Multi-scale numerical analysis of damage modes in 3D stitched composites

3D stitching composites have already sparked great interest among researchers due to their ability to effectively improve the weak inter-laminar properties in composite laminate structures. In this paper, a systematic analysis on mode I fracture and low-velocity impact damage of 3D stitched composites has been demonstrated via a novel multi-scale model which is capable of efficiently characterizing the distribution of stitching threads. The damage modes including intra-laminar cracks, inter-laminar delamination and stitching thread breakage are identified via a combination of continuum damage mechanics (CDM), cohesive zone model (CZM) element and truss element. This novel modeling approach enables an association analysis of stitching thread breakage status and other damage morphologies, promoting an in-depth study for the influence of stitching thread on damage evolution, which has not been reported yet. The introduction of stitching threads effectively suppresses the sharp propagation of delamination after peak load, avoiding the catastrophic destruction during mode I fracture. The mechanism of damage inhibition from stitching threads is well revealed by the numerical simulation. Furthermore, the capability of 3D stitched composites to resist low-velocity impact damage is effectively predicted in terms of peak load, damage area and damage morphology, finding that the increase of stitch density inhibits the propagation of matrix crack, as well as delamination. The universality and the expanding application of the model are also evaluated, demonstrating that it is suitable for various 3D stitched composites.