Search Results (175)

The aim of this study was to explore the compactibility of unidirectional staple carbon fibre laminates in comparison with their uni- and biaxial continuous fibre counterparts. Resin-preimpregnated plies were inserted into a heated compression mould at an elevated mould temperature of 110 °C. By applying stepwise loading, the correlation between consolidation pressure and fibre volume content was derived and related to fibre orientation distribution. The fibre orientation distribution is obtained from photographic image analyses of 2D ply sections of the same samples using the structure tensor approach. For commonly used autoclave prepreg pressure of 6.8 bar results indicate that lower-oriented staple carbon fibre unidirectional laminates with a fibre orientation distribution factor η₀ = 0.74 can potentially reach a maximum of 39% fibre volume fraction, while higher-oriented laminates with η₀ = 0.78 end up at 43%. An exponential extrapolation suggests that a consolidation pressure of ≥90 bar is required to achieve 60% fibre volume content with highly oriented unidirectional staple carbon fibre laminates. 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

Real-time monitoring of surface cracks in reinforced concrete (RC) beams is critical to structural safety and service performance evaluation. Current structural crack monitoring still faces prominent scientific and technical bottlenecks: conventional unidirectional sensors cannot achieve multi-directional collaborative sensing, rigid piezoelectric materials exhibit poor compatibility with the large deformation of concrete, and there is a lack of quantitative mapping relationships from sensing signals to crack parameters, making it difficult to simultaneously measure crack width, angle, and morphology. This paper presents a novel ring-shaped piezoelectric sensor based on polyvinylidene fluoride (PVDF) and an annular piezoelectric sensing mechanism for real-time monitoring of crack angle, width, and morphology. The sensor incorporates a laminated structure with four strip sensing units for multi-directional strain detection. Experiments were conducted on RC beams under various loading conditions, and finite element analysis was performed using COMSOL Multiphysics. An innovative crack damage index (B) was introduced to assess structural damage quantitatively. Results demonstrate high sensor sensitivity and stable output. Voltage signals increase both with crack width and crack angle, showing responses of 0.045 mV, 0.041 mV, and 0.023 mV for crack angles of 60°, 45°, and 30°, respectively, at a crack width of 9 mm. Strong consistency between experimental and simulation data validates the effectiveness of the mechanism in monitoring the direction, width, and types of cracks. The crack damage index B exhibits a positive correlation with the structural stress response, enabling a quantitative assessment of damage. This study is applicable to the prestressed concrete box girders and T-beams commonly used in large-span bridges, which are typically with a main span of 20–50 m, a beam length of 6–30 m, a section height of 1.2–2.5 m, and designed for Grade C35–C50 concrete. The findings provide a practical foundation for real-time crack monitoring in large-scale bridge beam members. Full article

In this study, the 33.5 J low-velocity impact (LVI) behaviour of unidirectional T300/5208 CFRP cylindrical shell panels with a 40-ply [45/0/−45/90]_5s layup was investigated using Abaqus/Explicit under the effect of the curvature-height parameter (f = 0–62.5 mm; a₁–a₆). To address the limitation of the previous single-block approach in not being able to represent delamination, the study was carried out on two models: an intralaminar-only (SC8R single-block) model and an intralaminar + interlaminar model containing nine cohesive interfaces. Quantitative results: In the intralaminar-only model, the maximum contact force peaks at a₃ (f = 25 mm), with 13,192 N, representing a 13.7% increase relative to the flat panel; whereas in the intralaminar + interlaminar model, the force is highest at a₂ (f = 12.5 mm), with 14663 N, and decreases monotonically with curvature (10,765 N at a₆). Failure mechanism: In the intralaminar-only model, the dominant intralaminar mode is matrix tensile damage (DAMAGEMT); in the intralaminar + interlaminar model, interlaminar separation (CSDMG) governs the total damage, and the initiated delamination area reaches its minimum at a₄ (f = 37.5 mm), with 7282 mm², and its maximum at a₅, with 9821 mm². Thus, a curvature-dependent delamination-minimum regime arises that differs from the a₃ optimum of the intralaminar-only model. An impact performance index (DPI) and its surface-area-corrected derivative, DPI* = DPI/ζ, were applied separately for both models. It was shown that delamination systematically lowers the performance level and shifts the optimum curvature window. All findings are comparative trends within a single numerical framework. Full article

