and final inspection. Recycled-fibre records must also identify feedstock origin and prior matrix, recovery route, recovered form and length, surface condition, contamination, orientation or alignment, and batch composition [41,48,49,69,70]. Natural-fibre records should include species, cultivation and harvest conditions, extraction and treatment, moisture content, storage, and processing temperature [27,28,29,51,52].

5.2. Joining, Repair, Inspection, and Return-to-Service Evidence

Joining and repair are controlled structural processes. Resistance, induction, and ultrasonic welding localise heat at thermoplastic interfaces; laser joining has also been reported for CFRTP-to-metal assemblies [44,47,71,72]. Joint quality depends on matrix chemistry, interface temperature, pressure, heating duration, heating-element or energy-director geometry, heat distribution, cooling history, fibre orientation, and surface condition. Resistance welding may cause current leakage into carbon reinforcement, non-uniform heating, resin squeeze-out or depletion, and corrosion risk when metallic heating elements remain in the joint [71]. Substantiation requires a defined process window, destructive mechanical tests, fatigue testing, environmental conditioning, and a validated non-destructive inspection method. Dissimilar thermoplastic-to-thermoset and composite-to-metal joints require separate evidence because thermal incompatibility and residual stress may govern failure [71,72].

Repair assessment combines strength recovery with failure mode, durability, repeatability, and field-process evidence. Khan et al. [73] used 1.43° and 5.71° scarfs in CFRP laminates repaired with a liquid thermoplastic resin at 25 °C or 210 °C. Four-point bending gave residual flexural strengths of 45% and 57% at 25 °C and 70% and 97% at 210 °C for the 1.43° and 5.71° scarfs, respectively. The 97% result approached the pristine flexural strength, although repaired specimens remained delamination-dominated and did not reproduce the pristine fibre-fracture and pull-out mechanisms. This E1 evidence applies to one laminate, repair resin, specimen geometry, and bending programme. Further validation is required for fatigue, environmental durability, repeated repair, and field-process variability.

Modi et al. [74] applied two induction-heating repair routes to CF/PEEK and CF/PEKK laminates damaged at 20 ± 1 J: a welded patch and local reheating/reconsolidation. Compression-after-impact-and-repair strength was 7–17% higher than the unrepaired compression-after-impact value, depending on material and route. Ultrasonic C-scans indicated only an approximate 3–5% reduction in damage area; matrix cracking, delamination, and broken fibres remained. The test separates matrix reconsolidation from fibre restoration. Local heating can improve residual compressive capacity by rebonding part of the matrix-dominated damage, while fractured fibres and failure outside the patch continue to limit recovery.

Return-to-service decisions require a damage map, restored load capacity, recovery of every assigned function, environmental durability, and repair repeatability [71,73,74,75]. Bonded thermoset repairs remain sensitive to surface preparation, cure quality, moisture, temperature, and combined hygrothermal exposure [75]. Thermoplastic repairs additionally require temperature, pressure, cooling, and crystallinity control [71,72,73,74]. For multifunctional parts, the repaired configuration must recover the relevant electrical continuity, shielding, dielectric or radio-frequency transmission, thermal transport, or fire-protection function. Ultrasonic C-scan, thermography, shearography, radiography, and local electrical or thermal measurements should be selected for the expected defect and component role. Visual closure of a damaged region is insufficient evidence for return to service.

5.3. Circular Routes and Component-Level Life Cycle Assessment

Circular value depends on the material form and function preserved by recovery. Mechanical recycling generally shortens fibres and produces mixed fibrous, flake, or powder fractions [50,69,70,76]. Elango et al. [50] identify milled recycled fibres as a separate material class that is usually excluded from primary structural use and requires dedicated qualification. Pyrolysis may retain more fibre length than mechanical size reduction, but can alter sizing, surface chemistry, and strength; the matrix becomes gases, oils, and char [70,77,78]. Solvolysis can recover relatively clean fibres and, in some systems, matrix-derived products, but requires solvent recovery, separation, purification, and process control [70,76,78]. Thermoplastic dissolution or remelting may retain more polymer value when contamination, molecular degradation, and mixed-material assemblies are controlled [39,40,41,45]. Recovery specifications should identify the intended output: continuous fabric, aligned fragments or platelets, nonwoven mat, chopped fibre, milled filler, recovered polymer, matrix-derived chemicals, or energy.

Life cycle assessment should compare alternatives that provide the same component function over a defined service life [79,80]. Results depend on the functional unit, system boundary, production yield, scrap rate, electricity and heat sources, transport, component mass, service life, inspection, repair, replacement, recovery yield, and virgin-material displacement [79,80,81]. Manufacturing variability also matters: rejected parts increase production burden, while statistically driven overdesign can increase component mass and aircraft use-phase energy demand [82]. A heavier repairable panel can have lower life cycle impact only when longer service life and avoided replacement offset added production and use-phase burdens. A recyclable laminate may offer little benefit if coatings, fasteners, adhesives, functional layers, or mixed skins prevent economical separation. Sensitivity analysis is needed because allocation, energy mix, transport, recovery efficiency, and secondary-material substitution can materially change the comparison [79,80,81,82].

The circular strategy should be defined during component architecture, before final material and joint selection. Each component needs a planned disassembly route, accessible damage zones, compatible repair materials, and an intended second-life or recovery class. Traceability should follow recovered feedstock into the next product so that recycled content remains linked to property and processing history. Table 4 records the process, defect, repair, inspection, and circularity controls used in the remainder of this review. The entries guide component selection and do not rank manufacturing or recovery routes.

Table 4. Process, defect, inspection, repair, and circularity controls for sustainable composite systems.

