Meso-scale Modeling for Progression of Damage in Stitched and Unstitched Composites Subjected to Tensile and Compression Loads

This paper deals with progressive damage modeling of three-dimensional (3D) stitched composites at the mesoscale level. It uses a 3D solid elements representation for global analysis and incorporates a mesoscale constitutive unit-cell model to simulate local damage response and matrix nonlinearity. This unit-cell model is based on realistic input from x-ray tomography of the composite part. Stitching affects the composite's mechanical properties differently under various loads and changes its damage mechanisms due to irregularities caused by the stitching. Simulations are conducted to investigate the impact of stitching on 3D composites at the mesoscale and compare the results with experimental data on the damage characteristics of 3D stitched composites. It is discovered that in the stitched composites under tensile loading, fiber damage is initiated at the crack area, resulting from the stitching process, and resin damage occurs in the stitching gap, where the fiber threads enter, and expands diagonally throughout the resin with increasing strain. While in the case of compression loading, the fiber damage starts in the area of the gap caused by the passage of the sewing fibers in the thickness direction, and the matrix damage spreads faster in the fibers in the transverse direction. The results show that stitching yields to a 22% and 28% increase in ultimate tensile and compressive strength, respectively. In both stitched and unstitched composite specimens, the formation of transverse cracks in the matrices and at the fiber interface, especially around the stitching area, are observed.