In-situ strain measurement of ballistic fabrics during impact using fiber Bragg gratings

In previous experiments, the authors demonstrated that strain values collected from fiber Bragg gratings (FBG) integrated into a single layer of Kevlar fabric, placed between a soft armor test specimen and backing material, could be related to the time dependent back-face deformation (BFD) of the armor sample. In this paper, we investigate the specific fabric deformation and failure mechanisms that cause observed events in the FBG measured strain behavior and the FBG spectral profile throughout the impact event. For these experiments, the standard clay backing material was replaced with a 20% clear ballistic gel to provide visual access to the back-face. The test specimen was impacted by an 8.24 g steel ball bearing travelling at 248.8 m/s, during which strain was calculated from the measured full spectrum response of the FBG using a high-speed optical interrogation system. The strain response was compared to the BFD of the Kevlar sample. The BFD was measured through the clear ballistic gel using two high speed cameras recording at 100,000 fps. The results from these tests can be used for future testing using a non-transparent backing material to obtain a detailed strain–time history, back-face deformation history and an understanding of the time sequence of physical energy dissipation mechanisms in the fabric.