Route/System	Critical Process Variables and Defects	Verification and Inspection	Repair or Circular Route	Evidence Interpretation and Qualification Gap
AFP/in situ thermoplastic consolidation [66,67,68]	Heat distribution, nip temperature, speed, pressure, intimate contact, crystallinity, void content; gaps, overlaps, wrinkles, bridging, twisted, missing, or loose tows, position errors, and foreign-object inclusions.	Thermography, laser profilometry, machine vision, and force and temperature records; ultrasonic inspection or computed tomography for selected internal defects.	Local reconsolidation; production-scrap reuse where matrix identity, contamination, and retained properties permit.	REV; E1–E3. Acceptance limits require a measured relation between defect geometry and its effect on fatigue, impact, joint behaviour, or assigned function at element scale.
Thermoplastic welding [71,72]	Matrix chemistry, interface temperature, pressure, time, cooling, heating-element or energy-director geometry, current leakage, non-uniform heating, residual stress, and resin squeeze-out or depletion.	Thermal monitoring, microscopy, ultrasonic or thermographic inspection; lap-shear, fracture, fatigue, and environmental testing.	Rewelding or local patching may be possible; disassembly depends on the joint and retained heating-element design.	REV; E1–E4. Demonstrator evidence supports process maturity. Qualification remains specific to material, joint geometry, defect population, environment, inspection, and repair route.
Bonded thermoset repair [75]	Damage removal, taper or scarf geometry, surface preparation, adhesive thickness, cure cycle, moisture, and contamination.	Ultrasonic C-scan, thermography, shearography, or radiography; residual-strength and environmental-durability tests.	Scarf or stepped bonded patch; limited material recovery after cure.	REV; E2–E4. Hot-wet durability, workmanship, repeatability, and inspectability remain controlling.
Liquid-thermoplastic scarf repair [73]	Scarf angle, repair temperature, resin infiltration, and interfacial consolidation.	Four-point bending and fractography in the cited study.	Liquid-thermoplastic scarf repair; the cited study assessed one repair cycle.	EXP; E1; C2 within study. Residual flexural strength was 45% and 57% at 25 °C, increasing to 70% and 97% at 210 °C for 1.43° and 5.71° scarfs, respectively. Repaired failure remained delamination dominated; fatigue and environmental durability were not assessed.
Induction repair of impacted CF/PEEK and CF/PEKK [74]	Initial impact damage, patch geometry, heating frequency and power, surface temperature, and applied pressure.	ASTM impact and compression-after-impact-and-repair tests plus ultrasonic C-scan.	Welded patch or local matrix reconsolidation.	EXP; E2; C2 within study. CAI-R strength was 7–17% above unrepaired CAI, while C-scan damage area decreased by approximately 3–5%, depending on material and route; fibre fracture persisted.
Recycled-carbon-fibre manufacture [39,40,41,48,49,50,69,70,76,77,78]	Feedstock origin and prior matrix, recovered fibre form and length, sizing and surface condition, oxidation, contamination, orientation or alignment, and batch composition.	Feedstock records, microscopy and surface analysis, fibre-length and orientation distributions, contamination assessment, and batch mechanical data.	Mechanical, thermal, chemical, or thermoplastic recovery; second-life product matched to the recovered material class and retained properties.	EXP/REV; E1–E3. Design allowables require traceable feedstock, defined recovered-material classes, controlled processing, and quantified batch variability.
Natural-fibre and bio-based processing [27,28,29,38,51,52]	Species and cultivation or harvest conditions, extraction and treatment, moisture content and regain, thermal degradation, resin flow, porosity, and fire additives.	Material-provenance, moisture and storage records, porosity, microscopy, and mechanical and fire testing after environmental conditioning.	Repair or replacement for lower-consequence parts; recovery determined by matrix, coatings, and additives.	EXP/REV; E1–E2. Controlled coupon evidence does not establish service durability, batch repeatability, or compostability or recyclability of an assembled component.
Component-level circularity and LCA [79,80,81,82]	Functional unit, production yield, scrap, electricity and heat source, component mass, service life, inspection, repair, replacement, recovery yield, recovered material class, and actual virgin-material displacement.	Auditable inventory and sensitivity analysis tied to the specified component and production route, with verification of secondary-use properties where claimed.	Design for disassembly, reuse, remanufacture, fibre or matrix recovery, matrix-derived chemical recovery, or defined downcycling.	REV/SYN; C0–C2. Environmental preference depends on functional unit, boundaries, allocation, energy mix, service life, and substitution assumptions. Material labels and recovery yield do not establish circularity.

Note: EXP = direct experimental evidence; REV = review-level synthesis; SYN = author synthesis. E0–E5 and C0–C2 are defined in Section 2.4. Quantitative values apply only to the stated materials and test conditions and are not design allowables.

Table 4. Process, defect, inspection, repair, and circularity controls for sustainable composite systems.

Route/System	Critical Process Variables and Defects	Verification and Inspection	Repair or Circular Route	Evidence Interpretation and Qualification Gap
AFP/in situ thermoplastic consolidation [66,67,68]	Heat distribution, nip temperature, speed, pressure, intimate contact, crystallinity, void content; gaps, overlaps, wrinkles, bridging, twisted, missing, or loose tows, position errors, and foreign-object inclusions.	Thermography, laser profilometry, machine vision, and force and temperature records; ultrasonic inspection or computed tomography for selected internal defects.	Local reconsolidation; production-scrap reuse where matrix identity, contamination, and retained properties permit.	REV; E1–E3. Acceptance limits require a measured relation between defect geometry and its effect on fatigue, impact, joint behaviour, or assigned function at element scale.
Thermoplastic welding [71,72]	Matrix chemistry, interface temperature, pressure, time, cooling, heating-element or energy-director geometry, current leakage, non-uniform heating, residual stress, and resin squeeze-out or depletion.	Thermal monitoring, microscopy, ultrasonic or thermographic inspection; lap-shear, fracture, fatigue, and environmental testing.	Rewelding or local patching may be possible; disassembly depends on the joint and retained heating-element design.	REV; E1–E4. Demonstrator evidence supports process maturity. Qualification remains specific to material, joint geometry, defect population, environment, inspection, and repair route.
Bonded thermoset repair [75]	Damage removal, taper or scarf geometry, surface preparation, adhesive thickness, cure cycle, moisture, and contamination.	Ultrasonic C-scan, thermography, shearography, or radiography; residual-strength and environmental-durability tests.	Scarf or stepped bonded patch; limited material recovery after cure.	REV; E2–E4. Hot-wet durability, workmanship, repeatability, and inspectability remain controlling.
Liquid-thermoplastic scarf repair [73]	Scarf angle, repair temperature, resin infiltration, and interfacial consolidation.	Four-point bending and fractography in the cited study.	Liquid-thermoplastic scarf repair; the cited study assessed one repair cycle.	EXP; E1; C2 within study. Residual flexural strength was 45% and 57% at 25 °C, increasing to 70% and 97% at 210 °C for 1.43° and 5.71° scarfs, respectively. Repaired failure remained delamination dominated; fatigue and environmental durability were not assessed.
Induction repair of impacted CF/PEEK and CF/PEKK [74]	Initial impact damage, patch geometry, heating frequency and power, surface temperature, and applied pressure.	ASTM impact and compression-after-impact-and-repair tests plus ultrasonic C-scan.	Welded patch or local matrix reconsolidation.	EXP; E2; C2 within study. CAI-R strength was 7–17% above unrepaired CAI, while C-scan damage area decreased by approximately 3–5%, depending on material and route; fibre fracture persisted.
Recycled-carbon-fibre manufacture [39,40,41,48,49,50,69,70,76,77,78]	Feedstock origin and prior matrix, recovered fibre form and length, sizing and surface condition, oxidation, contamination, orientation or alignment, and batch composition.	Feedstock records, microscopy and surface analysis, fibre-length and orientation distributions, contamination assessment, and batch mechanical data.	Mechanical, thermal, chemical, or thermoplastic recovery; second-life product matched to the recovered material class and retained properties.	EXP/REV; E1–E3. Design allowables require traceable feedstock, defined recovered-material classes, controlled processing, and quantified batch variability.
Natural-fibre and bio-based processing [27,28,29,38,51,52]	Species and cultivation or harvest conditions, extraction and treatment, moisture content and regain, thermal degradation, resin flow, porosity, and fire additives.	Material-provenance, moisture and storage records, porosity, microscopy, and mechanical and fire testing after environmental conditioning.	Repair or replacement for lower-consequence parts; recovery determined by matrix, coatings, and additives.	EXP/REV; E1–E2. Controlled coupon evidence does not establish service durability, batch repeatability, or compostability or recyclability of an assembled component.
Component-level circularity and LCA [79,80,81,82]	Functional unit, production yield, scrap, electricity and heat source, component mass, service life, inspection, repair, replacement, recovery yield, recovered material class, and actual virgin-material displacement.	Auditable inventory and sensitivity analysis tied to the specified component and production route, with verification of secondary-use properties where claimed.	Design for disassembly, reuse, remanufacture, fibre or matrix recovery, matrix-derived chemical recovery, or defined downcycling.	REV/SYN; C0–C2. Environmental preference depends on functional unit, boundaries, allocation, energy mix, service life, and substitution assumptions. Material labels and recovery yield do not establish circularity.

Note: EXP = direct experimental evidence; REV = review-level synthesis; SYN = author synthesis. E0–E5 and C0–C2 are defined in Section 2.4. Quantitative values apply only to the stated materials and test conditions and are not design allowables.