Composite laminates experience static and fatigue delamination, presenting significant challenges for failure prediction. This is critical in curved composites, where delamination behavior is complex to predict. In this study, fatigue tests were conducted on curved composite laminates under non-constant mixed-mode conditions. The testing setup involved a four-point bending test using L-shaped, unidirectional carbon-fiber-reinforced polymer curved beam specimens. A Teflon insert placed at the bend was used to initiate delamination. Experimental data acquisition included digital image correlation (DIC) to monitor delamination length during testing. This is important since it enhances subsequent model correlation. A virtual crack closure technique (VCCT)-based method for simulating fatigue-driven delamination under variable mixed-mode conditions was validated against experiments. Delamination growth was modeled using a Paris-like power–law relationship based on the strain energy release rate. The approach was implemented in Abaqus as a user subroutine, incorporating load ratio and mode mixity effects through VCCT-based mode separation. This study demonstrates accurate fatigue delamination prediction and highlights the role of optical measurements in experiments. The model improves our understanding of delamination propagation under varying mode mixity and contributes to structural integrity analysis. The results show how mode mixity influences delamination, impacting the performance and lifecycle of composite structures. Full article

Compression testing of high-performance carbon fiber composites remains challenging due to premature failure modes in unidirectional laminates, which can underestimate true material strength. This study investigates the compressive behavior of T800-grade carbon fiber-reinforced polymer (CFRP) cross-ply ([90/0]_2s) and unidirectional ([0]₈) laminates using finite element simulation and experimental testing following the SACMA SRM-1R-94 standard, combined with macroscopic and microscopic failure analysis. The results show that cross-ply laminates consistently exhibit valid mid-gauge failure with lower data dispersion (coefficient of variation: 3.44%), whereas unidirectional laminates are prone to invalid root failures (crushing or shear). The compressive strength derived from cross-ply laminates using the back-out factor (2040 MPa) is 13% higher than that from direct unidirectional testing (1802 MPa), attributed to the in situ effect where adjacent 90-degree plies suppress fiber microbuckling. The cross-ply approach provides a more reliable and practical method for characterizing the true in situ compressive strength of high-performance CFRP composites. Full article

Single-lap shear joints made from fabric T300/polyphenylene sulfide (T300/PPS) and unidirectional T700/low-melt polyaryletherketone (T700/LM-PAEK) laminates are joined via induction and conduction welding at different processing temperatures. The joints are tested experimentally to investigate the influence of the processing temperature on the damage evolution in the specimens which is tracked using digital image correlation. Cracks grow rapidly in the unwelded parts of the joint interface but assume a stable steady-state propagation rate when reaching the fully welded overlap region. It is found that higher welding temperatures lead to longer weld lengths, which improve the strength and stiffness of the specimens and delay damage initiation. An accelerated crack growth rate indicates that the structure is close to its ultimate load after which the joint fails abruptly as the crack growth becomes unstable. Induction welding temperatures at the upper end of the recommended processing window (330 °C for T300/PPS and 385 °C for T700/LM-PAEK) result in the joints with the highest load-carrying capacity and slowest crack propagation, but also the least damage tolerance. Full article

Hybrid composite laminates combining pseudo-woven meso-architectured composite (MAC) and unidirectional (UD) sub-laminates manufactured via automated fiber (AFP) placement are attractive as they combine the increased toughness of MAC and higher stiffness of UD while also reducing the manufacturing time. MACs are manufactured via a modified AFP process involving tow skips to create a woven-like architecture using unidirectional tows and introduce shallow crimp angles and complex fiber angle distributions in the architecture. Previous studies on hybrid MAC laminates have shown increased impact damage resistance/tolerance under high- and low-velocity impacts. This work presents an experimental study on the open-hole tension (OHT) and open-hole compression (OHC) response of T800-SC-24k carbon/epoxy laminates of nominal thickness 4.55 mm manufactured via AFP manufacturing. Two hybrid laminate configurations consisting of a UD core and pseudo-woven MAC sub-laminates on the outer surfaces are compared against a traditional UD quasi-isotropic control laminate. One of the hybrid laminate configurations has a plain-woven-like architecture while the other has a complex 3D woven type architecture. The hybrid laminates exhibited a marginal 7% increase in OHT strength and up to a 16% reduction in normal loading direction strains around the hole relative to the control. All three configurations showed comparable OHC strengths. Despite the complex meso-architecture of the MAC sub-laminates, failure in both OHT and OHC is found to be governed primarily by the UD core, which dominates load-carrying capability and failure mechanisms. The results demonstrate that the hybrid laminates maintained or improved in-plane OHT/OHC performance while previously demonstrating better damage resistance and tolerance under impact. Full article

