J. Compos. Sci., Volume 9, Issue 8 (August 2025) – 53 articles

Fiber orientation is an important descriptor of the microstructure for short fiber polymer composite materials where accurate and efficient prediction of the orientation state is crucial when evaluating the bulk thermo-mechanical response of the material. Macroscopic fiber orientation models employ the moment-tensor form in representing the fiber orientation state, and they all require a closure approximation for the higher-order orientation tensors. In addition, various models have more recently been developed to account for rotary diffusion due to fiber-fiber and fiber-matrix interactions which can now more accurately simulate the experimentally observed slow fiber kinematics in polymer composite processing. It is common to use explicit numerical initial value problem-ordinary differential equation (IVP-ODE) solvers such as the 4^th- and 5^th-order Dormand Prince Runge–Kutta (RK45) method to predict the transient and steady-state fiber orientation response. Here, we propose a computationally efficient method based on the Newton-Raphson (NR) iterative technique for determining steady state orientation tensor values by evaluating exact derivatives of the moment-tensor evolution equation with respect to the independent components of the orientation tensor. We consider various existing macroscopic-fiber orientation models and several closure approximations to ensure the robustness and reliability of the method. The performance and stability of the approach for obtaining physical solutions in various homogeneous flow fields is demonstrated through several examples. Validation of our orientation tensor exact derivatives is performed by benchmarking with results of finite difference techniques. Overall, our results show that the proposed NR method accurately predicts the steady state orientation for all tensor models, closure approximations and flow types considered in this paper and was relatively faster compared to the RK45 method. The NR convergence and stability behavior was seen to be sensitive to the initial orientation tensor guess value, the fiber orientation tensor model type and complexity, the flow type and extension to shear rate ratio. Full article

The present study is a comprehensive experimental investigation of the uncertainties in the mechanical response of short fibre-reinforced composites. The reference material investigated is a short glass fibre-reinforced phenolic resin matrix material, manufactured in a thermoset injection-moulding process. The mechanical material response was investigated in experiments on three different scales, considering ISO-size, miniature, and micro tensile samples. Further to the tensile experiments, compressive and shear tests were performed on macroscopic samples. Experiments on breadboard samples featuring more complex stress and deformation states complete the study. The injection-moulding process was found to control the basic fibre orientation. However, the nominal local fibre orientation distribution is superimposed with a pronounced local uncertainty. This microstructural uncertainty strongly affects the uncertainty in the local mechanical properties as well as variations between samples taken from identical positions in different but nominally identical plates. Full article

Polymers have long been utilized in various industries due to their unique properties. Among the family of polymers, polysulfides are popular due to their strong adhesion and high resistance to fuels and solvents, and have been utilized in specific applications. In this study, polysulfide nanocomposites were prepared using methylene dichloride (MD), sodium tetrasulfide (Na₂S₄), and reduced graphene oxide (rGO) and then cured using a rheometer. Polymethylene tetrasulfide (PMTS) and nanocomposites were evaluated by Fourier transform infrared (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Also, the cured samples were evaluated using FTIR, XRD, SEM, thermogravimetric analysis (TGA), and tensile test. The results showed that PMTS has a completely amorphous structure. XRD and SEM results showed that with the addition of rGO, free sulfur accumulates in the matrix, which participates in the reaction during the curing process. The cured polymethylene tetrasulfide (CPMTS) and the cured nanocomposites have a completely amorphous structure. Also, the presence of rGO improved the final properties of the product. Full article

Natural fiber-reinforced polymers are gaining popularity as sustainable structural materials. However, their inherent variability can limit their reliability in load-bearing applications. To address this issue, we investigate a novel structural health monitoring method that leverages mechanoelectrical effects in flax fiber-reinforced epoxy composites. In our study, a contactless capacitive coupled measurement setup records electrical polarization during fatigue testing at four load levels. The polarization signals we observed increased with increasing load levels. Additionally, changes in polarization correlate with changes in dynamic modulus, providing early indicators of potential failure. This work lays the foundation for a new type of structural health monitoring in natural fiber-reinforced polymers. Full article

A series of studies was conducted on the functional and structural characteristics of polymer composite materials (PCMs) based on silicone polymers modified with multi-walled carbon nanotubes (MWCNTs) and metallic particles (CuAl or Al). The influence of the structural parameters of carbon and metallic inclusions in the polymer matrix on the electrophysical and thermophysical properties of the composites was demonstrated. Various conduction mechanisms dominating in the inverse temperature ranges of 50 K^–1–13 K^–1, 13 K^–1–6 K^–1, and 6 K^–1–2 K^–1 were identified. The operational modes of the polymer composites as active materials for thermoregulating coatings were established. The highest temperature of 32.9 °C in operating mode and the shortest warm-up time of 180 s were observed in the composite modified with 4 wt.% CNTs and 10 wt.% bronze particles at a supply voltage of 10 V. The characteristics of the composites under atomic oxygen (AO) exposure with a fluence of 3 × 10²¹ atoms/cm² was evaluated, confirming their functionality, particularly for potential space applications. The composites demonstrated nearly complete retention of their functional characteristics. The aim of this study was to develop electrically conductive functional composites based on silicone polymers containing MWCNTs and metallic particles inclusions for creating electric heating elements with tailored functional characteristics. Full article