6. Coupled Thermal, Fire, Electrical, and Environmental Performance

Functional performance should be assessed in the structural condition expected in service. Composite substantiation uses a building-block sequence that connects material and process controls with coupon, element, detail, subcomponent, and component evidence [83,84,85]. Sandwich structures introduce penetration, delamination, core crushing, facesheet debonding, damage-detectability, and residual-strength concerns [86]. Durability and damage-tolerance methods also address changes in material properties and the initiation, growth, and residual effects of damage during service [87]. Impact models should be validated by tests at relevant levels of the structural test pyramid [88]. Certification by analysis consequently remains tied to test evidence [89]. Cohesive-zone predictions of disbond and delamination require benchmark problems, process-zone resolution, and convergence checks before use in larger structural models [90].

Retained function provides the comparison basis used in this section. Examples of functional loss include a fire barrier detached by impact, an EMI shield with moisture-induced loss of continuity, and a repaired battery cover whose stiffness is recovered while thermal isolation remains impaired. The comparisons are organised by component function and by performance measured after relevant damage, conditioning, or repair. Component suitability is determined from retained functional performance after relevant damage, conditioning, and repair.

6.1. Fire, Smoke, Toxicity, and Post-Fire Residual Integrity

Fire performance is described by several independent responses. Fire-resistant and fireproof certification tests expose a material or component to prescribed flame conditions; the criteria include duration relative to the applicable metallic reference and continued fulfilment of the intended design purpose [91]. Reaction-to-fire assessment separately considers time to ignition, flame spread, heat-release rate, smoke, and combustion gases [92]. Burn-through testing adds a configuration-level barrier assessment [93]. Tests on aircraft-grade carbon/epoxy under calibrated fire exposure identified cracking, thermal degradation, evolved-gas migration, and delamination [93]. After resin degradation, thermally stable carbon fibres prevented hole formation even though mechanical properties had been lost [93]. Penetration resistance and post-fire structural capacity accordingly require separate measurements. Delayed ignition, low smoke generation, protective-char formation, and retained interlaminar integrity should each be reported against the corresponding test.

These distinctions apply directly to sustainable composite systems. Lignocellulosic fibres contain combustible organic constituents and often moisture-sensitive interfaces. Flame retardants, mineral or ceramic barriers, protective skins, and hybrid reinforcement should be evaluated with porosity, impact tolerance, and retained strength [51,52,58]. Renewable feedstock alone is insufficient to demonstrate compliant fire behaviour. Several recyclable or bio-based thermosets combine repair or recovery chemistry with improved small-scale fire response [55,56,57]. Published demonstrations remain concentrated at resin, coupon, and small-laminate scales. Aircraft substantiation should combine fire, smoke, gas-toxicity, and residual-strength tests with the relevant hot-wet ageing, impact damage, manufacturing defects, and representative repairs because these prior conditions can change heat transport, cracking, volatile release, and barrier continuity.

6.2. Battery-Adjacent Thermal Management, Containment, and Crash Functions

A battery enclosure must carry load, resist impact, remove heat during normal operation, limit abnormal heating, route vent gases, maintain electrical isolation, and remain inspectable. Normal operation favours heat rejection, whereas a cell fault requires enough thermal resistance to delay propagation. Flame retardancy, low-conductivity barriers, and phase-change buffering address complementary functions [61,62]. In the EV battery-module configuration tested by Samarasinghe et al., embedded copper pins reduced surface temperature by up to 16.62% [63]. The result quantifies passive heat spreading for that configuration only; crash resistance, fire response, electrical isolation, and thermal-runaway containment require separate evidence.

Aircraft battery integration also requires evidence at system, crash, thermal, and enclosure levels. Ma and Ding [94] treated electrical propulsion and battery technology as one eVTOL certification problem and proposed a platform covering propulsion, battery, and aircraft-level design. Lobitz et al. [95] identified battery location and prevention of mechanical deformation as unresolved crashworthiness concerns and noted the risk of thermal runaway after deformation. Harrison et al. [96] compared Vahana battery-pack predictions with notional-mission test data, then simulated triggered-cell thermal runaway under different cooling strategies. Liu et al. [97] used a reduced-order model to examine current, terminal voltage, state of charge, heat generation, and temperature non-uniformity for two UAM profiles while varying ambient temperature and payload.

Burner or steady-heat coupons address only part of the required evidence. A representative sequence may combine impact or crushing, internal cell or module heating, venting, flame exposure, and post-event inspection. Structural batteries introduce coupled mechanical, electrical, and electrochemical failure modes because the energy-storage material also carries load [15,98]. Evidence should progress from material thermal response to enclosure joints, seams, attachments, and penetrations, then to module- or pack-level abuse under realistic venting and structural boundary conditions.

6.3. EMI Shielding, Dielectric Transmission, and Electrical Protection

Electrical requirements depend on component function. Avionics and power-electronics housings may need broadband electromagnetic interference (EMI) shielding with controlled bonding and grounding. Radomes and antenna windows instead need low transmission loss and stable permittivity and dielectric loss over the operating band. CFRP shielding is anisotropic and varies with reinforcement continuity, fibre content and orientation, lay-up, thickness, frequency, processing, and conductive additions [35]. Bulk conductivity is insufficient as a sole predictor because discontinuities, joints, seams, apertures, fasteners, and surface treatments can govern leakage. An architecture that benefits an enclosure may compromise a radio-frequency-transparent part. Haris et al. [64] screened natural-fibre biocomposites as radome candidates using dielectric behaviour, density, toughness, and rain-erosion considerations. Their results provide material-screening evidence, while fabricated-radome insertion-loss, antenna-pattern, and environmental-qualification testing remains a separate requirement. Frequency- and geometry-specific dielectric and transmission tests are still required.

Lightning, high-intensity radiated fields (HIRF), grounding, and high-voltage isolation impose additional constraints. Malburg et al. [99] examined an all-electric aircraft architecture combining high-power converters, motors, batteries, extensive cabling, and safety-critical avionics. They identified a changing electromagnetic environment and recommended assessing the limitations of existing aerospace EMC standards. Metal coatings and carbon fillers can increase shielding, with results dependent on filler content and dispersion and with possible processability or mechanical penalties [65]. Electrical functionality should be remeasured after the applicable fatigue, impact, thermal cycling, moisture, lightning exposure and associated heating, and repair history. Component-level tests should include joints, seams, apertures, connectors, coatings, grounding paths, and local damage.

6.4. Environmental Conditioning and Coupled-Function Retention

Environmental exposure can alter the microstructure governing mechanical and functional performance. Moisture may plasticise polymer matrices, swell natural fibres, weaken interfaces, change dielectric response, and promote corrosion near metallic additions. Thermal cycling redistributes residual stress and may extend interfacial damage, alter crystallinity in semi-crystalline matrices, or change contact resistance in conductive joints and layers. Natural-fibre systems therefore need retained-property data after defined moisture, temperature, and weathering histories [27,28,29]. Ciardiello et al. [52] examined one controlled moisture condition and measured porosity and mechanical response, but not long-term durability across broader exposures. Bio-based matrices likewise require formulation-specific conditioning because available evidence ranges from hydrothermal neat-resin tests to unconditioned mechanical baselines [36,37,38]. Functional layers should be tested after relevant exposures, with retained performance reported against the defined exposure history and a dry baseline.

Test order can influence the measured response. Impact before fire may create delaminations and vent paths. Moisture before electromagnetic testing may alter permittivity, dielectric loss, and leakage. Thermal cycling before repair changes the substrate condition, while cycling after welding or repair probes joint retention. Repair can also interrupt conductive or barrier layers. The isolated coupon studies reviewed here seldom address these interactions. Table 5 summarises the coupled evidence for representative component classes. Evidence maturity records the published validation level, and the qualification-gap column specifies the sequenced tests still needed. These entries do not rank material systems.

Table 5. Coupled thermal, fire, electrical, and environmental evidence needs and qualification gaps for representative sustainable-composite component classes.

