Understanding effectiveness of stitching in suppression of impact damage: An empirical delamination reduction trend for stitched composites

In this paper, a novel empirical trend has been identified to understand the delamination reduction in stitched composites subjected to impact loading. This empirical-based Delamination Reduction Trend (DRT) is developed based on an extensive series of low-velocity impact tests using specimens of different laminate thicknesses, stitch densities and stitch thread linear (mass) densities, subjected over a range of impact energy levels. The DRT simply relates two parameters: normalized delamination area (DelamNorm) and stitch fiber volume fraction (Vft), to characterize the effectiveness of stitching in impact damage suppression. DRT evidently shows a bi-linear behavior: first, an initial linearly decreasing relationship of DelamNorm with Vft; and second, a plateau which indicates maximum delamination area reduction limit of 40% by stitching. Experimentally observed mechanisms are presented and discussed with the aim to justify and explain the bi-linear behavior of DRT. The DRT is further validated with numerous published literature results and has demonstrated excellent agreement.