Composite laminates are susceptible to impact events during use and maintenance, affecting their safety performance. Edge-on impact is a more significant threat to laminates than central impact. In this work, the edge-on impact damage mechanism and residual strength in compression were investigated using experimental and simulation methods by considering variations in impact energy, stitching, and stitching density. The damage to the composite laminate after edge-on impact was detected in the test by visual inspection, electron microscopic observation, and X-ray computed tomography techniques. The fiber and matrix damage were determined according to the Hashin stress criterion, while the cohesive element was used to simulate the interlaminar damage. An improved Camanho nonlinear stiffness discount was proposed to describe the stiffness degradation of the material. The numerical prediction results matched well with the experimental values. The findings show that the stitching technique could improve the damage tolerance and residual strength of the laminate. It can also effectively inhibit crack expansion, and the effect increases with increasing suture density.

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.

In this work, experimental studies were conducted on the damage failure of laminated composite laminates under low-velocity impact and compressive failure behavior under compression after impact. The study primarily investigated the effects of stitch density, impact energy, and laminate thickness on the damage behavior of composite laminates. The experimental results indicate that at impact energies of 10 J, 15 J, and 20 J, the stitched specimens demonstrated higher impact resistance. When the stitch density was 10 × 10 mm, the average maximum impact force of the stitched specimens increased by 13.14%, 15.83%, and 21.48%, respectively, compared to the unstitched specimens. This was mainly attributed to the resin threads formed by the stitches, which enhance the through-thickness strength of the laminate, with the strengthening effect being positively correlated with stitch density. Under 20 J, the strength of the three groups of specimens with different stitching densities increased by 9.24%, 14.58%, and 21.48%, respectively, compared to the unstitched specimens. Under lower impact energies, the bending stiffness of the laminate itself was sufficient to resist the impact force, resulting in minimal differences in residual displacement among different specimens. Furthermore, the study found that under identical impact energy, stitch thread significantly suppressed delamination damage in thin specimens, whereas its effect on thick specimens was comparatively limited. The stitching also had a positive effect on the residual compressive strength of the specimens. Under 20 J impact energy, compared to the unstitched specimens, the residual compressive strength of the three groups of stitched specimens increased by 6.52%, 17.71%, and 27.48%, respectively. The mode of compression after impact failure also differed: unstitched laminated specimens mainly exhibited delamination damage, with cracks propagating along the width direction, while stitched laminated specimens demonstrated strength failure. Under axial compression, stress was released at the stitching points, leading to small-scale cracks along the fiber direction at these locations. Overall, the stitching process effectively enhances the impact resistance of laminated boards. Higher stitching density correlates with greater compressive residual strength, with this effect being more pronounced in thin-plate specimens.

Water-assisted injection molding (WAIM) is a promising molding process developed based on conventional injection molding (CIM). It has been a research hotspot in recent years and is still receiving extensive attention from many scholars and industries because of its significant potential advantages in practical applications. However, compared with CIM, since the additional water-related parameters are involved, the process moldability of thermoplastics is significantly reduced, especially for fiber-reinforced thermoplastics, which stunts the development of WAIM process. In this work, short-shot WAIM with an overflow cavity (OSSWAIM) was developed to address the problems and broaden the application scope of WAIMs. The results showed that compared with overflow WAIM (OWAIM) and short-shot WAIM (SSWAIM), OSSWAIM could significantly improve the process moldability and part quality of fiber-reinforced thermoplastics, especially for thermoplastic composites with a high fiber weight fraction. Besides, it was also found that water penetration had a slight influence on the fiber orientation near the water inlet, but had a significant influence on the fiber orientation near the end of mold cavity. Finally, three processing parameters affecting the water penetration, i.e., water pressure, melt temperature, and water injection delay time were investigated in terms of their influences on the fiber orientation within OSSWAIM.