High-Velocity Impact Behavior of Stitched CFRP Laminates

Abstract

In this paper, high velocity impact (HVI) behavior of the through-the-thickness stitched CFRP laminates is investigated. Both rigid body impact tests and soft body impact tests were conducted. The authors developed finite element model, in which the effect of the stitching was considered. Tests and simulation results revealed that through-the-thickness stitching significantly affects the damage process, and that it can effectively suppress the propagation of the delamination. Through thickness stitched CFRP laminates have prominent mechanical properties such as higher delamination toughness and higher CAI strength than that of non-stitched laminates. However, HVI behavior of the stitched CFRP laminate has not been investigated sufficiently. In this study, effect of stitching on HVI damage of stitched composites is investigated. HVI tests were conducted using single-stage air gun. 7.5mm-diameter steel spheres and 20 mm gelatin spheres are used as projectiles. The impact velocity ranges from 130 to 240m/s. HVI damage of each specimen was investigated by NDI techniques. Authors developed finite element model in which the damage was considered by using stress criterion, continuum damage mechanics (CDM), and cohesive zone model (CZM). The effect of the stitch threads were introduced by specially developed nonlinear spring elements. The simulation results revealed that delamination bridging of the stitch threads significantly affects the damage behavior of the laminate. Delamination area was reduced by crack closure traction generated by stitch threads. Experimental and simulation results demonstrated that through-the-thickness stitching is effective to suppress the delamination under high velocity impact load.