This study investigates the residual stresses developed during the curing process of polymer fiber-reinforced composites and their influence on fracture behavior, particularly the initiation and propagation of interlaminar cracks. The main objective is to quantify how different curing histories, including incomplete cure, alter the spatial distribution of residual stresses and, in turn, affect the mode-I fracture response of carbon-fiber/epoxy laminates. A transient thermal–structural finite element framework incorporating an autocatalytic cure kinetics model was used to simulate the curing process and predict residual stress development in a unidirectional carbon-fiber/epoxy laminate with an edge crack, considering thermal, chemical, and geometric effects. The cure model was calibrated using isothermal differential scanning calorimetry data to determine the degree of cure under different thermal conditions. The key novelty of this work is the integration of a validated cure-kinetics-based curing simulation with fracture analysis, enabling direct correlation of thermal history and degree of cure with spatially varying residual stresses at the crack front and their effect on fracture toughness. Numerical load–displacement predictions were compared with double cantilever beam experimental results and showed good agreement for the curing profiles examined. The results demonstrate that residual stresses generated by different cure cycles, including hold conditions and incomplete curing, significantly influence fracture toughness. In particular, the incomplete-cure profile produced an approximately 40% reduction in toughness compared with profiles that achieved complete cure, highlighting the importance of cure history in determining final structural performance. Full article

A sandwich structure consists of a light core and two thin laminates bonded on both sides of the core. Sandwich structures have applications in structural constructions such as wind turbine blades and marine boats. These structures may experience low-velocity impacts from maintenance operations or during service conditions; thus, it is important to study these low-velocity impacts. In the current study, a sandwich structure was fabricated from PVC foam core and unidirectional glass fibres using the vacuum resin infusion method. The PVC foam core used was of 10–20 mm thickness while the face sheet had two different thicknesses. The panel was tested for impact strength using drop weight equipment at impact energies at three energy levels. The results were reported for damage area, force–time, force–displacement and energy–time curves. Full article

Composite materials have traditionally been employed in the aerospace sector due to their ability to withstand highly demanding service conditions. In recent years, their application has expanded significantly into other engineering domains, including wind energy, shipbuilding, and the automotive industry. The design of composite structures often involves geometric discontinuities, such as cut-outs for access or fastener holes for mechanical joining, which typically become critical regions under load. Consequently, the stress concentrations induced by notches represent a major concern, as they can lead to substantial reductions in strength compared with unnotched laminates. A comprehensive understanding of the behaviour of notched specimens is therefore essential for the design of complex composite assemblies, where components are commonly joined using bolts and rivets. The objective of this study is to examine the tensile response and notch sensitivity of carbon fibre-reinforced polymer (CFRP) laminates with different stacking sequences, through a comparative analysis of unnotched and open-hole specimens. A central circular hole was introduced to reproduce the geometric discontinuities frequently encountered in structural applications, enabling a detailed assessment of stress concentration effects. The experimental results indicate that unidirectional laminates exhibit the highest sensitivity to notches, whereas quasi-isotropic configurations among the multidirectional laminates display the most pronounced reduction in strength, approaching 50%. Moreover, the Point Stress Criterion (PSC) and the Average Stress Criterion (ASC) were employed to determine the characteristic lengths of the specimens, revealing significant differences among the values obtained for each lay-up configuration. Overall, the findings highlight the strong influence of stacking sequence on the mechanical response of notched CFRP laminates and underscore the need to further refine existing failure criteria to accommodate novel laminate architectures, including Bouligand-type helicoidal bioinspired stacking sequences. Full article

This paper investigates the fatigue and residual strength behavior of recycled carbon fiber reinforced plastics (rCFRPs) with different fiber architectures in an epoxy resin matrix: a unidirectional (UD) rCFRP and a non-crimp fabric (NCF) composite. Due to the research gap in fatigue testing of recycled carbon fiber-reinforced plastics with quasi-continuous fiber reinforcement, their fatigue properties are investigated in this article. The objective of the present study is to contribute to the broader goal of integrating recycled carbon fibers as quasi-continuous fiber reinforcement in structural applications by understanding their failure behavior. To determine suitable stress levels for fatigue testing, quasi-static tensile tests are conducted first. Subsequently, fatigue tests are performed with a stress ratio of 0.1. Damage evolution is documented by a continuous recording of the stiffness degradation. For the unidirectional material, an S-N_f curve is created based on three stress levels. The curve can be described with a logarithmic equation. Fatigue testing of the NCF laminate is performed at a single stress level. Subsequent residual strength tests using standard specimens show no clear correlation between the number of load cycles of pre-cycling and residual strength, but indicate a sudden-death behavior for both composites. For further investigation of the damage behavior, in situ residual strength tests are carried out using a combination of acoustic emission analysis and micro-computed tomography (µCT) imaging. This investigation is intended to illustrate crack initiation and propagation three-dimensionally after pre-cycling and during residual strength tests. The results demonstrate a significant influence of the microstructure on the failure behavior. Full article