The overuse of nonrenewable resources has motivated intensive research and the development of new types of green bio-based and degradable feedstocks derived from natural sources, such as cellulose derivates, also in nanoforms. The inclusion of such nanoparticles in bio-based polymers with the aim of providing reinforcement is a trend, which, when associated with the incorporation active compounds, creates active packaging suitable for the packaging of highly perishable food, thus contributing to the product’s shelf-life extension. Chitosan (Ch)/sage essential oil (SEO) bionanocomposite reinforced with nanocrystalline cellulose (CNC) was cast as active packaging for the preservation of fresh poultry meat. Meat samples were wrapped in different bioplastics (pristine chitosan, chitosan with commercial CNC, chitosan with CNC obtained from three different lignocellulosic crops, giant reed (G), kenaf (K), and miscanthus (M), chitosan with SEO, and chitosan with SEO and CNC), while unwrapped samples were tested as the control. Periodically, samples were evaluated in terms of their physicochemical properties and microbial growth. Additionally, bionanocomposites were also evaluated in terms of their in situ antimicrobial properties, as well as migration toward food simulants. Meat samples protected with bionanocomposites showed lower levels of microbiological growth (2–3 logs lower than control) and lipid oxidation (20–30% lower than in control), over time. This was attributed to the intrinsic antimicrobial capacity of chitosan and the high oxygen barrier properties of the films resulting from the CNC inclusion. The SEO incorporation did not significantly improve the material’s antimicrobial and antioxidant activity yet interfered directly with the meat’s color as it migrated to its surface. In the in vitro assays, all bionanocomposites demonstrated good antimicrobial activity against B. cereus (reduction of ~8.2 log) and Salmonella Choleraesuis (reduction of ~5–6 log). Through the in vitro migration assay, it was verified that the SEO release rate of phenolic compounds to ethanol 50% (dairy products simulate) was higher than to ethanol 95% (fatty food simulate). Furthermore, these migration tests proved that nanocellulose was capable of delaying SEO migration, thus reducing the negative effect on the meat’s color and the pro-oxidant activity recorded in TBARS. It was concluded that the tested chitosan/nanocellulose bionanocomposites increased the shelf life of fresh poultry meat. Full article

One of the main challenges in hydroxyapatite research is to develop cost-effective synthesis methods that consistently produce materials closely resembling natural bone, while maintaining high biocompatibility, phase purity, and mechanical stability for biomedical applications. Traditional synthetic techniques frequently fail to provide desirable mechanical characteristics and antibacterial activity, necessitating the development of novel strategies based on natural precursors and selective ion doping. The present study aims to explore the possibility of synthesizing hydroxyapatite through the co-precipitation method, followed by a microwave-assisted hydrothermal maturation process. The main CaO sources selected for this study are eggshells and mussel shells. Cu²⁺ and Sr²⁺ ions were added into the hydroxyapatite structure at concentrations of 1% and 5% to investigate their potential for biomedical applications. Furthermore, the morpho-structural and biological properties have been investigated. Results demonstrated the success of hydroxyapatite synthesis and ion incorporation into its chemical structure. Moreover, HAp samples exhibited significant antimicrobial properties, especially the samples doped with 5% Cu and Sr. Additionally, all samples presented good biological activity on MC3T3-E1 osteoblast cells, demonstrating good cellular viability of all samples. Therefore, by correlating the results, it could be concluded that the undoped and doped hydroxyapatite samples are suitable biomaterials to be further applied in orthopedic applications. Full article

Recent advancements in composite materials design have made glass fiber-reinforced polymer composites (GFRPC) a viable choice for a wide range of engineering and industrial applications. Although GFRPCs boast attractive characteristics such as low specific mass and high specific mechanical strength, identifying and characterizing damage mechanisms in these materials is challenging. Several scientific studies have examined the root causes of GFRPC failure using various methods, including non-destructive techniques and learning algorithms. Despite this, ongoing investigations aim to accurately detect mechanical defects in GFRPCs. This study explores the use of non-destructive testing (NDT) combined with unsupervised learning algorithms to identify and classify damage mechanisms in GFRPCs. The NDT method employed in this study is acoustic emission (AE), which identifies waveforms associated with various failure mechanisms during testing. These waveforms are categorized using unsupervised learning methods such as principal component analysis (PCA) and self-organizing maps. PCA selects the most appropriate AE descriptors for distinguishing between different damage mechanisms, while the self-organizing maps algorithm performs clustering analysis and classifies failure mechanisms. Scanning electron microscope images of the observed failures are provided to sup-port the findings derived from AE data. Full article

