Determination of interlayer crack resistance of stitched thin-sheet epoxy carbon composites using wedge delamination (wedging) technique

Abstract

The local indices of the crack resistance of a thin-sheet reinforced polymeric carbon composite (CCM) stitched with glass and aramid threads with a double lockstitch were determined by wedging in the framework of linear elastic fracture mechanics under loading mode I using standard samples in the form of a double cantilever beam (DCB). Different number of stitches and stitching lines were used for different directions of crack propagation relative to the laying of the reinforcing fabric and stitches. It is shown that regardless of the type of stitching thread and the crack propagation direction, the average and normalized (relative to non-stitched samples) values of the local crack resistance parameters of stitched CCM samples mostly depend on the crack propagation area and on the density (pitch) of stitching responsible for a pronounced locality and anisotropy of the crack resistance. The highest resistance to crack propagation is observed in the areas of interweaving of the stitching threads as a result of additional energy consumption for deformation and breaking of stitching threads, and the lowest one is characteristic of the areas between stitches along the stitch or step between lines, both in the longitudinal and perpendicular directions of the crack propagation relative to the laying of the fabric and stitching lines. When evaluating the specific average values of the parameters of the local crack resistance of CCM samples related to one line of stitching with a longitudinal crack propagation or to one stitch when a crack propagates across the stitching lines, the effect of increased local crack resistance in the areas of interlacing of threads turns out to be significantly weaker, whereas this decrease in stitch areas and between the stitching lines is significantly more pronounced. For more accurate assessment of the crack resistance of a thin-sheet stitched CCM, it is advisable to determine and use specific local parameters taking into account scale effects.