Studies On Low-velocity Impact Damage Of Stitched Composite Laminates

Composites laminates with weak inter-lamina mechanical properties are prone to delaminate under the low velocity impact loading, which makes them having low impact resistance. Due to the above-mentioned shortcoming of laminated composites, it makes difficult to play a full role in reducing the structure weight. Stitched laminates, as newly developed composite materials, which prominently improve inter-laminar toughness, impact resistance and damage tolerance, have received great attention in the domain of composites research. At present, the main studies on stitched composites are restricted to the mechanical analysis of in-plane properties. However, the works dealing with the dynamic response and progressive damage of stitched composites subjected to low velocity impact are very limited. There are a few test investigations on the impact damage. Therefore, it is important to establish an effective model to predict the low velocity impact damage of stitched laminates, which has become one of the main problems to be solved in the fields of composites design.As for the typical stitched laminated composites, a new 3D finite element model is proposed to predict the dynamic response and progressive damage based on the software ABAQUS. To effectively model the stitched threads, the spatial truss element is selected to be applied in the model. In particularly, the typical failure modes, such as fiber break, matrix crushing, matrix cracking and delamination, were adopted to simulate the occurring of damage events of stitched composites. Based on the user-defined material constitutive program of VUMAT, the degradation scheme of elastic mechanical properties for corresponding failure modes were applied to predict the dynamic response and the damage propagation of stitched composites subjected to low impact load. To verify the numerical model, some typical examples were selected to conduct the simulations. The results indicate that the stitched threads are not broken during the low velocity impact. The failure modes and damage evolution of stitched laminates are similar to the unstitched laminates, but the former has smaller damage areas. The calculated results by the model are well agreed with the measured results. Numerical results verify the effectiveness of the model. The effects of the impact velocity, the component material systems and the main stitching parameters on the dynamic response and damage evolution of stitched composites were discussed in detail, and some valuable conclusions are drawn herein. The current work has laid a foundation for the following research on residual strength of stitched composites containing impact damage.