This study focuses on producing and processing highly aligned tapes from recycled carbon fibers (rCFs). The rCFs are processed with a modified carding machine, oriented through a specialized subsequent process and consolidated into a semi-finished product. These rCF-tapes are placed onto a two-dimensional tool using an adapted automated fiber placement (AFP) technology to demonstrate a novel approach of producing composites from highly oriented recycled materials. The semi-finished stacks are consolidated in a heating press and test coupons are tested according to corresponding standards. The rCF-tapes are evaluated using methods such as tensile and flexural testing and determination of fiber volume content. Mechanical values are assessed by processing various generations of rCF-tapes and comparing them to each other and to virgin fiber tapes (vCF-tapes) made of the same type of carbon fiber and matrix. Microscopic imaging is also performed to analyze the quality of the resulting composites. In this study, a tensile strength of up to 1100 MPa in the fiber direction and stiffness of up to 80 GPa at a fiber volume content (FVC) of approximately 40% were achieved. The results highlight the strong potential and benefits of using highly oriented rCF-tapes and demonstrate the suitability of fiber placement technologies for those recycled materials. Full article

Regenerative endodontics seeks to restore the vascularized pulp–dentin complex following conventional root canal therapy, yet reliable neovascularization within the constrained root canal remains a key challenge. This study investigates the development of an injectable, dual-curing hydrogel based on methacrylated decellularized amniotic membrane (dAM-MA) and compares its performance to a conventional gelatin methacryloyl (GelMA). The dAM-MA platform was designed for biphasic release, incorporating both free vascular endothelial growth factor (VEGF) for an initial burst and matrix-metalloproteinase-cleavable VEGF conjugates for sustained delivery. The dAM-MA hydrogel achieved shape-fidelity via thermal gelation at 37 °C and possessed tunable stiffness (0.5–7.8 kPa) after visible-light irradiation. While showing high cytocompatibility comparable to GelMA (>125% hDPSC viability), the dAM-MA platform markedly outperformed the control in promoting endothelial tube formation (up to 800 µm total length; 42 branch points at 96 h). The biphasic VEGF release from dAM-MA matched physiological injury kinetics, driving both early chemotaxis and late vessel maturation. These results demonstrate that dAM-MA hydrogels combine native extracellular matrix complexity with practical, dual-curing injectability and programmable VEGF kinetics, offering a promising scaffold for minimally invasive pulp–dentin regeneration. Full article

Silicone oil-based magnetic liquids containing carbon nanotubes (CNTs) were prepared using an in situ chemical coprecipitation method. The surface modification of Fe₃O₄/CNT composite particles was carried out by using three silane coupling agents: γ-aminopropyltriethoxysilane (550), γ-methacryloxypropyltrimethoxysilane (570), and phenyltrimethoxysilane (7030). Infrared Spectroscopy (IR), Transmission Electron Microscopy (TEM), and X-ray Diffraction (XRD) were used to confirm the successful doping of CNTs and the effective coating of the coupling agents. The rheological behavior of the magnetic liquids was systematically studied using an Anton Paar Rheometer. The results show that viscosity decreases exponentially with increasing temperature (fitting the Arrhenius equation), increases and tends to saturate with rising magnetic field intensity, and exhibits shear-thinning characteristics with increasing shear rate. Among the samples, Fe₃O₄@7030 has the best visco-thermal performance due to the benzene ring structure, which reduces the symmetry of the molecular chains. In contrast, Fe₃O₄@570 shows the most significant magneto-viscous effect (viscosity variation of 161.4%) as a result of the long-chain structure enhancing the steric hindrance of the magnetic dipoles. Full article

Reinforcing PLA composites with natural fibres is a prominent strategy for improving PLA’s properties while benefiting from its intrinsic biodegradation. However, these composites may be susceptible to an inefficient stress-transferring process due to the weak intermolecular interactions between PLA and natural fibres. A well-known practice is to incorporate coupling agents to improve polymer–fibre adhesion, such as carboxylic acids (CAs) and grafted copolymers. CAs are a more affordable and biodegradable option for improving PLA/natural fibre interface strength, resulting in a material with superior mechanical and thermal properties. In this context, this research discusses the potential use of mono (C6 and C8) and di (CC6 and CC8) carboxylic acids as coupling agents in PLA/pecan nutshells (PN) composites. PLA/PN composites with four different CAs were processed in a twin-screw extruder and subsequently injection moulded. The results indicated an increase in the flexural strength of the PLA due to the presence of PN in the neat composite. The use of CAs increased the storage modulus of PLA/PN composites, while C6 and CC8 reduced the PLA composite tan δ peak height. The PLA’s T_g in PLA/PN composite shifted to lower temperatures after the incorporation of CAs while increasing the PLA crystallinity degree. These results strongly suggested that besides acting as efficient coupling agents, these acids also exerted roles as nucleating agents and plasticisers. Full article

