Search Results (381)

The development of fully biodegradable poly(3-hydroxybutyrate) (PHB)-based materials with improved mechanical performance remains a major challenge due to the limited ductility and processability of this highly crystalline polymer. Blending and plasticisation are viable strategies to enhance PHB toughness; however, the interactions governing polymer–plasticiser compatibility and their impact on structure–property relationships remain not fully understood. In this work, the compatibility and plasticisation mechanisms of two citrate-based plasticisers, tributyl citrate (TBC) and acetyl tributyl citrate (ATBC), were systematically investigated in biodegradable blends based on PHB, polylactic acid (PLA), and poly(butylene adipate-co-terephthalate) (PBAT). Polymer–plasticiser affinity was evaluated through Hansen Solubility Parameters and interaction radius, which indicated good compatibility of PHB with both plasticisers and a stronger affinity for ATBC. Differential scanning calorimetry showed that citrate plasticisers reduced the glass transition temperature, modified crystallisation kinetics, and altered the crystalline morphology of the blends. Dynamic mechanical analysis confirmed the reduction in the glass transition temperature of PHB–PLA systems, which is in agreement with the DSC results. Migration experiments showed equilibrium after approximately 72 h, with PHB–PLA blends exhibiting better plasticiser retention than PHB–PBAT systems. TBC consistently showed higher migration than ATBC, in line with its lower molecular weight and higher volatility. Mechanical testing demonstrated that plasticisation efficiency strongly depended on blend composition: TBC was more effective in enhancing ductility in PHB–PLA blends, whereas ATBC performed better in PHB–PBAT systems. It was also highlighted that the plasticisers had a remarkable ability to substantially increase the ductility of the blends compared with their unplasticised counterparts, as reflected by the pronounced decrease in stiffness and the marked increase in elongation at break. SEM analysis of tensile fracture surfaces evidenced a brittle failure mode for PHB–PLA blends, whereas PHB–PBAT systems exhibited a ductile fracture mode with fibrillar features and clear signs of phase separation. Finally, thermogravimetric analysis showed no appreciable thermal degradation within the processing temperature window used for mixing and hot pressing, confirming the thermal stability of the materials under the selected conditions. These findings establish clear correlations between thermodynamic compatibility, migration behaviour, thermal properties, fracture mechanisms, and mechanical performance, providing useful guidelines for the design of citrate-plasticised PHB-based biodegradable materials. Full article

The modification of cementitious binders using active mineral additives and nano-components represents a crucial pathway for developing high-performance, sustainable concrete composites. Nevertheless, unilateral modification of the matrix with highly reactive siliceous materials often leads to an undesirable increase in composite brittleness. This study investigates the synergistic effect of the concurrent application of nano-silica (NS), silica fume (SF), and Class F fly ash (FA) in ternary and quaternary binders, aimed at optimizing both load-bearing capacity and fracture toughness. The experimental program was conducted on seven concrete series, evaluating their mechanical parameters and non-linear fracture properties using the two-parameter fracture model (TPFM) on notched beams subjected to three-point bending. Additionally, a high-resolution energy partitioning framework was applied, decomposing the total fracture energy into four distinct components—fracture initiation energy in the elastic range (G_ini), pre-peak microcracking energy (G_pre), main material softening energy (G_soft), and residual tail energy dissipated at large crack openings (G_tail)—along with the determination of the characteristic length (l_ch). The results demonstrated that while purely siliceous systems (modified with NS and SF) generate high strength increments, they simultaneously trigger a “brittleness trap,” manifested by a 13.65% decrease in the l_ch parameter. The introduction of FA effectively mitigates this hazard, transforming the failure mode into a quasi-ductile behavior. The concrete series modified with the NS+FA hybrid (Mix-5) exhibited a spectacular 107% increase in G_f and an increase in l_ch of nearly 50%, while maintaining high fracture toughness. Energy decomposition analysis in quaternary concretes confirmed a desirable reduction in the initiation energy share in favor of the softening and tail phases (G_tail reaching a record 13.1% for Mix-7), suggesting the probable activation of macroscopic crack-bridging mechanisms driven by the delayed hydration of FA particles. The research indicates that precise design of multi-component binders allows for achieving an optimal technological equilibrium point—the “sweet spot”—combining high structural capacity with safe material ductility. Full article

