Search Results (1,854)

Polybenzoxazines (PBz) represent a versatile class of high-performance thermosetting polymers that have attracted significant attention for advanced composite applications due to their unique combination of properties including high glass transition temperatures, low polymerization shrinkage, excellent thermal stability, and molecular design flexibility. This comprehensive review examines the structure–property relationships governing PBz composite performance, from molecular design principles through network formation, composite reinforcement strategies, and ultimate application performance. The review systematically addresses benzoxazine monomer structure and its influence on polymer network architecture, explores the polymerization mechanism, and critically evaluates composite design strategies incorporating carbon-based nanofillers, fiber reinforcements, and hybrid filler systems. Detailed analysis of structure–property relationships reveals how molecular and composite architecture control thermal stability (glass transition temperatures exceeding 350 °C and char yields up to 92%), mechanical performance, electrical properties (dielectric constants as low as 2.67), and chemical durability. Processing techniques ranging from conventional compression molding to emerging additive manufacturing approaches are discussed in the context of morphological control and property optimization. Applications spanning aerospace structures, high-frequency electronics and protective coatings demonstrate the technological relevance of PBz composites. Critical challenges including network brittleness, high cure temperatures, and recyclability limitations are addressed alongside recent advances in dynamic covalent networks, vitrimer chemistry, and self-healing systems that promise to overcome these barriers. This review provides a comprehensive framework for understanding and engineering polybenzoxazine composites for next-generation advanced applications. Full article

Lamb waves propagation analysis is among the leading-edge approaches for the Non-Destructive Testing of composite thin-walled structures. The underlying diagnostic principle relies on the dependence of elastic waves wavelength on the plate thickness through which they propagate. Therefore, critical defects in composite panels, such as delaminations, can be detected by a non-contact, automated and high-resolution method such as Laser Doppler Vibrometry. The general approach applied in signal processing is referred to as Local Wavenumber Estimation. It consists of creating an image of the average wavelength over the acquisition frequency band in which the pixel element corresponds to a point of the vibrometer scan grid. The standard signal processing approach is typically implemented via the Short Space Fourier Transform. While effective at creating an average wavelength image over the acquisition frequency band, a major drawback of the Short Space Fourier Transform is its heavy reliance on the a priori selection of a spatial window size; a suboptimal choice can severely degrade detection robustness and spatial resolution. To overcome this limitation, we propose a novel method based on the 2D Continuous Wavelet Transform for the detection of defects in composite structures based on the isotropic Morlet wavelet. Thereafter, the proposed method is experimentally validated exploiting a glass fiber reinforced plate with an induced delamination. The proposed method proves superior on average by eliminating parameter dependency while preserving the original image resolution, resulting in a 66% Intersection over Union metric, comparable with the values of the standard method, but with a lower computational time. Full article

Improving resource efficiency in structural polymer components requires manufacturing strategies capable of enhancing mechanical performance without increasing material consumption. In this study, the influence of processing-induced fiber architecture on the dynamic fracture toughness of short-glass-fiber-reinforced polyamide 6.6 (PA6.6-GF30) was investigated as a potential strategy for improving structural efficiency through microstructural design. Two materials with identical polymer matrix and glass-fiber content, manufactured by compression molding and injection molding, were evaluated using instrumented Single Edge Notched Bend (SENB) impact tests under Linear Elastic Fracture Mechanics (LEFM) conditions. The effects of processing route, specimen orientation, and notch preparation method were systematically assessed. Compression-molded specimens exhibited pronounced anisotropy, with fracture toughness increasing from 2.30 MPa·m^1/2 at α = 90° to 7.37 MPa·m^1/2 at α = 0°, whereas injection-molded specimens showed a comparatively uniform response (4.19–4.77 MPa·m^1/2). In contrast, notch preparation had only a minor influence on the measured fracture toughness. These results demonstrate that processing-induced fiber architecture is the dominant factor governing fracture toughness in PA6.6-GF30 and provide a mechanical basis for improving structural efficiency through optimized manufacturing. Although component-level structural optimization and life-cycle assessment were beyond the scope of this work, the findings indicate that tailoring the processing-induced fiber architecture may support future resource-efficient and circular design strategies for recyclable thermoplastic composites. Full article