In this study, a pH-responsive molecularly imprinted polymer (MIP) drug carrier was developed utilizing boric acid-functionalized mesoporous silica nanoparticles (MSNs) as the substrate. The carrier was engineered for controlled drug release, with capecitabine (CAPE) being selected as the template molecule due to its structural characteristics and clinical relevance. In vitro drug release studies demonstrated the pH-responsive release behaviors of the fabricated carrier, highlighting its promising applicability in the controlled release of pharmaceutical compounds containing cis-diols, particularly for site-specific therapy where pH variations serve as physiological triggers. Full article

This study examined the impact of storage and operational aging on the thermal stability, structural degradation, and electrical properties of styrene–butadiene rubber (SBR) compound by analyzing three distinct materials: a laboratory-stored sample, an operationally aged one, and an original unaged reference. Thermal degradation was analyzed through thermogravimetric analysis (TGA), which examined weight loss as a function of temperature and time at different heating rates. Results showed that the onset temperature and peak position in the 457 °C to 483 °C range remained stable. The activation energy (Ea) was determined using the Kissinger–Akahira–Sunose (KAS), Flynn–Wall–Ozawa (FWO), and Friedman methods, with the original unaged sample’s (OUS) Ea averaging 203.7 kJ/mol, decreasing to 163.47 kJ/mol in the laboratory-stored sample (LSS), and increasing to 224.18 kJ/mol in the operationally aged sample (OAS). The Toop equation was applied to estimate the thermal degradation lifetime at a 50% conversion rate. Since the material had been exposed to electricity, the evolution of electrical conductivity was studied and found to have remained stable after storage at around 0.070 S/cm. However, after operational aging, it showed a considerable increase in conductivity, to 0.321 S/cm. Scanning Electron Microscopy (SEM) was employed to analyze microstructural degradation and chemical changes, providing insights into the impact of aging on thermal stability and electrical properties. Full article

The growing interest in probiotic bacteria within the food industry is driven by their recognized health benefits for consumers. However, preserving their therapeutic viability and stability during gastrointestinal transit remains a formidable challenge. Hence, this research aimed to enhance the viability of Lactobacillus reuteri through microencapsulation using a binary polysaccharide mixture composed of low acyl gellan gum (LAG), high acyl gellan gum (HAG), and calcium for the microencapsulation of L. reuteri. To achieve this, the Box–Behnken design was applied, targeting the optimization of L. reuteri microencapsulated to withstand simulated gastrointestinal conditions. The microcapsules were crafted using the internal ionic gelation method, and optimization was performed using response surface methodology (RSM) based on the Box–Behnken design. The model demonstrated robust predictive power, with R² values exceeding 95% and a lack of fit greater than p > 0.05. Under optimized conditions—0.88% (w/v) LAG, 0.43% (w/v) HAG, and 24.44 mM Ca—L. reuteri reached a viability of 97.43% following the encapsulation process. After 4 h of exposure to simulated gastric fluid (SGF) and intestinal fluid (SIF), the encapsulated cells maintained a viable count of 8.02 log CFU/mL. These promising results underscore the potential of biopolymer-based microcapsules, such as those containing LAG and HAG, as an innovative approach for safeguarding probiotics during gastrointestinal passage, paving the way for new probiotic-enriched food products. Full article

Background. Physicomechanical properties and clinical service of bulk-fill composites depend on their adequate polymerization and depth of cure. Some manufacturers claim that these composites can be adequately cured when used in bulks exceeding 4 mm. Objective. The aim of this study was to compare Vickers microhardness (VMH) and depth of cure (DOC) of six contemporary bulk-fill resin composites at depths of 4 mm and 6 mm. Material and methods. Six bulk-fill composites were evaluated in this study: 1. Tetric EvoCeram Bulk (Ivoclar Vivadent, Schaan, Liechtenstein), (TEC); 2. Filtek Bulk Fill Posterior (3M ESPE Dental Products Division, St. Paul, MN, USA), (FBF); 3. Filtek One Bulk Fill (3M ESPE Dental Products Division, St. Paul, MN, USA, (FOB); 4. SonicFill 2 (Kerr, Orange, CA, USA), (SF2); 5. Admira Fusion X-tra (Voco, GmbH, Cuxhaven, Germany), (AFX); 6. GrandioSO X-tra (Voco, GmbH, Cuxhaven, Germany), (GSX). The 18 specimens (3 of each composite) were prepared in split Teflon moulds of 4 mm diameter and 6 mm thickness. All composites were cured in standard mode for 20 s using LED LCU (D-Light Duo, RF-Pharmaceuticals Sarl, Geneva, Switzerland; 1200–1300 mW/cm). The VMH was measured using a digital Micro Hardness Tester Shimadzu (HMV-2T E, Shimadzu Corporation, Kyoto, Japan). A 50 g (0.5 N) load force was applied for 30 s. Each specimen was measured at five places selected by chance at each level (N = 15). The hardness ratio or DOC was calculated for all samples as the ratio of bottom and surface microhardness at levels of 4 and 6 mm. Data were analysed using one-way ANOVA and Tukey’s post hoc test. Results. Significant reduction in VMH was observed for all tested materials when comparing top surface and bottom (p < 0.01). The highest VMH was obtained for GSX and AFX, and the lowest for TEC. The results show that the degree of polymerization was adequate for all tested materials at a depth of 6 mm, since the hardness ratio exceeded 0.80 in all cases. The hardness ratio at 4 mm was high for all tested composites ranging from 0.91 for TEC to 0.98 for GSX. All composites showed adequate DOC at the bottom of the 6 mm bulk samples. However, the hardness ratio was the highest for Admira Fusion X-tra (0.96) and GrandioSO X-tra (0.97). Conclusions. All tested materials showed a significant decrease in microhardness from the top surface to the bottom. The DOC was adequate for all bulk-fill composites at a depth of 6 mm cured under standard mode for 20 s. All bulk-fill resin composites evaluated in this study can be used in bulk, up to 6 mm. Full article

