Dynamic Behavior of Carbon Fiber Reinforced Polymer (CFRP) Composites at Higher Strain Rates

Abstract

Carbon Fiber Reinforced Polymer (CFRP) composites are known to have highly variable modulus and strength based on fiber direction. This presents significant challenges when attempting to identify their mechanical properties. In particular, the composite strength and failure envelope in multi-axial loading is expected to have a complex nature due to anisotropy. Furthermore the heterogeneity of CFRP composites makes it even more difficult to model their failure modes and behavior. These intricacies become more pronounced at higher strain rates. In this study specimens with varying layup, geometry, and fiber volume fractions were tested in different loading conditions. Fiber volume fractions of the samples have been determined using thermogravimetric analysis (TGA) in nitrogen gas environment burnout tests. High strain rate response of CFRP composites are of scientific and technological interest. They are used extensively in aerospace (due to their high specific strength and stiffness) which necessitates their characterization for high velocity impact. The polymeric resins are of course expected to demonstrate rate dependence. Therefore split Hopkinson pressure bar (SHPB) experiments were used to determine the high strain rate response of CFRP composites in this study. The dependence of failure stress and strain on the strain rate was examined and summarized based on different loading conditions, geometries and layups. The failure stress is not very sensitive to strain rate in the range of this study, however comparisons with quasi-static data is done to further analyze this effect. The failure strains are higher when bidirectional specimens are loaded in the transverse direction (normal to the plane of fibers) compared to the axial loading of the unidirectional specimens. Meanwhile it was observed that the failure stresses of both unidirectional and bi-directional fiber specimens are close to each other. This has led to proposing a resin strength dominated failure mode for CFRP composites.