Thickness effects on the compressive stiffness and strength of stitched composite laminates

A finite element unit cell model for predicting the 3-D mechanical properties of laminates with through-thickness reinforcement has been developed using a unique voxel mesh approach. As part of its development, the model is being verified to other models and validated with experimental data. The model has been validated to tensile experimental data with much success. For further validation of the model and its assumptions, an experimental program was conducted to investigate the compressive properties of stitched laminates for three different thicknesses. It was found that the unstitched modulus remained independent of thickness, while the stitched modulus increased with thickness due to spreading and kinking of the outer surface plies during the stitch formation process. The model predicted the change in modulus well, however did not capture the thickness effect observed in the experimental results. While it was found that stitching did have an influence on the in-plane compressive properties of the test specimens, the known damage tolerance and manufacturing advantages of stitched laminates are considered to outweigh any penalties for certain critical applications.