Poly(trimethylene terephthalate) (PTT) is a thermoplastic polyester with a unique structure due to having three methylene groups in the glycol unit. PTT competes with poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) in carpets, textiles, and thermoplastic materials, primarily due to the development of economically efficient synthesis methods. PTT is widely utilized in textiles, carpets, and engineering plastics because of its advantageous properties, including quick-drying capabilities and wrinkle resistance. However, its low melting point, resistance to chemicals, and brittleness compared to PET, have limited its applications. To address some of these limitations for targeted applications, PTT nanocomposites incorporating clay, carbon nanotube, silica, and ZnO have been developed. The distribution of nanoparticles within the PTT matrix remains a significant challenge for its potential applications. Several techniques, including sol–gel blending, melt blending, in situ polymerization, and in situ forming methods have been developed to obtain better dispersion. This review discusses advancements in the synthesis of various PTT nanocomposites and the effects of nanoparticles on the isothermal and nonisothermal crystallization of PTT. Full article

To study the influence of curing agent structure on the properties of epoxy resin, four types of aromatic diamines with the structure of diphenyl methane (4,4′-methylenedianiline (MDA), 4,4′-methylenebis(2-ethylaniline) (MOEA), 4,4′-methylenebis(2-chloroaniline) (MOCA), and 4,4′-methylenebis(3-chloro-2,6-diethylaniline) (MCDEA)) and a high-performance epoxy resin, 3-(oxiran-2-ylmethoxy)-N,N-bis(oxiran-2-ylmethyl)aniline (AFG-90MH), were used in this study. The resulting resin systems were designated as AFG-90MH-MDA, AFG-90MH-MOEA, AFG-90MH-MOCA, and AFG-90MH-MCDEA. After curing, these systems were named AFG-90MH-MDA-C, AFG-90MH-MOEA-C, AFG-90MH-MOCA-C, and AFG-90MH-MCDEA-C. The influence of the structure of the diamines on the processability, curing reaction activity, and thermal and mechanical properties (including flexural and tensile properties) of the epoxy resins were investigated. These systems demonstrate excellent processability with wide processing windows ranging from 30 °C to 110–160 °C while maintaining low viscosity. Consistent apparent activation energy (E_a) trends via both Kissinger and Flynn-Wall-Ozawa methods were observed. The epoxy systems exhibit the following increasing E_a sequence: AFG-90MH-MDA < AFG-90MH-MOEA < AFG-90MH-MOCA < AFG-90MH-MCDEA. The processability and curing reaction kinetic results indicate that the reactivities of the diamines decrease in the order: MDA > MOEA > MOCA > MCDEA. Polar chlorine substituents in diamines strengthen intermolecular interactions, thereby enhancing mechanical performance. The flexural strength of cured epoxy systems decreases as follows with corresponding values: AFG-90MH-MOCA-C (165 MPa) > AFG-90MH-MDA-C (158 MPa) > AFG-90MH-MCDEA-C (148 MPa) > AFG-90MH-MOEA-C (136 MPa). Diamines with substituents like chlorine or ethyl groups reduce the glass transition temperatures (T_g) of the cured resin systems. However, the cured resin systems with the diamines containing chlorine demonstrate superior thermal performance compared to those with ethyl groups. The cured epoxy systems exhibit the following descending glass transition temperature order with corresponding values: AFG-90MH-MDA-C (213 °C) > AFG-90MH-MOCA-C (190 °C) > AFG-90MH-MCDEA-C (183 °C) > AFG-90MH-MOEA-C (172 °C). Full article

