Strain rate effects on the transverse compressive and shear behavior of unidirectional composites

Methods for dynamic characterization of composite materials were extended and applied to the study of strain rate effects under transverse compression as well as shear. Falling weight impact and Split Hopkinson Pressure Bar systems were developed for dynamic characterization of composite materials in compression and shear at strain rates up to 1800 s(-1). Strain rates below 10 s(-1) were generated using a servohydraulic testing machine. Strain rates between 10 s(-1) and 300 s(-1) were generated using the drop tower apparatus. Strain rates above 500 s(-1) were generated using the Spilt Hopkinson Pressure Bar. Seventy-two and forty-eight ply unidirectional carbon/epoxy laminates (IM6G/3501-6) loaded in the transverse direction were characterized. Off-axis (15 degrees, 30 degrees, 45 degrees and 60 degrees) compression tests of the same unidirectional material were also conducted to obtain the inplane shear stress-strain behavior. Strain rates over a wide range, from 10(-4) s(-1) (quasistatic) up to 1800 s(-1), were recorded. The 90-degree properties, which are governed by the matrix, show an increase in modulus and strength over the static values but no significant change in ultimate strain. The stress-strain curve stiffens as the strain rate increases. This stiffening behavior is very significant in the nonlinear region for strain rates between 10(-4) s(-1) and 1 s(-1). For strain rates above 1 s(-1), the stress-strain behavior continues this stiffening trend until it is almost linear at a strain rate of 1800 s(-1). The shear stress-strain behavior, which is also matrix-dominated, shows high nonlinearity with a plateau region at a stress level that increases significantly as the strain rate increases.