Explore the vast range of titles available.

In this study, a progressive damage model was developed for the mechanical response and damage evolution of carbon fiber stitched composite laminates under low-velocity impact (LVI). The three-dimensional Hashin and Hou failure criteria were used to identify fiber and matrix damage. The cohesive zone model was adopted to simulate the delamination damage, combined with the linear degradation discounting of the equivalent displacement method to characterize the stiffness degradation of the material, and the corresponding user material subroutine VUMAT was coded. The finite element analysis of the LVI of stitched composite laminates under different energies was finished in Abaqus/Explicit. Furthermore, the simulation predictions matched well with the results of the experimental tests. Based on this, composite laminates’ mechanical response and damage forms with different thicknesses and stitch densities were analyzed. The findings show that the main damages of composite laminates were matrix tensile damage and delamination. The stitching process could improve the impact tolerance of composite laminates, inhibiting delamination and reducing the area of the delamination damage. The higher the density of the stitching, the more noticeable its inhibition would be. The thickness of the laminate also had a more significant effect on the damage to the laminate. Thin plates were more prone to matrix tensile damage due to their lower flexural rigidity, whereas thick plates were more susceptible to delamination because of their higher flexural rigidity.

Delamination in laminated composites originates from premature matrix cracking within the interlaminar region, ultimately leading to ply separation under indirect loading. Among the techniques proposed to mitigate this failure mode, through-thickness stitching has emerged as a localized reinforcement strategy capable of enhancing interlaminar performance without modifying the in-plane laminate architecture. However, previous studies report that stitching can either improve or degrade the mechanical properties of the composite, with stitch density identified as a critical variable. This work aims to keep the tensile strength of a stitched composite at levels comparable to its unstitched counterpart. The reinforcement was applied using an eight-strand polyethylene thread (0.28 mm in diameter) embedded in a low-viscosity epoxy infusion system (MAX 1618 A/B) combined with a 90° biaxial fiberglass woven fabric. The tensile behavior of laminates was examined for three longitudinal stitching configurations consisting of 2, 3, and 5 continuous stitch lines. Results show that increasing stitch count produces a progressive reduction in tensile strength, attributed to stress concentration around stitch sites and microstructural effects such as resin-rich zones and fiber waviness.

In bonded composite lap joints, inter-laminar strength is critical to prevent the delamination. In this paper, the phenomenon of progressive delamination in lap joints is simulated in ABAQUS software using stress based composite failure model. It is observed that in all the joints, failure initiation is due to high inter-laminar stresses at the edges of the lap joint. Use of through the thickness stitching results in a significant improvement in inter-laminar strength in joints. These joints are further analyzed for “selective stitching” wherein the stitching is restricted to the failure regions. The results show that selective stitching is able to provide failure strengths close to that of fully stitched model.

Delamination is a major failure Mode in laminated composites, typically triggered by premature interlaminar matrix cracking and leading to severe structural degradation. To address this, various through-thickness reinforcement strategies have been explored, including three-dimensional woven architecture. Although these designs significantly improve delamination resistance, their industrial adoption stays limited due to reproducibility challenges and the high cost and operational complexity of advanced manufacturing systems needed for controlled through-thickness reinforcement. This study investigates an alternative interlaminar reinforcement method, through-thickness stitching, aimed at enhancing Mode-I delamination resistance of a commercial fiberglass laminate without changing its native architecture. Composites were manufactured using a low-viscosity epoxy infusion system (MAX 1618 A/B) and a [0/90] biaxial fiberglass fabric. An eight-filament polyethylene thread (Ø = 0.12 mm) was introduced in predefined stitch architectures consisting of three longitudinal patterns having two, three, and five continuous stitch lines, referred to as AV, BV and CV samples, respectively. Results show that stitching highly increases Mode-I interlaminar fracture toughness GIC by 0.3808, 0.4152 and 0.5192 kJ/m2 for AV, BV and CV respectively, compared to 0.0265 kJ/m2 for the unstitched composite O, highlighting the strong influence of stitch orientation and spacing on interlaminar performance. But scanning electron microscopy revealed added failure mechanisms in stitched specimens, including localized fiber misalignment of up to 33° and resin-rich regions approximately 0.6 mm in length, suggesting that while stitching enhances delamination resistance, it may also influence other mechanical properties.

Effects of four kinds of stitch yarns (carbon fiber (CF), aramid fiber, PBO fiber, and silk fiber (SF)) on interlaminar shear strength (ILSS) and failure behavior of the three-dimensional stitched composites (3DSCs) were investigated by double-notch shear test at room temperature (RT) and 100 ℃. The failure process was recorded by the high-speed camera system. The interfacial strength of stitch yarns/resin at 100 ℃ was quantitatively measured for the first time. The results revealed that the mechanical properties of stitch yarns play a dominant role in determining the ILSS of composites at RT, and 3DSCs stitched with PBO stitch yarn show the best ILSS at RT, which can reach about 82.6 MPa. At 100 ℃, the stitch yarns/resin interface performance is the decisive factor to the ILSS of composites, 3DSCs stitched with CF stitch yarn show the best interlaminar shear performance, and the ILSS is about 52.4 MPa. Moreover, the improvement of the ILSS by stitching was more obviously at 100 ℃, increasing by at least 42%. The low-cost SF has a great advantage in replacing CF at RT on the premise of guaranteeing mechanical properties.Graphical abstractThe effects of four kinds of stitch yarns on interlaminar shear damage law and failure mechanism of the 3DSCs were revealed at room temperature and 100 ℃.

