Inelastic behavior of fiber composites subjected to out-of-plane high strain rate shearing

A new pressure-shear recovery experiment for investigating out-of-plane dynamic shear resistance of composites has been developed. The technique was used to investigate failure mechanisms during dynamic multiaxial loading of an S-2 glass fiber woven composite with 60% fiber volume fraction. Velocity profiles of the target surface were measured with a Variable Sensitivity Displacement Interferometer (VSDI) yielding normal and transverse velocity-time histories. A dynamic shear resistance of approximately 200 MPa was measured when axial stress in the range 2.5-4.2 GPa and strain rates as high as 1.57 x 10(5) s(-1) were imposed on the thin samples. Unlike metals and other traditional materials, the measured shear resistance decreases with the accumulation of shear deformation resulting from inelasticity and damage in the heterogeneous composite microstructure. The records show that the shear softening rate increases with an increase in axial stresses owing to stress-induced damage. Microscopy studies performed on recovered samples clearly show fiber breakage, matrix inelasticity, and matrix-fiber debonding as the major failure modes in these composites. Microstructural analyses revealed that at low impact velocities, in which normal stresses of about 2 GPa are attained, matrix cracking and matrix-fiber debonding are the primary damage mechanisms. At higher impact velocities, resulting in normal stresses in excess of 4 GPa, fiber microfracturing becomes significant in addition to matrix cracking and matrix-fiber debonding. These observations show that the experimentally measured dependence of the dynamic shear resistance on axial stresses is the result of induced damage and inelasticity in the composite constituents. (C) 1997 Acta Metallurgica Inc.