Wood polymer composites (WPCs) represent a rapidly growing class of sustainable materials, formed by combining lignocellulosic fibers with thermoplastic or thermoset polymeric matrices. This review summarizes the state of the art in WPC development, emphasizing the use of recyclable (or recycled) and biodegradable polymers as matrix materials. The integration of waste wood particles into the production of WPCs addresses global environmental challenges, including plastic pollution and deforestation, by offering an alternative to conventional wood-based and petroleum-based products. Key topics covered in the review include raw material sources, fiber pre-treatments, compatibilizers, mechanical performance, water absorption behavior, thermal stability and end-use applications. Full article

Carbon fiber-reinforced plastics (CFRPs) offer excellent in-plane mechanical performance, but their relatively low interlaminar fracture toughness makes them vulnerable to delamination, particularly around intralaminar discontinuities such as resin-rich regions or fiber gaps. This study investigates the effectiveness of polyamide (PA) mesh inserts in improving interlaminar toughness and suppressing delamination in CFRP laminates with such features. Two PA mesh configurations were evaluated: a fully embedded continuous layer and a 20 mm cut mesh strip placed between continuous and discontinuous plies near critical regions. Fracture toughness tests showed that PA mesh insertion improved interlaminar toughness approximately 2.4-fold compared to neat CFRP, primarily due to a mechanical interlocking mechanism that disrupts crack propagation and enhances energy dissipation. Uniaxial tensile tests with digital image correlation revealed that while initial matrix cracking occurred at similar stress levels, the stress at which complete delamination occurred was approximately 60% higher in specimens with a 20 mm mesh and up to 92% higher in specimens with fully embedded mesh. The fully embedded mesh provided consistent delamination resistance across the laminate, while the 20 mm insert localized strain redistribution and preserved global mechanical performance. These findings demonstrate that PA mesh is an effective interleaving material for enhancing damage tolerance in CFRP laminates with internal discontinuities. Full article

Phosphogypsum, a byproduct of orthophosphoric acid production, is one of the large-tonnage wastes. Since phosphogypsum mainly consists of CaSO₄ 2H₂O, it can be considered as an alternative gypsum-bearing raw material in the production of gypsum binders. However, its features, such as particle morphology and the presence of impurities, can negatively affect the characteristics of phosphogypsum-based binders. Identification of these factors will allow us to develop methods for their minimization and increasing the efficiency of phosphogypsum use from the required source as a raw material for the production of phosphogypsum-based binders. In this regard, the manuscript contains a comprehensive and comparative analysis of phosphogypsum and natural gypsum, which makes it possible to establish their differences in chemical composition and structural and morphological features, which subsequently affect the properties of the phosphogypsum-based binder. It has been established that the key factor negatively affecting the strength of phosphogypsum-based paste (2.58 MPa) is its high water demand (0.89), which is due to the high values of the specific surface area of the particles and the presence of a large number of conglomerates with significant porosity in phosphogypsum. It has been suggested that preliminary grinding of phosphogypsum can help reduce the amount of water required to obtain fresh phosphogypsum-based paste with a standard consistency and improve its physical and mechanical properties. Full article

This study examines the energy behavior of a strengthening system consisting of a Fiber Reinforced Polymer (FRP) plate bonded to a rigid substrate and subjected to tensile loading, where the adhesive interface is governed by a bilinear bond–slip law with a vertical descending branch. The investigation focuses on the interaction between the elastic energy stored in the FRP and the adhesive interface, as well as the characteristic lengths that control the debonding process. Analytical expressions for the strain energy stored in both the FRP plate and the adhesive interface are derived, enabling the identification and evaluation of two critical characteristic lengths as the bond stress at the loaded end approaches its maximum value $l_c$, at which the elastic energies of the FRP and the adhesive interface converge, signaling energy saturation; and $l_{max}$, where the adhesive interface attains its peak energy absorption. Upon reaching the energy saturation state, the system undergoes failure through the sudden and complete debonding of the FRP from the substrate. The onset of unstable debonding is rigorously analyzed in terms of the first and second derivatives of the total potential energy with respect to the bond length. It is further demonstrated that abrupt debonding may also occur in cases where the length exceeds $l_c$ when the bond stress reaches its maximum, and the bond–slip law is characterized by a vertical branch. The findings provide significant insights into the energy balance and stability criteria governing the debonding failure mode in FRP-strengthened structures, highlighting the pivotal role of characteristic lengths in predicting both structural performance and failure mechanisms. Full article