Integration of composites into the fabrication process of structural assemblies within the aerospace, automotive, marine or civil engineering industries represents a rational solution adopted by leading companies which are guided by the necessity for novel low-weight, high-strength, and high-stiffness materials. During the manufacturing process and throughout the service life, fiber-reinforced polymer structures are subjected to impact loading, either accidentally or as an inherent requirement of the operational cycle. Firstly, general aspects regarding impact loading and some parameters used for its characterization are briefly described. Recent progress regarding the influence of the stacking sequence, fiber type, and impactor geometry on the impact performance of Kevlar and glass fiber reinforced composite materials is emphasized. Additionally, the effects of environmental factors (such as temperature, UV radiation, or humidity) on the impact energy absorbed by polymers reinforced with each of the two types of synthetic fibers are presented. Finally, the importance of directing the researcher’s judgment towards improving the characteristics of materials subjected to impact, from a sustainable perspective, is motivated through the presentation of the impact behavior of polymer composites reinforced with Kevlar fibers or glass fibers hybridized with vegetable fibers. Full article

Glass fiber-reinforced polymer (GFRP) is increasingly used in civil engineering because of its high strength-to-weight ratio, corrosion resistance, durability, and low maintenance requirements. However, its inherently brittle behavior and limited ductility restrict its application in seismic regions due to poor energy dissipation capacity. To address this limitation, this study proposes a novel hybrid system comprising pultruded GFRP profiles and a replaceable steel link with ductile pipe sections. The pipe element confines inelastic deformation to the steel components while keeping the GFRP members elastic. Numerical results demonstrate stable hysteretic behavior with no significant degradation in strength or stiffness, confirming the effectiveness of the proposed system. Also, increasing the ratio of the pipe thickness to the flange thickness of the steel link (β) ensures suitable performance provided that plastic hinge formation remains confined to the ductile pipe element and replaceable steel link. Adding the pipe element to the I-shaped steel link increases web stress when β ≤ 1.0 (leading to web yielding), while stresses in the flange, GFRP beam, and GFRP columns are reduced by 46–51%, 17–60%, and 15–40%, respectively. However, for β > 1.0, stresses in GFRP components are not reduced but slightly increase by 1–9% (negligible), making β > 1.0 not recommended. Also, by changing the $\beta =0.50$ to $0.75$, $1.00$, $1.25$, and $1.50$, the flexural capacity, stiffness, and energy dissipation are enhanced by 1.51 times to 2.46 times, 1.18 times to 1.38 times, and 1.35 times to 1.68 times, respectively. Finally, the necessary design equations for the proposed system are presented. Full article

CAD/CAM reinforced resin composites are increasingly used in restorative dentistry because of their favorable mechanical and esthetic properties. This in vitro study evaluated the effect of acidic exposure on surface properties and Streptococcus mutans biofilm formation on three CAD/CAM resin composites: a short-fiber-reinforced composite (SFRC), a woven glass fiber-reinforced composite (TRINIA; TR), and a resin nanoceramic-reinforced composite (Lava Ultimate; LU). Twenty-four specimens from each material were prepared and assessed for surface roughness and wettability using an optical profilometer and contact angle goniometer before and after 24 h of storage in neutral (pH 7) or acidic (pH 4) buffer solutions (n = 12 per group). Surface morphology was further examined using scanning electron microscopy (SEM). Biofilm formation was quantified using S. mutans colony-forming unit (CFU) counts, followed by confocal laser scanning microscopy (CLSM) for biofilm architecture and viability. At baseline, TR exhibited the highest roughness values (0.744 to 0.785 µm, p < 0.001), whereas no significant differences in contact angle were observed among materials. Acidic exposure significantly increased surface roughness and reduced contact angle values in all examined materials (p < 0.001). Compared to storage in neutral conditions, bacterial adhesion was significantly reduced in TR and LU stored in acid (p < 0.001), but SFRC showed no significant change (p = 0.079). Among specimens exposed to acidic conditions, TR exhibited the highest bacterial adhesion (3.24 log₁₀ CFU/mm²) relative to SFRC and LU (p < 0.001). CLSM analysis demonstrated denser biofilm accumulation on TR and LU compared with SFRC. These findings demonstrate that acidic exposure significantly affects the surface properties and bacterial adhesion behavior of CAD/CAM resin composites in a material-dependent manner. SFRC showed no statistically significant difference in S. mutans CFU/mm² between neutral and acidic storage conditions despite measurable surface changes, highlighting its potential suitability for clinical applications associated with acidic oral environments. Full article