Graphene is a novel material that can bring several advantages in the composite materials manufacturing field, such as improved electrical and thermal properties, and high performance. In particular, functionalizing current composite materials can bring advantages in the aerospace field in thermal management for electric aircraft engines. This paper studies the addition of graphene particles into carbon fiber composites manufactured by the Resin Transfer Molding Process (RTM). Thermal and mechanical properties are evaluated and compared with a conventional composite laminate. Major improvements were achieved on the thermal behavior of the composite material while maintaining general properties, but in particular, the addition of graphene had a negative impact on transverse tensile and mode II fracture toughness due to agglomerates present in the fiber–resin interface. Full article

Polymer-based sandwich structures are widely used for their lightweight and tailorable properties, but interfacial failure phenomena often govern their performance. Among these, Mode I skin/core debonding is a critical mechanism that limits structural reliability. This review provides a unified and critical assessment of experimental methodologies for Mode I fracture characterisation, focusing on the ASTM D8637/D8637M standard and alternative setups, including Double Cantilever Beam (DCB), Single Cantilever Beam (SCB), and Climbing Drum Peel (CDP) tests. Alongside the influence of geometrical factors, processing conditions and intrinsic polymer properties on Mode I characterisation are detailed. Conventional DCB setups are shown to introduce mixed-mode effects due to asymmetric loading. In contrast, the modified DCB-UBM setup achieves near-pure Mode I conditions at the expense of increased complexity. Comparative analysis indicates that the SCB configuration with a roller base outperforms the standardised flexible-rod setup, particularly for specimens with non-linear responses. The review also indicates that Mode I debonding behaviour is strongly influenced by several factors, including interfacial adhesion quality, constituent material properties, manufacturing-induced defects, specimen configurations, and environmental factors. Therefore, the interpretation of debonding performance requires a comprehensive structure–property–processing framework. Moreover, geometric constraints imposed by ASTM D8637/D8637M are also revisited, demonstrating that reduced-dimension specimens can yield comparable fracture toughness, thereby enabling greater design flexibility. Additionally, while the standard prescribes Modified Beam Theory (MBT) and Area Method (AM) for initiation and propagation, both methods provide comparable propagation toughness under linear conditions. For non-linear systems, alternative data reductions based on CDP concepts, with the SCB–roller base setup, are effective. Based on this assessment, key challenges and potential improvements are identified, guiding the development of more accurate and reliable testing methodologies for polymer sandwich structures. Full article

Hydraulic fracturing is the primary technology for extracting unconventional oil and gas resources. Rock apparent fracture toughness is the most critical parameter in hydraulic fracturing processes. Rock apparent fracture toughness exhibits characteristics distinct from those of metallic materials, particularly as field-estimated values of rock apparent fracture toughness in hydraulic fracturing exceed laboratory-measured values by 1–2 orders of magnitude. Existing interpretation models assume a constant stress distribution in the fracture process zone (FPZ), which contradicts the softening behavior of rock. To address this gap, and based on the assumption of a power-law softening stress distribution in the FPZ of quasi-brittle rock, we develop a mode I apparent fracture toughness model for rock under far-field tensile and compressive stress configurations. This model considers both the softening characteristics of rock and the fluid lag effect. A comparative analysis was conducted on the differences in rock apparent fracture toughness between far-field compressive stress and tensile stress configurations. The results reveal that the difference in configuration between far-field compressive stress and tensile stress constitutes the fundamental reason for the order-of-magnitude discrepancy between the rock apparent fracture toughness estimated from hydraulic fracturing field tests and that measured in laboratory experiments. The influence of the ratio of in situ stress to tensile strength, the ratio of FPZ length to fracture length, the ratio of fluid lag zone length to fracture length, and stress distribution within the FPZ on rock apparent fracture toughness was analyzed, and these factors are found to have decisive effects on the rock apparent fracture toughness. Additionally, the size effect on rock apparent fracture toughness was discussed. This research contributes to more precise hydraulic fracturing parameter design. Full article