Non-autoclave-cured wet layup composites are used extensively in applications ranging from civil and marine infrastructure to offshore components and in transmission power systems. In many of these applications the composites can be exposed to elevated temperatures for extended periods of time. While residual tensile characteristics have been used traditionally to assess the integrity of the composite after a thermal event/exposure, it is emphasized that fiber-dominated characteristics such as longitudinal tensile strength are not affected as much as those associated with shear. This paper reports on the investigation of shear related characteristics through off-axis and short-beam shear testing after exposure to temperatures between 66 °C and 260 °C for periods of time up to 72 h. It is shown that the use of shear test results in conjunction with tensile tests enables better assessment of the competing effects of postcure, which results in an increase in performance, and thermal degradation, which causes drops in performance. Off-axis-to-tensile strength and short-beam shear strength-to-tensile strength ratios are used to determine zones of influence and mechanisms. It is shown that temperatures up to 149 °C can lead to advantageous postcure related increases in performance whereas temperatures above 232 °C can lead to significant deterioration at time periods as low as 4 h. The use of shear tests is shown to provide data critical to performance integrity showing trends otherwise obscured by just the use of longitudinal tensile tests. A phenomenological model developed based on effects of the competing mechanisms and grouping based on phenomenon dominance and temperature regimes is shown to model data well providing a useful context for deign thresholds and determination of remaining structural integrity. Full article

Carbon fiber (CF) and carbon fiber-reinforced polymers (CFRPs) are widely used in the aerospace, automotive, and energy sectors due to their high strength, stiffness, and low density. However, significant waste is generated during manufacturing and after the use of CFRPs. Traditional disposal methods like landfilling and incineration are unsustainable. CFRP machining processes, such as drilling and milling, produce fine chips and dust that are difficult to recycle due to their heterogeneity and contamination. This study investigates the oxidation behavior of CFRP drilling waste from two types of materials (tube and plate) under oxidative (non-inert) conditions. Thermogravimetric analysis (TGA) was performed from 200 °C to 800 °C to assess weight loss related to polymer degradation and carbon fiber integrity. Scanning electron microscopy (SEM) was used to analyze morphological changes and fiber damage. The optimal range for removing the polymer matrix without significant fiber degradation has been identified as 500–600 °C. At temperatures above 700 °C, notable surface and internal fiber damage occurred, along with nanostructure formation, which may pose health and environmental risks. The results show that partial fiber recovery is possible under ambient conditions, and this must be considered regarding the harmful risks to the human body if submicron particles are inhaled. This research supports sustainable CFRP recycling and fire hazard mitigation. Full article

This study investigates the mechanical, morphological, and wear properties of SiO₂-filled tri-axial warp-knitted (TWK) glass fiber-reinforced vinyl ester matrix composites, with a focus on void fraction, tensile, flexural, hardness, and wear behavior. Adding SiO₂ fillers reduced void fractions, enhancing composite strength, with values ranging from 1.63% to 5.31%. Tensile tests revealed that composites with 5 wt% SiO₂ (GV1) exhibited superior tensile strength, Young’s modulus, and elongation due to enhanced fiber–matrix interaction. Conversely, composites with 10 wt% SiO₂ (GV2) showed decreased tensile performance, indicating increased brittleness. Flexural tests demonstrated that GV1 outperformed GV2, showcasing higher flexural strength, elastic modulus, and deflection, reflecting improved load-bearing capacity at optimal filler content. Shore D hardness tests confirmed that GV1 had the highest hardness among the specimens. SEM analysis revealed wear behavior under various loads and sliding distances. GV1 exhibited minimal wear loss at lower loads and distances, while higher loads caused significant matrix detachment and fiber damage. These findings highlight the importance of optimizing SiO₂ filler content to enhance epoxy composites’ mechanical and tribological performance. Full article

This article analyzes statistics on the failure of technological equipment, assemblies, and mechanisms of agricultural (and other) machines associated with the breakdown or failure of gear pumps. It was found that the leading causes of gear pump failures are the opening of gear teeth contact during pump operation, poor assembly, wear of bushings, thrust washers, and gear teeth. It has also been found that there is a problem related to the restoration, repair, and manufacture of parts in the conditions of enterprises serving the agro-industrial complex of the Republic of Kazakhstan (AIC RK). This is due to the lack of necessary technological equipment, tools, and instruments, as well as centralized repair and restoration bases equipped with the required equipment. This work proposes to solve this problem by applying AM technologies to the repair and manufacture of parts for agricultural machinery and equipment. The study results on the stress–strain state of support bushings under various pressures are presented, showing that a fully filled bushing has the lowest stresses and strains. It was also found that bushings with 50% filling and fully filled bushings have similar stress and strain values under the same pressure. The difference between them is insignificant, especially when compared to bushings with lower filling. This means that filling the bushing by more than 50% does not provide a significant additional reduction in stresses. In terms of material and printing time savings, 50% filling may also be the optimal option. Full article