Component/Material System	Required Functional Evidence	Conditioning and Coupled Tests	Evidence Interpretation	Unresolved Qualification Gap
Natural-fibre or bio-based interior or secondary panel [51,52]	Time to ignition, flame spread, heat-release rate, smoke and gas toxicity, burn-through where applicable, and post-fire residual strength.	Hot-wet ageing and moisture cycling; impact or fastener damage before fire; coatings, joints, and exposed-edge conditions.	EXP/REV; E1–E2; C1. The reported static, impact, moisture, and fire results apply to their tested material and configuration.	Aircraft fire, smoke, and toxicity tests after ageing; batch variability; representative joints; repair followed by structural and fire retesting.
Recyclable or bio-based CFRP with fire-modified matrix [55,56,57]	Fire, smoke, and gas-toxicity response; glass-transition temperature; toughness; repair or reprocessing; and retained laminate properties.	Thermal cycling and hot-wet ageing; repeated repair or reprocessing; impact before fire.	EXP; mainly E1–E2. The sources demonstrate laboratory-scale fire response, repair, or recycling; aircraft-configuration evidence remains unavailable.	Laminate-, joint-, and component-scale smoke and toxicity, fatigue, repeated-cycle property retention, and post-fire residual-strength data.
Composite battery enclosure with passive thermal-management features [61,63]	Impact/crush resistance, normal heat rejection, abnormal heat isolation and propagation resistance, vent-gas management, electrical isolation, flame response, and attachment integrity.	Mechanical abuse followed by cell- or module-level heating; vibration and ageing; representative seams, fasteners, ports, access panels, and attachments included.	EXP/COMP/REV; E1–E3. Reference [63] quantifies passive heat spreading for one EV battery-module configuration. The other listed functions lie outside that experiment.	Representative enclosure- or pack-level abuse tests with controlled venting, post-event structural and functional inspection, repeated builds, and process repeatability.
Low-conductivity flame-retardant barrier or phase-change composite [62]	Heat-flux delay, thermal-runaway propagation resistance, flame response, dimensional stability, and compatibility with enclosure loads and interfaces.	Repeated heating, compression, vibration, impact, vent-gas exposure, and realistic contact with cells or modules.	EXP; E1–E2. Propagation delay was demonstrated for one material and configuration. Transfer depends on geometry, thermal boundary conditions, and heat-sink capacity.	Module- or pack-level validation, ageing, barrier attachment, repeatability, and post-event structural integrity.
Conductive CFRP avionics or power-electronics enclosure [35,65]	Shielding effectiveness versus frequency and direction; continuity across joints, seams, and apertures; electrical bonding and grounding; and residual structural performance.	Moisture, thermal cycling, fatigue, impact, repair, and galvanic-corrosion exposure where metallic additions are present, followed by EMI retesting.	REV; E1–E2; C0–C2. Reported shielding depends on method, thickness, lay-up, frequency, and specimen configuration.	Enclosure-level broadband testing with seams, connectors, apertures, grounding paths, repaired regions, damage, and production variability.
Natural-fibre biocomposite radome candidate [64]	Permittivity, dielectric loss, insertion loss, antenna-pattern retention, moisture sensitivity, rain-erosion resistance, and structural integrity.	Rain and moisture, temperature cycling, erosion, impact, coatings, bonded joints, lightning-protection integration, and repair.	REV; E0–E1. Evidence is limited to a preliminary material-screening review; fabricated-radome, insertion-loss, radiation-pattern, and environmental-qualification data remain unavailable.	Frequency- and geometry-specific radome and antenna tests after environmental ageing, impact, coatings, lightning-protection integration, and repair.
Hybrid lightning/EMI/fire multifunctional laminate [58,65,99]	Lightning-current distribution, shielding, thermal-barrier and fire response, interlaminar strength, and electrical isolation between functional layers.	Representative lightning-current or high-current exposure combined with fatigue, moisture, impact, and a galvanic environment.	REV/SYN; E0–E2. The combined assessment is author synthesis. Added functions introduce new interfaces and coupled failure modes.	Coupled structural–electrical–fire validation, inspectability, restoration of each function after repair, and end-of-life separation.
Aged or repaired multifunctional component [52,71,72,73,74]	Restored structural, thermal, fire, shielding, or dielectric function, with strength recovery reported separately.	Representative ageing and damage; repair; repeated inspection; then structural and assigned-function retesting.	EXP/REV; E1–E3. Current repair evidence is mainly structural and specific to the tested material, geometry, and process.	Return-to-service limits, repeated-repair capability, functional-layer restoration, and validated non-destructive inspection.

Note: EXP = direct experimental evidence; COMP = validated analytical or computational evidence; REV = review-level synthesis; SYN = author synthesis. E0–E5 and C0–C2 are defined in Section 2.4. Source roles describe the cited support, and evidence maturity applies to the claim being assessed. The entries identify evidence needs and limitations; they are not design allowables or certification findings.

Table 5. Coupled thermal, fire, electrical, and environmental evidence needs and qualification gaps for representative sustainable-composite component classes.

Component/Material System	Required Functional Evidence	Conditioning and Coupled Tests	Evidence Interpretation	Unresolved Qualification Gap
Natural-fibre or bio-based interior or secondary panel [51,52]	Time to ignition, flame spread, heat-release rate, smoke and gas toxicity, burn-through where applicable, and post-fire residual strength.	Hot-wet ageing and moisture cycling; impact or fastener damage before fire; coatings, joints, and exposed-edge conditions.	EXP/REV; E1–E2; C1. The reported static, impact, moisture, and fire results apply to their tested material and configuration.	Aircraft fire, smoke, and toxicity tests after ageing; batch variability; representative joints; repair followed by structural and fire retesting.
Recyclable or bio-based CFRP with fire-modified matrix [55,56,57]	Fire, smoke, and gas-toxicity response; glass-transition temperature; toughness; repair or reprocessing; and retained laminate properties.	Thermal cycling and hot-wet ageing; repeated repair or reprocessing; impact before fire.	EXP; mainly E1–E2. The sources demonstrate laboratory-scale fire response, repair, or recycling; aircraft-configuration evidence remains unavailable.	Laminate-, joint-, and component-scale smoke and toxicity, fatigue, repeated-cycle property retention, and post-fire residual-strength data.
Composite battery enclosure with passive thermal-management features [61,63]	Impact/crush resistance, normal heat rejection, abnormal heat isolation and propagation resistance, vent-gas management, electrical isolation, flame response, and attachment integrity.	Mechanical abuse followed by cell- or module-level heating; vibration and ageing; representative seams, fasteners, ports, access panels, and attachments included.	EXP/COMP/REV; E1–E3. Reference [63] quantifies passive heat spreading for one EV battery-module configuration. The other listed functions lie outside that experiment.	Representative enclosure- or pack-level abuse tests with controlled venting, post-event structural and functional inspection, repeated builds, and process repeatability.
Low-conductivity flame-retardant barrier or phase-change composite [62]	Heat-flux delay, thermal-runaway propagation resistance, flame response, dimensional stability, and compatibility with enclosure loads and interfaces.	Repeated heating, compression, vibration, impact, vent-gas exposure, and realistic contact with cells or modules.	EXP; E1–E2. Propagation delay was demonstrated for one material and configuration. Transfer depends on geometry, thermal boundary conditions, and heat-sink capacity.	Module- or pack-level validation, ageing, barrier attachment, repeatability, and post-event structural integrity.
Conductive CFRP avionics or power-electronics enclosure [35,65]	Shielding effectiveness versus frequency and direction; continuity across joints, seams, and apertures; electrical bonding and grounding; and residual structural performance.	Moisture, thermal cycling, fatigue, impact, repair, and galvanic-corrosion exposure where metallic additions are present, followed by EMI retesting.	REV; E1–E2; C0–C2. Reported shielding depends on method, thickness, lay-up, frequency, and specimen configuration.	Enclosure-level broadband testing with seams, connectors, apertures, grounding paths, repaired regions, damage, and production variability.
Natural-fibre biocomposite radome candidate [64]	Permittivity, dielectric loss, insertion loss, antenna-pattern retention, moisture sensitivity, rain-erosion resistance, and structural integrity.	Rain and moisture, temperature cycling, erosion, impact, coatings, bonded joints, lightning-protection integration, and repair.	REV; E0–E1. Evidence is limited to a preliminary material-screening review; fabricated-radome, insertion-loss, radiation-pattern, and environmental-qualification data remain unavailable.	Frequency- and geometry-specific radome and antenna tests after environmental ageing, impact, coatings, lightning-protection integration, and repair.
Hybrid lightning/EMI/fire multifunctional laminate [58,65,99]	Lightning-current distribution, shielding, thermal-barrier and fire response, interlaminar strength, and electrical isolation between functional layers.	Representative lightning-current or high-current exposure combined with fatigue, moisture, impact, and a galvanic environment.	REV/SYN; E0–E2. The combined assessment is author synthesis. Added functions introduce new interfaces and coupled failure modes.	Coupled structural–electrical–fire validation, inspectability, restoration of each function after repair, and end-of-life separation.
Aged or repaired multifunctional component [52,71,72,73,74]	Restored structural, thermal, fire, shielding, or dielectric function, with strength recovery reported separately.	Representative ageing and damage; repair; repeated inspection; then structural and assigned-function retesting.	EXP/REV; E1–E3. Current repair evidence is mainly structural and specific to the tested material, geometry, and process.	Return-to-service limits, repeated-repair capability, functional-layer restoration, and validated non-destructive inspection.