Grouting and sealing in gas drainage boreholes are two of the critical measures to ensure efficient coal seam gas extraction. However, traditional cement grouting often leads to debonding and cracking of the slurry–coal cemented body under external load, resulting in poor sealing performance. To suppress crack propagation and achieve borehole reinforcement and efficient sealing, this study compares the mechanical properties and crack evolution characteristics of slurry–coal cemented samples grouted with different modified materials. Five types of cement-based sealing materials, including ordinary Portland cement, were used for grouting coal rock in boreholes. By employing an acoustic emission signal acquisition system and a non-contact full-field strain measurement system, the tensile mechanical properties of coal before and after grouting were compared. The influence of material properties on the reinforcement capacity of borehole coal was analyzed, along with the failure process characteristics and final failure morphology of the slurry–coal cemented body under Brazilian splitting load. Finally, the effects of material toughness and bond strength on the brittleness index and failure mode of the slurry–coal cemented samples under Brazilian splitting conditions were discussed. The results show that the tensile strength improvement rates of the samples were 26.9%, 55.3%, 48.4%, 8.6%, and 45.6%, respectively. Distinct from previous studies focusing on fractured grouting or intact coal rock, this work for the first time systematically reveals the non-monotonic influence of the combination of material toughness and bond strength on the reinforcement effect of borehole coal samples and proposes an evaluation framework based on quantitative acoustic emission crack type analysis and the concept of effectiveness threshold. The varying degrees of tensile strength enhancement indicate differences in the reinforcement capabilities of grouting materials with different properties. The acoustic emission signals during the failure process of the slurry–coal cemented body exhibited typical stage-specific characteristics, though material properties altered the failure modes. By quantifying the intrinsic properties and crack characteristics of the slurry–coal cemented body using the brittleness index and grayscale histograms, this study provides a theoretical basis for guiding efficient sealing of gas drainage boreholes through an in-depth understanding of the mechanical behavior and crack evolution of borehole coal samples before and after grouting under Brazilian splitting conditions. Full article

Aluminum is undoubtedly a key material in modern industry. Flat plates made of aluminum alloys are widely used in construction, aeronautics, automotive, and others. The current paper presents an analysis of the behavior of a thin plate made of Al7075-T651 aluminum alloy, subjected to a uniaxial stress, and clamped at one end. The results of the numerical simulation with FRANC2D software have been used for accurate determination of the stress intensity factors (K_I, K_II) and being validated for the simple cases using analytical calculations. The Failure Assessment Diagram (FAD) based on the toughness ratio Kr and the load ratio Lr has been used to evaluate the structural integrity of cracked components based on the load, its position, crack size, and the fracture properties of the material. The FAD analysis results highlight the significant influence of crack position on the values of the K factor. The edge and inclined cracks lead to increases in stress intensity factors and to the occurrence of mixed-mode loading conditions. The study demonstrates the effectiveness and usefulness of the proposed methodology in the analysis of structures with discontinuities and emphasizes the importance of crack positioning in assessing the safety of engineering components. Full article