Recycled aggregate concrete (RAC) offers an effective approach to reducing the environmental burden associated with construction and demolition waste. In this study, a fiber-reinforced RAC was developed by replacing part of the cement with fly ash and ground granulated blast-furnace slag, while glass fibers or basalt fibers were incorporated as reinforcing materials. A systematic experimental program was conducted to evaluate the mechanical behavior of the proposed concrete under different saturation conditions. The results show that the best toughness performance was achieved in the natural moisture state. In comparison, compressive and flexural strengths reached their maximum values under dry conditions, whereas splitting tensile strength peaked in the natural state. Based on the experimental data, prediction equations were established for the splitting tensile and flexural strengths by considering both saturation degree and fiber content. A stress–strain model under uniaxial compression was also developed. In addition, scanning electron microscopy (SEM) was employed to examine the fiber–matrix interface and hydration products, thereby clarifying the microstructural characteristics of the concrete at different saturation levels. Full article

Novel fiber-reinforced polysiloxane composites (FRPCs) using various fibers infiltrated with a high char yield polysiloxane resin were developed and tested under extreme aerothermal environments. Techneglas manufactures the preceramic polysiloxane resin. FRPCs using glass, silica, carbon, graphite, carbon/polybenzimidazole, alumina, and quartz fibers in different architectures were manufactured by the Koo Research Group. Characterization of thermal stability, flammability, ablation, thermophysical, and mechanical properties of these FRPCs was performed. Thermal stability properties of these FRPCs were characterized using thermogravimetric analysis, and flammability properties using microscale combustion calorimetry. Ablation properties using an oxy-acetylene test bed with advanced diagnostics were performed at several heat fluxes and exposure times. Recession rate, mass loss rate, front surface temperature, and back-face heat-soaked temperature are criteria to compare material performance. The microstructures of these pre- and post-tested FRPCs were investigated using scanning electron microscopy and micro-computed tomography. Thermophysical properties of these FRPCs in virgin and char states were characterized at elevated temperatures. Using these material properties and surface thermochemistry analysis, material response modeling was performed and validated with aerothermal test data. The composites’ tensile, compression, and flexural properties were conducted via ASTM standards. These FRPCs compared favorably with legacy phenolic-based ablatives under similar extreme aerothermal environments. Full article

The use of Glass Fiber-Reinforced Polymer (GFRP) bars to reinforce concrete beams can provide high resistance to corrosion, high performance, high sustainability, and reasonable strength but with low ductility. This study focused on the structural behavior and indirect evaluation of the ductility index of hollow and solid beam specimens reinforced with GFRP bars, steel bars, or both (hybrid). Eight simply supported beam specimens with dimensions of 1200 mm (length), 150 mm (height), and 100 mm (width) were made using self-compacted concrete (SCC) and tested using two-point repeated loading. The tests results showed that the ultimate load capacity of the tested solid and hollow beams reinforced with GFRP bars were 78% and 67% higher than that of the corresponding solid and hollow beam specimens with steel-bar reinforcement. The measured energy absorption is “instantaneous” energy absorption because the residual stress disappears after the load is removed at the end of the test and any cracks will close due to the semi-linear response of the beam specimens reinforced with GFRP bars. Regarding the solid beam specimens, those containing GFRP bars showed an increase in energy absorption of 64–127% compared with the corresponding reference beams. The hollow beam specimens containing GFRP bars showed an increase in energy absorption of 21–68% compared with the corresponding reference beam containing three steel bars. Full article