This study investigates the bond behavior of fabric-reinforced cementitious matrix (FRCM) composites with three common masonry substrates—solid clay bricks (SBs), perforated bricks (PBs), and concrete hollow blocks (HBs)—using knitted polyester grille (KPG) fabric. Through uniaxial tensile tests of the KPG fabric and FRCM system, along with single-lap and double-lap shear tests, the interfacial debonding modes, load-slip responses, and composite utilization ratio were evaluated. Key findings reveal that (i) SB and HB substrates predominantly exhibited fabric slippage (FS) or matrix–fabric (MF) debonding, while PB substrates consistently failed at the matrix–substrate (MS) interface, due to their smooth surface texture. (ii) Prism specimens with mortar joints showed enhanced interfacial friction, leading to higher load fluctuations compared to brick units. PB substrates demonstrated the lowest peak stress (69.64–74.33 MPa), while SB and HB achieved comparable peak stresses (133.91–155.95 MPa). (iii) The FRCM system only achieved a utilization rate of 12–30% in fabric and reinforcement systems. The debonding failure at the matrix–substrate interface is one of the reasons that cannot be ignored, and exploring methods to improve the bonding performance between the matrix–substrate interface is the next research direction. HB bricks have excellent bonding properties, and it is recommended to prioritize their use in retrofit applications, followed by SB bricks. These findings provide insights into optimizing the application of FRCM reinforcement systems in masonry structures. Full article

The incorporation of residues into rubber composites has gained attention as a sustainable strategy to address waste management challenges while replacing commercial fillers. In this study, we investigated the potential use of water filter cartridge residue after exhaustion, composed of activated carbon, as a reinforcing filler in vulcanized natural rubber composites. Samples were prepared with 5, 10, 15, and 20 phr (per hundred rubber) of residue and compared to unfilled natural rubber. Stress vs. strain tests reached 13.9 MPa of tension at rupture for composites containing 10 phr of carbon-activated residues, representing a 21.9% increase compared to natural rubber. Interestingly, the tension at rupture for NR/AC_10phr reached values close to those of NR/CB_5phr (with carbon black N330) attaining 14.4 MPa. These results indicate that, even at relatively low concentrations, the carbon filter can offer partial substitution for commercial fillers. Moreover, the use of activated carbon from filter cartridges as filler in rubber composites provides an environmentally favorable alternative to energy-intensive regeneration processes for activated carbon. Full article

This study aimed to develop experimental filler-reinforced resin composites for vat-photopolymerization 3D printing and to evaluate the effects of filler addition on their mechanical, physicochemical, and bonding properties for dental restorative applications. Silanized nano- and/or micro-fillers were incorporated into acrylic resin monomers to formulate photocurable resins suitable for vat-photopolymerization. The rheological behavior of these liquid-state resins was assessed through viscosity measurements. Printed resin composites were fabricated and characterized for mechanical properties—including flexural strength, flexural modulus, and Vickers hardness—both before and after 8 weeks of water immersion. Physicochemical properties, such as water sorption, water solubility, and degree of conversion, were also evaluated. Additionally, shear bond strength to a resin-based luting agent was measured before and after artificial aging via thermocycling. A commercial dental CAD-CAM resin composite served as a reference material. Filler incorporation significantly improved the mechanical properties of the printed composites. The highest performance was observed in the composite containing 60 wt% micro-fillers, with a flexural strength of 168 ± 10 MPa, flexural modulus of 6.3 ± 0.4 GPa, and Vickers hardness of 63 ± 1 VHN, while the commercial CAD-CAM composite showed values of 152 ± 8 MPa, 7.9 ± 0.3 GPa, and 66 ± 2 VHN, respectively. Filler addition did not adversely affect the degree of conversion, although the relatively low conversion led to the elution of unpolymerized monomers and increased water solubility. The shear bond strength of the optimal printed composite remained stable after aging without silanization, demonstrating superior bonding performance compared with the CAD-CAM composite. These findings suggest that the developed 3D-printed resin composite is a promising candidate for dental restorative materials. Full article

There is insufficient evidence regarding the cytotoxicity of restorative 3D-printing resins, used as part of the digital workflow in dentistry. This study presents a novel comparative evaluation of cell viability and adhesion using human Wharton’s jelly-derived mesenchymal stem cells (WJ-MSCs), a less commonly used but clinically relevant cell line in dental biomaterials research. The aim of this study was to evaluate the cell viability of WJ-MSCs seeded on 3D-printed resins intended for temporary restorations. Resin discs of three commercial 3D-printing resins (NextDent C&B, Leaf Dental C&B, and UNIZ Temp) and a conventional self-curing acrylic resin (NicTone) were used. WJ-MSCs were cultured on the specimens for 1, 4, and 10 days. Cell viability was assessed using the PrestoBlue assay, Live/Dead immunofluorescence staining, and 7AAD/Annexin V staining. Cell adhesion was evaluated using scanning electron microscopy. Direct exposure to the 3D-printed resins and the self-curing acrylic caused slight reductions in cell viability compared to the control group in both microscopic analyses. 7AAD/Annexin V showed the highest percentage of viable WBCs for the conventional acrylic (34%), followed by UNIZ (35%), NextDent (42%), and Leaf Dental (36%) (ANOVA p < 0.05 Tukey’s post-hoc test p < 0.05). These findings suggest that 3D-printed resins could be considered safe for use in temporary restorations. Full article