There is a lack of knowledge concerning the interlaminar fracture toughness of 3D-printed composite materials using both commercial filament composites and fused deposition modeling (FDM) technology from Markforged^®. In this investigation, additive manufacturing (AM) continuous fiber-reinforced thermoplastic (cFRT) specimens have been tested to characterize the initiation and propagation of interlaminar fracture toughness in mode I (G_I). Unidirectional glass fiber (GF)-reinforced polyamide 6 (PA) laminates were characterized by means of the double cantilever beam (DCB) test. These specimens were manufactured using a MarkTwo^® printer and tested without doublers, following a laminate configuration selected according to appropriate experimental findings reported in the state of the art, ensuring reliable fracture characterization. The experimental results exhibited repeatability and strong agreement between the modified compliance calibration (MCC) and modified beam theory (MBT) reduction methods. The resistance curve (R-curve) indicated a progressive increase in fracture resistance during crack propagation. To analyze the experienced failure mechanism during testing, the fracture surfaces of representative post-mortem DCB specimens were observed using a scanning electron microscope (SEM), revealing characteristic morphological features at two magnification levels. Moreover, representative cross-sections of the tested DCB specimens were electronically observed to analyze the interlaminar morphologies, showing an irregular and random distribution of the matrix, fiber, and voids between consecutive plies and adjacent deposited rasters. Compared with previously reported Markforged^® continuous fiber-reinforced systems, the GF/PA composite material exhibited intermediate initiation fracture toughness but lower propagation toughness. This study contributes to filling the existing gap in fracture toughness data for glass fiber-reinforced additively manufactured composites. Full article

Interlaminar fracture toughness (ILFT) is a key factor governing the damage tolerance and service reliability of carbon fiber-reinforced polymer (CFRP) laminates. This study aims to clarify how the deformation capability of epoxy resin affects the Mode I and Mode II ILFT of carbon fiber/epoxy laminates under comparable fiber, resin-content, and laminate-configuration conditions. Two epoxy systems were compared: a high-strength/high-modulus (HSHM) resin system, designated as Group B, and a high-toughness (HT) resin system, designated as Group T. Neat resin castings were characterized by tensile and flexural tests, and the corresponding CFRP laminates were evaluated using double cantilever beam (DCB) and end-notched flexure (ENF) tests. Although Group T showed slightly lower tensile strength and modulus than Group B, its elongation at break increased from 4.0% to 6.5%, corresponding to an increase of approximately 62.5%. The Mode I ILFT (GIC) increased from approximately 279 J/m² for Group B to 487 J/m² for Group T, while the Mode II ILFT (GIIC) increased from approximately 530 J/m² to 708 J/m², corresponding to improvements of approximately 74.6% and 33.6%, respectively. Scanning electron microscopy (SEM) observations indicated that Group T promoted more resin-covered fibers, resin tearing, crack-tip blunting, crack deflection, shear deformation features, and crack-path reconstruction. These results indicate that, within the present two-system comparison, resin ductility-related deformation capability and local crack-tip deformability should be considered together with strength and modulus when evaluating interlaminar crack resistance. The toughening effect also showed loading-mode dependence, with Mode I improvement mainly related to crack-tip blunting and resin tearing, whereas Mode II improvement was mainly associated with matrix shear deformation, resistance to interfacial sliding, and crack-path deflection. Full article

This work investigates the Mode I fracture behavior of adhesive joints manufactured from unidirectional carbon fiber-reinforced epoxy composites (CFRP) under static and fatigue loading. Specimens were exposed to two degradation environments: hygrothermal conditions (60 °C, 70% RH) and saline conditions (35 ± 2 °C, 89% RH), for 1 and 12 weeks, and compared with non-exposed material. Double Cantilever Beam (DCB) tests were conducted to evaluate the influence of aging on fracture toughness. Thermal (Differential Scanning Calorimetry, DSC) and spectroscopic (Fourier Transform Infrared Spectroscopy, FTIR) analyses were performed to identify degradation mechanisms. DSC results showed no significant variation in glass transition temperature (T_g) under saline exposure, whereas hygrothermal aging increased T_g, indicating post-curing effects. FTIR analysis revealed moisture uptake and oxidation under saline conditions, while hygrothermal exposure mainly led to structural rearrangement. Critical energy release rate (G_IC) values were used to define fatigue test conditions, enabling the construction of fatigue initiation (ΔG–N) and crack propagation (G–da/dN) curves. A Weibull-based model was applied to describe fatigue initiation behavior. Results show that saline exposure promotes progressive degradation, whereas hygrothermal conditions may enhance performance due to post-curing effects. Full article