Composite structures are extensively used in several industries such as aerospace, automotive, sports, and construction due to their many advantages, including tailorable mechanical properties, high strength-to-weight ratios, and high specific stiffness. However, due to their low interlaminar tensile and shear strength, composites are prone to delaminations, which can degrade the overall mechanical performance of the structure. Through-thickness stitching provides a third-direction reinforcement to enhance the interlaminar tensile and shear strengths. In this study, quasi-isotropic composite test specimens were manufactured with a novel through-thickness robotic chain stitching with different patterns and tested under uniaxial tensile and three-point bend loadings. A design of experiments (DoE) approach was used to investigate the influence of stitch parameters (stitch density, stitch angle, and linear thread density) on the tensile strength, tensile modulus, and flexural strength of stitched composites. Experimental results are then used to develop a statistically informed response surface model (RSM) to find optimal stitching parameters based on a maximum predicted tensile strength, tensile modulus, and flexural strength. This study reveals and discusses the optimum selection of stitch processing parameters to improve the in-plane and out-of-plane mechanical properties.

Due to the excellent mechanical properties and heat resistance, bismaleimide matrix composite materials have been widely used in aircraft. However, they are susceptible to low-energy impacts, such as bird hits, gravel, tools falling, etc., which can easily result in delamination. The delamination can significantly reduce the compression performance of composites and become a potential hazard for aircraft in service. In this paper, a stitching method developed from the Z-pin manufacturing process was proposed to repair delaminated laminates. Firstly, the delaminated area was stitched by fiber bundles that were pre-impregnated with glue. Then, the fiber bundles threading through the laminate become the pins after the curing process, thus producing the bridging effect between delaminated layers. As a result, the in-plane compressive properties of the laminate are enhanced. The parameters, including the size, number, and position of the stitching hole, for the stitching repair were optimized, and the factors affecting the repair effect were discussed through both finite element analysis and experiments. The results showed that for a carbon fiber/bismaleimide composite plate with a circular delamination roughly 30 mm in diameter, the in-plane compressive strength can be recovered from 54.45% to 84.23% of the pristine plate, and the modulus was fully recovered.

Laminated composites with complex structures, such as L, C, or T-shaped geometries, are used at junction parts in the automotive and aerospace industries. Due to delamination failure in the curved or angled region, these structures are relatively weak in the through-thickness direction when subjected to complex loading. To overcome the issue of weak delamination resistance, this study is aimed at investigating the effect of various reinforcements on the strength properties and delamination resistance of glass/epoxy curved laminates under a four-point bending test. Experimental research was carried out on glass/epoxy curved laminates strengthened by combining three different types and geometries of reinforcement. These included chopped short fibers and glass fiber patches between each ply at the corner region of curvature, and stitching of aramid filament at the corner region of 90-degree bending. A 4 mm pitch interval was observed to enhance curved beam strength, interlaminar tensile strength and delamination resistance. The obtained findings show that glass/epoxy curved laminates reinforced with chopped kenaf short fiber, 45° glass fiber and a single stitch at the corner show greater enhancement in curved beam strength by 52.7, 34.3, and 5.6%, respectively, compared to the baseline sample. Similarly, interlaminar tensile strength improved by 69.3, 40, and 78.5% compared to the baseline sample. On the other hand, curved laminates needled with a single stitch at the corner show higher delamination resistance than other reinforcements. The experimental results correlate well with the damage morphology indicated in SEM fractographic images.

In industrial production, carbon fiber composite laminates are stitched to improve the delamination resistance of materials. In the service process of parts, fatigue damage caused by cyclic load is one of the important reasons for failure of parts. How to predict the fatigue life of stitched composites simply, efficiently and accurately is the focus of research. In this paper, the residual stiffness model of stitched composites is established by introducing the concept of fatigue stiffness and stitching strengthening coefficient. The fatigue life of unstitched carbon fiber composites under complex cyclic stress is predicted by using the tensile fatigue properties of unstitched carbon fiber composites. The tensile fatigue test and three-point bending fatigue test of unstitched T300 carbon fiber composite are carried out, and the fatigue life of the same kind of carbon fiber composite is estimated according to the proposed model. The reliability of the proposed model is verified by comparing the calculated results with the experimental data. The analysis shows that the proposed model could well predict the fatigue life of the same kind of composites after suturing by using the fatigue properties of the unstitched carbon fiber composites. The fatigue stiffness model based on suture strengthening coefficient can effectively predict the fatigue life of suture laminates, which provides a theoretical analysis method for the life prediction of suture composite materials.

Various yarn-shaped flexible strain sensors have recently been developed. However, research is lacking on additive manufacturing for smart clothing for integrating yarn sensors with commercial garments. Herein, a strain-sensing yarn is sewn into a piece of fabric through a novel stitching technique, and the influence of the stitching method and needle pitch on the sensing performance is investigated using finite element analysis (FEA). The sensing performance could be improved when the sensing yarn is self-locked in the fabric at the needle eyes, and the needle pitch was reduced to 0.5 cm, which is attributed to the enhanced stress and strain concentration. Meanwhile, the composite sensing fabric featured outstanding performance, including a low detection limit (0.1%), rapid response (280 ms), excellent durability (10000 cycles), and high stability (negligible drift and frequency independence). In addition, the remarkable wear resistance, washability, and anti-interference to ambient humidity and perspiration were obtained. Therein, the optimal stitch trace lengths of sensing yarn for detecting elbow motion, breathing, and heartbeats are discussed. Finally, a smart clothing system composed of smart clothing, data acquisition unit, and mobile APP was developed to simultaneously detect human movement and physiological signals. This work provides a reference to produce intelligent garments based on yarn sensors for health monitoring.