Strain-Rate Dependence of Mode II Delamination Initiation in a Polyester-Fiberglass Composite Using Dynamic ENF Testing

Abstract The inter-laminar crack growth resistance in fiber-reinforced composites under Mode II (shear) loading is a critical factor for structural integrity in dynamic environments. Traditional End Notched Flexural (ENF) tests have been extensively used under static conditions, yet limited studies exist on high strain rate effects using dynamic setups. The objective of this study is to investigate the influence of varying strain rates on the Mode II strain energy release rate G IIi at damage initiation for polyester-fiberglass composites using a dynamic loading approach. ENF specimens composed of ± 45° glass fiber mats and polyester resin were subjected to three-point bending under different strain rates ranging from 70 s⁻¹ to 150 s⁻¹ using a Split Hopkinson Pressure Bar (SHPB) system. High-speed imaging was employed to capture damage progression, and Mode II SERR was computed using beam theory formulations based on force and displacement data. The results demonstrate a clear trend of increasing G IIi with elevated strain rates, rising from 0.64 to 7.19 kJ/m². The average increase in dynamic crack propagation energy was observed to exceed values from static testing. However, observed failure modes included fiber bridging and non-planar crack paths, contributing to variability in measured results. The dynamic ENF testing shows that Mode II fracture energy in polyester-fiberglass composites is strongly strain rate dependent. The results suggest the need for longer specimen geometries to ensure stable inter-laminar propagation in future investigations.