Note: EXP = direct experimental evidence; COMP = validated analytical or computational evidence; REV = review-level synthesis; SYN = author synthesis. E0–E5 and C0–C2 are defined in Section 2.4. Source roles describe the cited support, and evidence maturity applies to the claim being assessed. The entries identify evidence needs and limitations; they are not design allowables or certification findings.

7. Qualification Evidence and Component-Specific Adoption

Qualification evidence applies to a defined material–process specification and component application. High specific performance, recycled content, renewable feedstock, weldability, or repairable chemistry may justify material screening. Design allowables, defect acceptance criteria, damage-tolerance limits, approved repair procedures, and production conformity require further evidence. Substantiation should address the manufacturing route, service environment, component failure consequence, defect and inspection limits, repair, and continued-airworthiness strategy [84,85]. An analytical method must have a domain of validity established by tests at the relevant building-block levels [89].

7.1. Building-Block Substantiation and Design Allowables

Composite substantiation progresses from material and coupon characterisation to joints, elements, representative subcomponents, and full components. Coupon tests characterise material response and variability, including sensitivity to processing and environmental knockdowns. At the element level, testing examines load transfer, stress concentrations, joint behaviour, and the effects of representative defects. At the subcomponent and full component levels, the assessment incorporates manufacturing representativeness, inspection access, boundary conditions, load introduction, and interactions among failure modes [84,85,89]. The scope of the evidence programme is determined by the functions assigned to the component. For primary airframes and rotor supports, the programme must provide statistically based structural allowables, fatigue and damage tolerance data for representative load paths, and validated methods for inspection and repair [84,85,89]. Battery enclosure evidence must address structural response to impact or crash, thermal runaway propagation and containment, vent gas management, fire performance, electrical isolation, and attachment integrity [5,61,62,63,91,92,93,94,95,96,97]. Structural batteries require evidence of load-bearing and electrochemical performance together with damage detection, thermal runaway containment, and electrical isolation [15,98]. Table 7 sets out the evidence gates for these three component classes. Under the E0 to E5 framework, each claim is assigned the highest maturity level demonstrated by its supporting evidence. Certification planning and compliance findings remain subject to the applicable certification programme.

Required controls differ by material class. Thermoplastic composites require control of thermal history, crystallinity, porosity, forming, and welded interfaces [42,43,44,45,46,47]. Recycled-carbon-fibre systems require feedstock identity, recovered architecture, surface condition, and statistically characterised property classes [39,40,41,48,49,50]. Natural-fibre systems add biological and moisture-driven variability [27,28,29], while bio-based and dynamic-network matrices need long-term thermal, chemical, creep, and repeated-repair data [36,37,38,53,54,55,56,57]. Load-bearing applications also require statistically defensible allowables, environmental knockdowns, defect limits, fatigue and impact evidence, and defined repair restrictions.

Validated analysis can reduce selected repeated tests only within a demonstrated domain; it does not replace measured variability, defect data, or representative failure evidence. Impact and damage-tolerance models require calibration and validation against representative tests [88], while analysis-based substantiation must remain supported by test evidence at the relevant building-block levels [89]. Before transfer to larger structures, cohesive-zone models for disbond and delamination require benchmark verification, adequate process-zone resolution, appropriate element and solution controls, and demonstrated convergence [90].

7.2. Process Conformity, Inspection, Repair, and Continued Airworthiness

Qualification controls the incoming material and the as-manufactured or repaired component state. AFP and in situ consolidation specifications should link path planning and steering, heat input and thermal history, deposition speed, compaction, void content, interlaminar bonding, and crystallinity to the laminate condition [66,67,68]. Welding specifications should bound the material and joint configuration, interface temperature, pressure, heating time or energy input, cooling history, and current leakage where applicable [71,72]. Process-conformity and inspection criteria should connect production records and inspection indications to defect geometry and its effect on structural or functional performance.

Where repair is permitted, the process and its inspection requirements form part of the controlled component configuration. Khan et al. [73] reported that a repair made at 210 °C with a 5.71° scarf angle recovered 97% of pristine flexural strength, while delamination remained the dominant failure mode. Modi et al. [74] found that induction-welded repairs increased compression-after-impact strength by 7–17% relative to the impacted controls and reduced the measured damage area by approximately 3–5%. These results quantify structural recovery for the tested materials and geometries. Budhe et al. [75] identified temperature and moisture as environmental threats to bonded-joint durability. Continued-airworthiness evidence should define inspection capability, detectable-damage limits, inspection intervals, repair limits, environmental durability, repeated-repair effects, and return-to-service criteria. Assigned shielding, thermal, electrical-isolation, fire, venting, or radio-frequency functions should be retested after repair.

Traceability content should follow the dominant variability sources. Recycled fibres require records of feedstock origin, prior matrix, recovery route, recovered fibre form and length, surface condition, contamination, orientation or alignment, and batch variability [39,40,41,48,49,69,70]. Natural fibres require records of species, cultivation and harvest conditions, extraction and treatment, moisture content, storage, and processing history [27,28,29,51,52]. Bio-based and reversible matrices require verified chemistry, bio-based content, cure or exchange conditions, ageing history, and property retention [13,36,37,38,53,54,55,56,57]. These records enable production conformity, root-cause analysis, controlled repair or replacement, and preservation of property history through service and recovery.

7.3. Evidence-to-Adoption Framework

The framework starts with the component application. It uses five steps: define the structural and functional roles; classify the consequence of losing each role; identify the mechanical, environmental, fire, thermal, electrical, inspection, and repair requirements; assess the available evidence by source role, evidence maturity (E0–E5), and test comparability (C0–C2); and identify the missing evidence before assigning an adoption classification. Four classifications are used: near-term candidate, conditional candidate, research-stage candidate, and not assessable. Each classification applies to the stated material–process–component combination, operating context, and failure consequence.

