INELASTIC BEHAVIOR OF FIBER COMPOSITES SUBJECTED TO OUT-OF-PLANE HIGH-STRAIN RATE SHEARING

A new pressure-shear recovery experiment for investigating out-of-plan e dynamic shear resistance of composites has been developed. The techn ique was used to investigate failure mechanisms during dynamic multiax ial loading of an S-2 glass fiber woven composite with 60% fiber volum e fraction. Velocity profiles of the target surface were measured with a Variable Sensitivity Displacement Interferometer (VSDI) yielding no rmal and transverse velocity-time histories. A dynamic shear resistanc e 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 impos ed on the thin samples. Unlike metals and other traditional materials, the measured shear resistance decreases with the accumulation of shea r deformation resulting from inelasticity and damage in the heterogene ous composite microstructure. The records show that the shear softenin g rate increases with an increase in axial stresses owing to stress-in duced damage. Microscopy studies performed on recovered samples clearl y show fiber breakage, matrix inelasticity, and matrix-fiber debonding as the major failure modes in these composites. Microstructural analy ses revealed that at low impact velocities, in which normal stresses o f about 2 GPa are attained, matrix cracking and matrix-fiber debonding are the primary damage mechanisms. At higher impact velocities, resul ting in normal stresses in excess of 4 GPa, fiber microfracturing beco mes significant in addition to matrix cracking and matrix-fiber debond ing. These observations show that the experimentally measured dependen ce of the dynamic shear resistance on axial stresses is the result of induced damage and inelasticity in the composite constituents. (C) 199 7 Acta Metallurgica Inc.