With the increasing demand for tunnel construction under extreme geological conditions such as high geo-stress, rock bursts, and fault zones, the performance requirements for shotcrete in initial support systems have become more stringent. This study develops a cross-scale structured hybrid fiber-reinforced shotcrete by incorporating alkali-resistant glass fibers including HP and HD types with different lengths and carbon nanotubes (CNTs) into a conventional shotcrete matrix. An orthogonal experimental design at four factors and four levels was adopted to investigate the effects of fiber and CNT contents on the mechanical properties and microstructure of shotcrete. Uniaxial compressive strength, splitting tensile strength, slumping, rebound rate, and microscopic characteristics such as SEM were evaluated at 3, 7, and 28 days. Results show that the optimal mix proportion is 4% HP fiber (24 mm), 2% HD fiber (18 mm), 2% HD fiber (6 mm), and 0.2% CNT. Under this formulation, the 28-day compressive and splitting tensile strengths reached 43.53 MPa and 4.85 MPa, respectively, with a rebound rate as low as 3.85%. The enhanced performance is attributed to the multi-scale reinforcement mechanism. Long fibers suppress macroscopic cracks, short fibers bridge micro-cracks, and CNTs densify the interfacial transition zone. This study provides a parametric reference for the development of high-performance shotcrete and its engineering application in complex underground excavations. Full article

Crack detection in composite structures remains a central challenge in structural health monitoring, particularly when sensing must rely on a small number of embedded multiplexed fiber Bragg gratings (FBGs). Here, we present a physics-guided convolutional neural network (CNN) framework for crack-associated event detection from multiplexed FBG interrogator signals acquired during the three-point bending of glass-fiber-reinforced polymer (GFRP) beams. The dataset was constructed from raw interrogator recordings and synchronized force–displacement metadata while preserving the cracked and non-cracked loading stages present in the experiments. Each candidate response was encoded by 13 synchronized optical, loading, and mechanics-guided descriptors, including Euler–Bernoulli expected strain and residual terms, where the residual denotes the difference between the measured response and the elastic response predicted by beam theory. A compact one-dimensional CNN operating on 30-response sequences was evaluated on 64 experimental runs under strict leave-one-run-out validation. At the selected operating point, the model reached window-level precision of 0.900, recall of 0.910, F1 score of 0.905, and balanced accuracy of 0.942, while the corresponding run-level decision reached a precision of 0.833, a recall of 1.000, an F1 score of 0.909, and a balanced accuracy of 0.969. Bootstrap resampling over runs yielded 95% confidence intervals of 0.787–0.978 for window-level F1 and 0.769–1.000 for run-level F1. To probe generalization beyond the initial fabrication batch, the final frozen pipeline was also tested once on seven later-batch runs from two newly manufactured specimens, where it reached a window-level precision of 0.908, a recall of 1.000, an F1 score of 0.952, a balanced accuracy of 0.969, an ROC-AUC of 0.979, a PR-AUC of 0.955, and perfect run-level classification. These results show that a compact sequence CNN, enriched with mechanics-guided strain interpretation, can extract robust crack-event signatures from multiplexed FBG measurements while preserving a simple and reproducible modeling pipeline. Full article

