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

Methods for dynamic characterization of composite materials were extended a nd applied to the study of strain rate effects under transverse compression as well as shear. Falling weight impact and Split Hopkinson Pressure Bar s ystems were developed for dynamic characterization of composite materials i n compression and shear at strain rates up to 1800 s(-1). Strain rates belo w 10 s(-1) were generated using a servohydraulic testing machine. Strain ra tes between 10 s(-1) and 300 s(-1) were generated using the drop tower appa ratus. Strain rates above 500 s(-1) were generated using the Spilt Hopkinso n Pressure Bar. Seventy-two and forty-eight ply unidirectional carbon/epoxy laminates (IM6G /3501-6) loaded in the transverse direction were characterized. Off-axis (1 5 degrees, 30 degrees, 45 degrees and 60 degrees) compression tests of the same unidirectional material were also conducted to obtain the inplane shea r 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, w hich are governed by the matrix, show an increase in modulus and strength o ver the static values but no significant change in ultimate strain. The str ess-strain curve stiffens as the strain rate increases. This stiffening beh avior 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 st rain rate of 1800 s(-1). The shear stress-strain behavior, which is also ma trix-dominated, shows high nonlinearity with a plateau region at a stress l evel that increases significantly as the strain rate increases.