Delamination is a critical failure mode in composite laminates that degrades the structural performance and load-carrying capacity. This study investigates the improvement of Mode I and Mode II interlaminar fracture toughness of carbon fiber-reinforced polymer (CFRP) laminates through the interleaving of electrospun thermoplastic nanofiber mats. Nanofiber veils were inserted between carbon fiber plies to enhance resistance to delamination under tensile opening (Mode I) and in-plane shear (Mode II) loading. The effects of nanofiber interleaving were evaluated using double cantilever beam (DCB) tests for Mode I and end notch flexure (ENF) tests for Mode II. Both tests were conducted on a symmetric quasi-isotropic laminate [-45/45/90/0₅]_s containing a thick unidirectional 0° ply at the mid-plane. Thermally induced residual stresses resulting from mismatches in ply coefficients of thermal expansion and unsymmetric arm lay-ups were accounted for in the experimental determination of fracture toughness. These stresses, generated during cooling from the cure temperature, influence the effective strain energy release rate and were included in the fracture toughness calculations to ensure accurate toughness evaluation and consistency with numerical predictions. The results demonstrate improved delamination fracture toughness, highlighting the potential of nanofiber interleaving for aerospace and wind energy applications. Full article

Carbon fiber-reinforced polymer (CFRP) composites modified with alumina (Al₂O₃) and silicon carbide (SiC) nanoparticles were developed to produce hybrid nanocomposites with improved mechanical and thermal characteristics. This study investigates the hot drilling behavior of unidirectional CFRP and hybrid nanocomposites by examining the effects of spindle speed, feed rate, drill diameter, and drill geometry (step, core, and twist). Response Surface Methodology (RSM) and Analysis of Variance (ANOVA) were used to identify the most influential parameters governing drilling-induced damage. ANOVA results revealed that drill geometry was the most dominant factor, contributing more than 89% to delamination, burr formation, and surface roughness, followed by drill diameter with over 7% contribution. For temperature rise, drill geometry accounted for more than 50% of the total variation, while drill diameter contributed over 17%. Among the tools evaluated, the step drill produced the minimum drilling-induced damage, followed by the twist drill. In terms of material performance, the Al₂O₃-reinforced hybrid nanocomposite exhibited superior drilling behavior compared to the SiC-reinforced and neat CFRP laminates. Overall, the results demonstrate that drilling-induced damage under hot drilling conditions can be effectively minimized through appropriate selection of tool geometry and process parameters, confirming the suitability of hot drilling for machining aerospace-grade CFRP hybrid nanocomposites. Full article

One of the most critical damage modes affecting the structural performance of traditional composite materials, and therefore their durability, is the occurrence of interlaminar cracks (delamination), which are prone to grow under different loading conditions. In this study, the feasibility of repairing carbon fiber reinforced polymer (CFRP) laminates using structural adhesives was experimentally investigated by evaluating the Mode I interlaminar fracture toughness. Two unidirectional AS4 CFRP systems were analyzed, manufactured with epoxy 8552 and epoxy 3501-6 matrix resins. Mode I delamination behavior was characterized using Double Cantilever Beam (DCB) specimens. Three commercial structural adhesives were used in the repair process: two epoxy-based systems, (Loctite^® EA 9460™, manufactured by Henkel adhesives (Düsseldorf, Germany), and Araldite^® 2015 manufactured by Huntsman Advanced Materials (The Woodlands, TX, USA) and one low-odor acrylic adhesive, 3M Scotch-Weld^® DP8810NS manufactured by 3M Company (St. Paul, MN, USA). Adhesive joints were applied to previously fractured specimens, and the results were compared with those obtained from baseline composite specimens. The results indicate that repaired joints based on the 8552 matrix exhibited higher strain energy release rate (G_Ic) values, approaching those of the original material. The 3501-6 system showed increased fiber bridging, contributing to higher apparent fracture toughness. Among the adhesives evaluated, the acrylic-based adhesive provided the highest delamination resistance for both composite systems. Full article