Conical nosed projectile perforation of polyethylene reinforced cross-ply laminates: Effect of fiber lateral displacement

Fiber lateral displacement refers to fibers being pushed aside transversely rather than being strained to tensile failure; a phenomenon that lowers the ballistic resistance of fabric composites. In this study, the effect of lateral displacement was examined for cross-ply laminates. Ballistic impact tests were performed on ultra-high molecular weight polyethylene (UHMWPE) cross-ply laminates of different thicknesses with a 90° or 60° conical-nosed projectile. Two types of laminates, HB26 and HB50 that contain the same fibers but different matrices, were tested. The number of fibers undergoing lateral displacement in each ply of perforated laminates was quantitatively determined and found to generally decrease from the impact surface to the rear surface of laminates. Following perforation by a projectile with 90° nose angle, fewer fibers lying in the projectile path failed in the weaker matrix HB50 laminate than in the stronger matrix HB26 laminate. On the other hand, the HB50 laminate exhibited a larger ballistic limit velocity than the HB26 laminate. For a projectile with 60° nose angle, more fibers failed during perforation of HB26 laminates but the ballistic limit was smaller than that of the 90° projectile. These contrary results demonstrate that for fibrous cross-ply laminates, the kinetic energy dissipated during projectile penetration is not proportional to the number of failed fibers and the ballistic resistance cannot be evaluated solely by the magnitude of lateral displacement. Localized direct tension along with membrane stretching, makes important contributions to fiber failure. The longer nose of the 60° projectile resulted in more extensive fiber breakage through this localized mechanism and thereby resulted in a smaller ballistic resistance than the 90° projectile.