The intralaminar and interlaminar mode I initiation fracture toughness of unidirectional IM7/8552 carbon/epoxy composites were re-evaluated using only the published experimental data. Classical statistics, two-parameter Weibull analysis (location fixed at zero), non-parametric kernel density estimation (KDE), bootstrap resampling (10,000 replications), and bootstrap-based uncertainty quantification were applied to the fatigue-precracked (FPC) initiation values (n = 12) and the corresponding R-curves. The pooled FPC mean initiation toughness was 0.1982 kJ/m² (COV = 8.50%). Weibull fitting yielded a shape parameter β = 12.33 and scale η = 0.2058 kJ/m², providing a B-basis value of 0.1715 kJ/m² (90% reliability) and an A-basis value of 0.1417 kJ/m² (99% reliability). The Kolmogorov–Smirnov test confirmed statistical equivalence between intralaminar and interlaminar groups (p > 0.05), validating the use of a single initiation toughness for both crack planes when sharp fatigue-precracked starter cracks are employed. Intralaminar R-curves exhibited significantly steeper propagation, rising to approximately 0.385 kJ/m² at Δa = 30 mm due to extensive fiber bridging, whereas interlaminar R-curves reached a near-plateau after 12–15 mm. Bootstrap 95% confidence bands quantified the higher uncertainty associated with the intralaminar R-curve. Teflon-insert data produced artificially high initiation values and unstable growth, confirming that only fatigue-precracked results are suitable for design allowables. This study demonstrates that a single, statistically robust initiation toughness (B-basis = 0.1715 kJ/m²) can be used interchangeably for intra- and interlaminar cracking in progressive-damage models and preliminary design analysis of IM7/8552 structures. The open-source statistical workflow (KDE + bootstrap) developed here is transferable to other small-sample composite datasets, though the numerical B-basis value (0.1715 kJ/m²) is specific to IM7/8552 and should not be generalized without validation. Full article

In order to verify the feasibility of in situ repair of underwater local dry laser welding (ULDLW) on nuclear power reactor components, this work investigates the microstructure and mechanical properties of 304L austenitic stainless steel repaired by ULDLW using ER308L filler metal. Comprehensive comparison would be made between the ULDLW and conventional in-air laser welding to evaluate their applicability. The results demonstrate that the rapid cooling rate inherent to the underwater environment significantly influences solidification behavior and microstructural evolution. The weld metal (WM) solidifies in the ferritic–austenitic (FA) mode, with an increased proportion of lathy δ-ferrite at the expense of skeletal morphology compared to the in-air welds. Electron backscatter diffraction (EBSD) analysis reveals the substantial grain refinement in underwater welds, with average grain sizes of 39.4 μm versus 47.3 μm for in-air weld bead, accompanied by a higher fraction of low-angle grain boundaries (LAGBs). These microstructural modifications yield superior mechanical properties: underwater weld bead exhibits ultimate tensile strength (UTS) of 685.6 MPa, elongation of 57.5%, and impact toughness of 22.6 J, significantly exceeding the corresponding values for in-air welds (663.9 MPa, 51.8%, and 18.6 J, respectively). Fractographic analysis confirms ductile fracture mechanisms in both conditions. The enhanced performance is attributed to grain refinement strengthening via the Hall–Petch relationship and the increased LAGBs fraction, which impedes dislocation motion and crack propagation. Full article