Samarasinghe et al. [63] reduced surface temperature by up to 16.62% in one EV battery-module configuration using embedded copper pins. Their study quantifies passive heat spreading for that configuration; aircraft-enclosure assessment additionally requires crash-loading, electrical-isolation, vent-gas, fire-containment, and thermal-runaway-propagation evidence. An aircraft battery enclosure therefore remains research stage until its assigned functions are demonstrated together at representative enclosure or pack scale. Butenegro et al. [49] support a material-specific E1 assessment for the recycled-prepreg/PA11 system. Haris et al. [64] support screening at E0–E1 for radome materials; fabricated-component transmission and environmental evidence remains to be generated.

Evidence thresholds rise with component consequence and functional coupling. Table 5 defines the coupled structural, thermal, fire, electrical, and environmental evidence for each functional component class, and Table 6 applies these requirements to the component adoption assessment. Operational hazards, infrastructure, standards, and procedures at airports and vertiports provide the surrounding operating context [100]. High aspect ratio composite wings also require integrated aerostructural, aeroelastic, manufacturing, and model correlation evidence [101,102,103,104].

7.4. Component-Specific Adoption Assessment

Table 6 applies the framework to representative component classes and reports the evidence range, adoption classification, and next evidence gate for each class. Thermoplastic CFRP has the broadest joining and repair evidence among the emerging systems reviewed [42,43,44,45,46,47,66,67,68,71,72,73,74,75]. The assigned classification remains specific to the material, process, component function, operating context, and failure consequence.

Functional component classifications combine the structural evidence with the assigned functions defined in Table 5. Conductive composite enclosures remain conditional, radomes and structural batteries remain at the research stage, and battery enclosures range from research stage to conditional, based on the representative component evidence [15,35,58,61,62,63,64,65,94,95,96,97,98,99]. Emerging sustainable systems remain at the research stage for occupied aircraft primary structures until repeatability, component, and qualification evidence reaches E4 to E5.

Table 6. Component-specific evidence, component consequence, adoption classification, and qualification needs for sustainable composites in electric aircraft, eVTOL vehicles, and UAVs.

Component Class and Consequence	Candidate Material Systems and Present Evidence	Current Evidence Range	Adoption Classification	Required Next Evidence
UAV covers, fairings, and payload housings; low to moderate consequence [14,30,49,51,52]	Thermoplastic composites, aligned or long-fragment recycled-CF systems, and natural-fibre hybrids have coupon and limited panel or element evidence.	E2–E3	Near-term candidate for a defined unoccupied mission; conditional when fire, EMI, or loss-of-control consequences are assigned.	Hot-wet and impact retention; fasteners and joints; process repeatability; field inspection and repair; mission-specific failure assessment.
Occupied-aircraft interior and acoustic panels; moderate consequence [12,13,14,51,52,58,91,92,93]	Natural-fibre, bio-based, recycled-CF, and fire-modified thermoplastic systems have material and small-panel evidence.	E1–E2	Conditional candidate.	Full-panel fire, smoke, and toxicity testing after ageing; joints and edges; batch variability; cleaning-agent and aircraft-fluid exposure; repair; production conformity.
Access panels, fairings, and secondary skins; moderate consequence [42,43,44,45,46,47,66,67,68,69,70,71,72,73,74,75]	Thermoplastic CFRP has the broadest joining and repair evidence among the emerging systems reviewed; selected recycled-CF systems remain geometry- and feedstock-specific.	E2–E4	Conditional; a near-term candidate only for a defined material–process system with component evidence.	Weld and joint fatigue; impact; environmental ageing; defect limits; non-destructive inspection; repair durability; allowables; repeated-build validation.
Avionics and power-electronics enclosures; moderate consequence [35,65,99]	Conductive CFRP, thermoplastic CFRP, and hybrid conductive layers have review-level synthesis of experimental shielding studies.	E2–E3	Conditional candidate.	Broadband shielding across seams and apertures; electrical bonding and grounding; thermal management; fire; vibration; moisture; impact and repaired-region performance; and enclosure-level repeatability.
Radomes and antenna windows; moderate consequence [35,64,65]	Natural-fibre biocomposites have preliminary screening evidence. Conductive CFRP and filler-modified polymers need direct dielectric and transmission data for radio-frequency-transparent applications.	E0–E2	Research stage.	Permittivity, dielectric loss, insertion loss, and antenna-pattern retention over the operating band after moisture, thermal cycling, impact, rain erosion, coatings, joints, lightning-protection integration, and repair.
Battery enclosures and crash/fire barriers; high consequence [5,61,62,63,95,96,97,98]	Cited fire-resistant glass-fibre and thermoplastic systems, ceramic or mineral barriers, and multifunctional laminates are demonstrated mainly at coupon or element level.	E1–E3	Research stage to conditional, depending on enclosure-scale evidence.	Impact/crush followed by module- or pack-level thermal-runaway testing; vent-gas management; electrical isolation; flame, smoke, and toxicity; attachment integrity; post-event structural and functional inspection; and repeated builds.
UAV primary wing, spar, or boom; high and mission-dependent consequence [42,43,44,47,84,85,89,101,102,103,104]	Among the emerging systems reviewed, thermoplastic CFRPs and selected hybrid CFRPs have the broadest structural evidence. Recycled- and natural-fibre evidence remains below representative component level.	E2–E3	Conditional for bounded unoccupied missions; research stage where failure threatens people or critical operations.	Spectrum fatigue; vibration; impact; joints; manufacturing defects; environmental ageing; non-destructive inspection; repair; statistically defensible allowables.
eVTOL rotor-support structures and occupied-aircraft primary airframe; very high consequence [42,43,44,47,84,85,89,101,102,103,104]	Qualified incumbent CFRP provides the reference baseline. A specific thermoplastic-CFRP system may progress conditionally; recycled, natural-fibre, and most reversible-matrix systems remain immature.	E1–E4 for emerging systems	Research stage for emerging sustainable systems.	E4–E5 representative component- or full-scale evidence, production conformity, A- and B-basis allowables, damage tolerance, applicable crash/fire interaction, repair, and continued-airworthiness substantiation.
Load-bearing structural-battery module; very high consequence [15,98]	Multifunctional structural-battery concepts couple electrochemical storage with the primary load path.	E0–E2	Research stage.	Coupled electrochemical and structural performance under fatigue, crash, thermal runaway, electrical isolation, damage detection, repair, safe degradation, and full aircraft-level safety integration.

Note: E0–E5 evidence maturity and C0–C2 test comparability are defined in Section 2.4. The ranges report current evidence maturity for the stated component application. Adoption classifications are component-specific author synthesis based on the reviewed evidence. Certification approval and universal material ranking lie outside their scope.

Table 6. Component-specific evidence, component consequence, adoption classification, and qualification needs for sustainable composites in electric aircraft, eVTOL vehicles, and UAVs.