Evaluation of dentin-replacement materials (DRMs) is essential for optimizing material selection and restorative outcomes. Four restorative groups, each comprising a DRM (SDR^® Plus, group A; EverX Posterior^™, group B-control; Filtek^™ Z250, group C; Fuji II LC^®, group D) overlayed with a microhybrid composite (Filtek Z250) to replace enamel layer, were evaluated for mechanical, optical, and handling characteristics. Standardized specimens were prepared for color change (ΔEab/E00) and fracture toughness (K_IC) testing (n = 8/group/test), with application time and handling evaluated by two independent assessors. Data were collected and analyzed statistically. The greatest color difference was observed in group B, which was significantly higher than that in groups A and C (p < 0.05), while group D did not differ significantly from any other group. Groups A and B exhibited significantly higher (p < 0.001) fracture toughness than groups C and D with no significant differences within either pair of groups. Group A required the least application time (p < 0.001), followed sequentially by groups B, C, and D. Handling ratings varied by material, with moderate inter-rater reliability (κ = 0.53). Within the study’s limitations, restorations with SDR Plus composites offered clinical application efficiency with improved fracture toughness and favorable optical integration. Full article

The wood-based grid sandwich structure with a high load-to-mass ratio and specific strength was prepared with the core of KH-560-modified jute (Corchoruscapsularis) fabric-reinforced epoxy laminated composite (JFRELC). The compressing behavior and energy absorption characteristics of pure grid cores (GC50#, GC80#) and grid sandwich structures (GS50#, GS80#) were analyzed and compared. The failure mechanism of the fracture surfaces of jute fabrics of grid sandwich cores was clarified by SEM. The results showed that the core made of JFRELC-80# had a good performance for the grid sandwich structure by tenon-and-mortise linking. The load-bearing capacity and energy absorption performance of this wood-based grid sandwich structure can be comparable to that of some glass and carbon fiber reinforced composite sandwich structures, and even show certain advantages. The failure modes of the grid sandwich structure were panel cracking, core buckling and core collapse. The failure mechanisms of jute fabrics in epoxy resin were fiber pull-out and fiber splitting. Full article

This study investigates the mechanical and thermal properties of epoxy composites reinforced with aluminum nanoparticles (Al NPs), titanium nanoparticles (Ti NPs), and chopped E-Glass fibers, individually and in hybrid combinations. Thirteen compositions were systematically fabricated and characterized, spanning pure epoxy (PRC0), Al NP-series (PRA1–3), Ti NP-series (PRT1–3), Al NP/E-Glass hybrid series (PRAG1–3), and Ti NP/E-Glass hybrid series (PRTG1–3). The investigation evaluates the effects of these reinforcements on tensile strength, flexural strength, Shore D hardness, thermogravimetric stability, and microstructure. The PRTG2 composite (2 wt% Ti NP + 2 wt% E-Glass fiber) achieved the highest tensile strength of 80 MPa (33.3% improvement over pure epoxy) and the highest flexural strength of 115 MPa (43.75% improvement). These results demonstrate the superior reinforcing efficiency of Ti nanoparticles over Al nanoparticles and the synergistic benefit of combining nanoparticle and fiber reinforcements within a single epoxy matrix. Full article

The aim of the presented study is to verify the possibility of forming hybrid adhesive-mechanical joints between aluminum sheet and PP matrix composites reinforced with carbon and glass fibers using thermal drilling technology. The research responds to weight reduction trends in the automotive and aerospace industries, seeking joining methods that preserve the continuity of reinforcing fibers. The methodology included applying an experimental organosilicate agent to the aluminum, sequential thermal drilling, and an innovative modification of bushing geometry using a 9.3 mm diameter tool. Joint quality was evaluated via tensile shear testing and non-destructive analysis using computed tomography (CT). Results showed that the organosilicate layer significantly increased the load-bearing capacity and adhesion of glass fiber joints. Hybrid joints exhibited higher energy absorption than purely adhesive joints. The proposed bushing geometry modification led to a statistically significant increase in total dissipated energy (by 28% to 37%) and a desired change in the failure mechanism from composite pull-out to bushing shear. CT analysis confirmed the preservation of fiber integrity through radial deflection. Consequently, hybrid joining via thermal drilling with modified geometry effectively utilizes the mechanical properties of metallic materials in multi-material structures. Full article