This work reveals the interlaminar fracture behavior and failure modes of carbon nanotube (CNT) film toughening composite laminates under Mode I and Mode II fractures. Experiment results display that the Mode I fracture toughness increases to its maximum value when a 2-layer CNT film is added, then it decreases with the increase in CNT layers. However, the trend changes with the number of CNT layers under Mode II fracture, that is, the fracture toughness gradually increases with the increase in CNT layers. This result indicates that compared to a Mode II fracture, the toughening effect of multi-layer CNT under a Mode I fracture has not been effectively produced. A novel micro-mechanical model, based on a Voronoi diagram, is established to identify the failure mode within the CNT toughening region. It is shown that the crack propagation paths of the two kinds of fracture modes are different: cracks propagate along the CNT/resin interface for Mode I fracture, while propagating simultaneously at both the interface and resin for Mode II fracture. The change in failure mode of the CNT toughening region is the reason for the various effects under the two-fracture loading. This work innovatively utilizes finite element simulation and cross-sectional micro characterization methods to reveal the differences in interlayer failure modes of CNT film interlayer toughening materials under different fracture modes, aiming to provide guidance for the application of CNT films in the field of interlayer toughening. Full article

Despite extensive advancements in Engineered Cementitious Composites (ECCs), mixture design remains predominantly empirical, due to the absence of a quantitative parameter directly linking fiber–matrix interfacial mechanics to strain-hardening performance. This study identifies fiber–matrix interfacial friction as a quantifiable parameter and establishes a micromechanics-guided interfacial regulation framework to enhance the toughness of ECC by regulating fiber failure modes. First, a critical fiber–matrix interfacial frictional stress, (τ₀)_crit, corresponding to the transition between fiber pull-out and fracture, was theoretically derived based on energy dissipation maximization during crack propagation. A back-calculation approach was further developed to determine interfacial frictional stress (τ₀) directly from tensile stress–crack opening responses under single-crack tension, eliminating reliance on single-fiber pull-out testing. Then, τ₀ was tuned toward (τ₀)_crit through interfacial regulation using fly ash. Experimental results demonstrate that the toughness of ECC is maximized when τ₀ approaches (τ₀)_crit, confirming the validity of the proposed toughness enhancement mechanism. The study establishes an explicit mechanistic linkage between interfacial micromechanics and macroscopic strain-hardening performance, providing a predictive and quantitative design pathway that transcends empirical mixture adjustment. Full article

Carbon fiber-reinforced polymers (CFRP) are widely employed in advanced manufacturing sectors such as aerospace, wind energy, and new energy vehicles owing to their high specific strength and stiffness. The growing demand for lightweight, high-performance, and multifunctional materials has accelerated the development of structurally and functionally integrated CFRP. Introducing functional interlayers between composite laminates is an effective strategy to impart additional functionalities; however, such interlayers are often multi-component and structurally complex. A critical challenge remains to integrate functionality without compromising, and preferably enhancing, the load-bearing capability of CFRP, particularly their resistance to interlaminar delamination. In this study, electrically heated CFRP incorporating a sandwich-structured interlayer composed of glass fiber mesh fabric/CNT veils doped with carbon nanotubes/glass fiber mesh fabric (GF/CNTs-CNTv/GF) was investigated. The effects of interlayer architecture and CNT loading on the Mode II interlaminar fracture toughness were systematically examined. Delamination failure modes and interlaminar toughening mechanisms were analyzed using scanning electron microscopy and ultra-depth-of-field three-dimensional microscopy. The results demonstrate that an optimal CNT pre-impregnation concentration of 1.0 wt% yielded a maximum G_IIC of 1644.8 J/m², corresponding to a 103.06% increase relative to the reference laminate. The enhanced performance is attributed to simultaneous optimization of interfacial “nano-engineering” effects, including matrix toughening and a pronounced “nano-anchoring” mechanism induced by CNT. These effects promote a transition in failure mode from weak interfacial debonding to a mesh-block composite delamination pattern, thereby activating multiple energy-dissipation mechanisms such as crack deflection, fiber pull-out, rupture, and bridging. This work highlights the effectiveness of CNT-modified sandwich interlayers in improving delamination resistance and provides both theoretical insight and experimental validation for the design of multifunctional CFRP with superior interlaminar fracture toughness. Full article