Component Class and Consequence	Candidate Material Systems and Present Evidence	Current Evidence Range	Adoption Classification	Required Next Evidence
UAV covers, fairings, and payload housings; low to moderate consequence [14,30,49,51,52]	Thermoplastic composites, aligned or long-fragment recycled-CF systems, and natural-fibre hybrids have coupon and limited panel or element evidence.	E2–E3	Near-term candidate for a defined unoccupied mission; conditional when fire, EMI, or loss-of-control consequences are assigned.	Hot-wet and impact retention; fasteners and joints; process repeatability; field inspection and repair; mission-specific failure assessment.
Occupied-aircraft interior and acoustic panels; moderate consequence [12,13,14,51,52,58,91,92,93]	Natural-fibre, bio-based, recycled-CF, and fire-modified thermoplastic systems have material and small-panel evidence.	E1–E2	Conditional candidate.	Full-panel fire, smoke, and toxicity testing after ageing; joints and edges; batch variability; cleaning-agent and aircraft-fluid exposure; repair; production conformity.
Access panels, fairings, and secondary skins; moderate consequence [42,43,44,45,46,47,66,67,68,69,70,71,72,73,74,75]	Thermoplastic CFRP has the broadest joining and repair evidence among the emerging systems reviewed; selected recycled-CF systems remain geometry- and feedstock-specific.	E2–E4	Conditional; a near-term candidate only for a defined material–process system with component evidence.	Weld and joint fatigue; impact; environmental ageing; defect limits; non-destructive inspection; repair durability; allowables; repeated-build validation.
Avionics and power-electronics enclosures; moderate consequence [35,65,99]	Conductive CFRP, thermoplastic CFRP, and hybrid conductive layers have review-level synthesis of experimental shielding studies.	E2–E3	Conditional candidate.	Broadband shielding across seams and apertures; electrical bonding and grounding; thermal management; fire; vibration; moisture; impact and repaired-region performance; and enclosure-level repeatability.
Radomes and antenna windows; moderate consequence [35,64,65]	Natural-fibre biocomposites have preliminary screening evidence. Conductive CFRP and filler-modified polymers need direct dielectric and transmission data for radio-frequency-transparent applications.	E0–E2	Research stage.	Permittivity, dielectric loss, insertion loss, and antenna-pattern retention over the operating band after moisture, thermal cycling, impact, rain erosion, coatings, joints, lightning-protection integration, and repair.
Battery enclosures and crash/fire barriers; high consequence [5,61,62,63,95,96,97,98]	Cited fire-resistant glass-fibre and thermoplastic systems, ceramic or mineral barriers, and multifunctional laminates are demonstrated mainly at coupon or element level.	E1–E3	Research stage to conditional, depending on enclosure-scale evidence.	Impact/crush followed by module- or pack-level thermal-runaway testing; vent-gas management; electrical isolation; flame, smoke, and toxicity; attachment integrity; post-event structural and functional inspection; and repeated builds.
UAV primary wing, spar, or boom; high and mission-dependent consequence [42,43,44,47,84,85,89,101,102,103,104]	Among the emerging systems reviewed, thermoplastic CFRPs and selected hybrid CFRPs have the broadest structural evidence. Recycled- and natural-fibre evidence remains below representative component level.	E2–E3	Conditional for bounded unoccupied missions; research stage where failure threatens people or critical operations.	Spectrum fatigue; vibration; impact; joints; manufacturing defects; environmental ageing; non-destructive inspection; repair; statistically defensible allowables.
eVTOL rotor-support structures and occupied-aircraft primary airframe; very high consequence [42,43,44,47,84,85,89,101,102,103,104]	Qualified incumbent CFRP provides the reference baseline. A specific thermoplastic-CFRP system may progress conditionally; recycled, natural-fibre, and most reversible-matrix systems remain immature.	E1–E4 for emerging systems	Research stage for emerging sustainable systems.	E4–E5 representative component- or full-scale evidence, production conformity, A- and B-basis allowables, damage tolerance, applicable crash/fire interaction, repair, and continued-airworthiness substantiation.
Load-bearing structural-battery module; very high consequence [15,98]	Multifunctional structural-battery concepts couple electrochemical storage with the primary load path.	E0–E2	Research stage.	Coupled electrochemical and structural performance under fatigue, crash, thermal runaway, electrical isolation, damage detection, repair, safe degradation, and full aircraft-level safety integration.

Note: E0–E5 evidence maturity and C0–C2 test comparability are defined in Section 2.4. The ranges report current evidence maturity for the stated component application. Adoption classifications are component-specific author synthesis based on the reviewed evidence. Certification approval and universal material ranking lie outside their scope.

Figure 3 plots the evidence ranges and adoption classifications from Table 6 against component consequence.

A material system can occupy different adoption categories when the component consequence, exposure history, inspection access, repair route, or available evidence changes. The framework should be revised as element-, component-, and service-level data become available.

8. Research Priorities and Staged Adoption Roadmap

Research priorities are determined by the evidence gap preventing a defined material, process, and component combination from reaching the next adoption stage. Each synthesis table serves a distinct analytical purpose. Table 3 compares material families using common performance indicators; Table 4 defines manufacturing, inspection, repair, and circularity controls; Table 5 defines coupled functional evidence; Table 6 reports component adoption classifications and next evidence gaps; and Table 7 specifies the staged evidence gates for high-consequence applications. E2 is established through material and process baselines. Progression to E3 depends on evidence from elements or demonstrators, whereas E4 requires representative component evidence. Qualification or service evidence provides the basis for E5.

8.1. Near-Term Priority: Reproducible Material and Process Evidence

For UAV covers, fairings, payload housings, and selected interior panels with limited failure consequences, the first research requirement is a reproducible E2 baseline for each defined combination of material, manufacturing process, and configuration. The baseline must document the material state, processing conditions, and traceability records needed to reproduce that configuration. Section 4 and Section 5 identify the relevant material and process variables, while Table 4 specifies the associated controls and traceability requirements. Representative demonstrators should then be evaluated against the structural and functional requirements in Table 3 and Table 5. Testing at this scale provides E3 evidence by documenting the demonstrator response under defined conditions and against the assigned requirements. Progression to E4 requires repeatable performance in the specified component geometry and intended operating environment. Primary structural applications must follow the separate substantiation pathway defined in Table 7.

8.2. Intermediate Priority: Coupled-Function Component Validation

Access panels, secondary skins, enclosures, radomes, battery covers, and thermal barriers require evidence for coupled functions as validation progresses from E3 to E4. At E3, representative demonstrators should be tested for the assigned functions using the conditioning sequences specified in Table 5. Progression to E4 requires evidence from a representative component configuration that reproduces the relevant geometry and interfaces. Table 6 identifies the next evidence gate for each component class. Validation should follow the prescribed exposure sequence and use defined acceptance metrics. Component life cycle assessment should accompany technical validation and apply the manufacturing and circularity controls defined in Table 4 [14,39,76,77,78,79,80,81,82,105].

8.3. Longer-Term Priority: Qualification and High-Consequence Adoption

For components with severe failure consequences, coupon data provide only the initial material basis for substantiation. The assessment should then follow the building block sequence described in Section 7.1. Evidence supporting each progression step must be traceable to the material definition, manufacturing process, geometry, environmental exposure, representative defects, and relevant failure modes [84,85,89,90]. Table 7 applies this sequence to primary airframes and rotor supports, battery enclosures, and structural battery modules. For each application class, it identifies the evidence required at every stage and the gate governing progression to the next level.

Each evidence record should identify the configuration to which the result applies and retain enough information to reconstruct how that result was obtained. Required information includes provenance, test method, conditioning history, defects, repair history, uncertainty, and conformity status. If a validated model replaces a repeated test, its record must specify the calibration data, validation cases, uncertainty, and domain of applicability [89,90,101,102,103,104]. Negative and nonconforming results remain part of the evidence base because they define process limits and prevent unsupported transfer between stages.

Table 7. Component-specific experimental and qualification pathway for high-consequence composite applications.

Evidence Stage	Primary Airframes and Rotor Supports	Battery Enclosures and Thermal Barriers	Structural Battery Modules	Required Gate
1. Material and process definition; E0 to E1	Fix constituents, architecture, processing route, critical defects, traceability, and the initial analysis basis. Define load cases and failure consequences [84,85].	Define enclosure geometry, attachments, barriers, vents, electrical isolation, and credible normal and abnormal thermal conditions [5,61,91,92,93,94,95,96,97].	Define load path, cell and matrix architecture, electrical isolation, sensing, thermal management, and safe degradation functions [15,98].	Controlled material, process, component functions, hazards, and acceptance metrics.
2. Conditioned coupons and process baselines; E2	Generate statistically based strength and stiffness data, fatigue and impact response, environmental knockdowns, defect sensitivity, and process variability.	Measure laminate and barrier response for impact, heat flux, ignition, heat release, smoke, gas toxicity, electrical isolation, and thermal ageing.	Measure structural, electrochemical, thermal, and electrical response under fatigue, damage, temperature, state of charge, and ageing.	Repeatable coupon evidence and bounded process windows sufficient to design elements.
3. Joints, elements, and coupled functions; E3	Test joints, load introduction, representative defects, bonded or welded interfaces, local buckling, damage growth, inspection capability, and repair [71,72,73,74,75,88,89,90].	Test seams, attachments, vents, seals, barriers, grounding paths, and post-impact thermal and fire response, including inspection after an event.	Test current paths, interfaces, sensing, damage detection, fault initiation, containment, and retained structural and electrochemical capability.	Validated failure modes and analysis methods for a representative element configuration.
4. Representative subcomponents; E4	Use manufacturing-representative panels, beams, wing boxes, frames, or rotor support details with realistic boundaries, load paths, defects, conditioning, inspection, and repair.	Use an enclosure section or pack-level article for sequential impact or crush, thermal runaway, vent routing, fire containment, isolation, and attachment integrity.	Use a load-bearing module for combined fatigue, impact, thermal runaway, isolation, sensing, damage detection, and residual capability.	Repeatable subcomponent evidence that closes the dominant coupled failure modes.
5. Full component and system integration; E4 to E5	Test the complete primary structural or rotor support article under critical static, fatigue, damage tolerance, environmental, crash, and repair cases applicable to its function.	Test the complete enclosure and installed interfaces under representative module or pack loads, thermal runaway, venting, fire, electrical isolation, and post-event inspection.	Test the complete structural battery module within the aircraft load, electrical, thermal, control, and containment architecture.	Correlated full component evidence and verified interface requirements for the defined aircraft system.
6. Qualification, production, and service; E5	Establish the applicable qualification basis, production conformity, validated analysis domain, inspection intervals, repair limits, and continued airworthiness controls [84,85].	Control enclosure manufacture, barrier and vent configuration, event detection, inspection, replacement criteria, maintenance, and post-event disposition.	Control module manufacture, health monitoring, allowable degradation, fault response, replacement, repair restrictions, and aircraft-level safety integration.	Approved configuration with controlled production and service evidence. Progression remains subject to the applicable certification programme.

Note: E0 to E5 are defined in Section 2.4. The table identifies the evidence sequence and does not prescribe a regulatory category, calendar, or certification finding.

Table 7. Component-specific experimental and qualification pathway for high-consequence composite applications.

Evidence Stage	Primary Airframes and Rotor Supports	Battery Enclosures and Thermal Barriers	Structural Battery Modules	Required Gate
1. Material and process definition; E0 to E1	Fix constituents, architecture, processing route, critical defects, traceability, and the initial analysis basis. Define load cases and failure consequences [84,85].	Define enclosure geometry, attachments, barriers, vents, electrical isolation, and credible normal and abnormal thermal conditions [5,61,91,92,93,94,95,96,97].	Define load path, cell and matrix architecture, electrical isolation, sensing, thermal management, and safe degradation functions [15,98].	Controlled material, process, component functions, hazards, and acceptance metrics.
2. Conditioned coupons and process baselines; E2	Generate statistically based strength and stiffness data, fatigue and impact response, environmental knockdowns, defect sensitivity, and process variability.	Measure laminate and barrier response for impact, heat flux, ignition, heat release, smoke, gas toxicity, electrical isolation, and thermal ageing.	Measure structural, electrochemical, thermal, and electrical response under fatigue, damage, temperature, state of charge, and ageing.	Repeatable coupon evidence and bounded process windows sufficient to design elements.
3. Joints, elements, and coupled functions; E3	Test joints, load introduction, representative defects, bonded or welded interfaces, local buckling, damage growth, inspection capability, and repair [71,72,73,74,75,88,89,90].	Test seams, attachments, vents, seals, barriers, grounding paths, and post-impact thermal and fire response, including inspection after an event.	Test current paths, interfaces, sensing, damage detection, fault initiation, containment, and retained structural and electrochemical capability.	Validated failure modes and analysis methods for a representative element configuration.
4. Representative subcomponents; E4	Use manufacturing-representative panels, beams, wing boxes, frames, or rotor support details with realistic boundaries, load paths, defects, conditioning, inspection, and repair.	Use an enclosure section or pack-level article for sequential impact or crush, thermal runaway, vent routing, fire containment, isolation, and attachment integrity.	Use a load-bearing module for combined fatigue, impact, thermal runaway, isolation, sensing, damage detection, and residual capability.	Repeatable subcomponent evidence that closes the dominant coupled failure modes.
5. Full component and system integration; E4 to E5	Test the complete primary structural or rotor support article under critical static, fatigue, damage tolerance, environmental, crash, and repair cases applicable to its function.	Test the complete enclosure and installed interfaces under representative module or pack loads, thermal runaway, venting, fire, electrical isolation, and post-event inspection.	Test the complete structural battery module within the aircraft load, electrical, thermal, control, and containment architecture.	Correlated full component evidence and verified interface requirements for the defined aircraft system.
6. Qualification, production, and service; E5	Establish the applicable qualification basis, production conformity, validated analysis domain, inspection intervals, repair limits, and continued airworthiness controls [84,85].	Control enclosure manufacture, barrier and vent configuration, event detection, inspection, replacement criteria, maintenance, and post-event disposition.	Control module manufacture, health monitoring, allowable degradation, fault response, replacement, repair restrictions, and aircraft-level safety integration.	Approved configuration with controlled production and service evidence. Progression remains subject to the applicable certification programme.

Note: E0 to E5 are defined in Section 2.4. The table identifies the evidence sequence and does not prescribe a regulatory category, calendar, or certification finding.

9. Conclusions

This review assessed sustainable polymer- and fibre-composite systems for electric aircraft, eVTOL vehicles, and UAVs through a component-specific chain from chemistry and processing to retained structural and functional performance, repair, circularity, evidence maturity, and qualification. The evidence supports component-specific adoption decisions based on a defined material–process–component combination, its assigned functions and consequence, manufacturing route, exposure history, and available inspection, repair, and continued-airworthiness evidence.

Thermoplastic CFRP has the clearest near-term evidence base for selected secondary and semi-structural components, although fatigue, impact, weld durability, fire behaviour, process conformity, and geometry remain application-specific. Recycled-carbon-fibre systems are supported for fairings, covers, housings, interiors, and other lower-consequence components when feedstock provenance, recovered architecture, property scatter, and traceability are controlled. Natural-fibre and bio-based systems are better suited to interiors and low-consequence UAV components because moisture, porosity, thermal stability, fire behaviour, and biological variability limit higher-consequence use. Hybrid and multifunctional systems can provide shielding, dielectric transmission, thermal management, or fire barriers. Their component assessment must address added mass, interfaces, manufacturing variability, inspection, repair, and end-of-life separation.

Adoption should progress from stable E2 material–process baselines through E3 joint, repair, element, and coupled-function evidence to E4 representative-component validation under sequenced exposures. High-consequence applications require E5 allowables, accepted qualification, certified use, or service evidence. High-aspect-ratio wing studies provide integrated analytical methods and one laboratory-scale manufacturing and test correlation [101,102,103,104], collectively forming a laboratory-level evidence base. Battery enclosures and structural batteries require assessment of crash, fire, thermal runaway, venting, electrical isolation, electromagnetic compatibility, and coupled structural–electrochemical failure [91,92,93,94,95,96,97,98,99]. Heterogeneous geometries, processes, conditioning histories, and test methods prevent numerical pooling and broad readiness claims. Research should therefore prioritise statistically defensible allowables, integrated structural and functional tests, production conformity, validated inspection and repair, and component-level life cycle assessment. Sustainability claims must include manufacturing yield, service durability, repair, recovery yield, and actual displacement of virgin material without loss of the required component performance.