Impact compressive failure of a unidirectional carbon/epoxy laminated composite in three principal material directions

Abstract
The impact compressive failure behavior of a unidirectional T700/2521 carbon/epoxy laminated composite in three principal material directions or fiber (1-), in-plane transverse (2-) and through-thickness (3-) directions is investigated on the conventional split Hopkinson pressure bar (SHPB). Cubic and rectangular block specimens with the same square cross section are machined from an about 10 mm thick composite laminate. The uniaxial compressive stress-strain curves up to failure at quasi-static and intermediate strain rates are measured on an Instron testing machine. A pair of steel rings are attached at both ends of the rectangular block specimens to prevent end-crushing or brooming only in the 1-direction tests, except at high strain rates. A modified sleeved SHPB is applied to allow recovery of the failed specimen without being subjected to repeated compressive loading during the test. It is shown that the ultimate compressive strength (or failure stress) exhibits no strain-rate effect in the 1-direction, but a slight strain-rate effect in the 2- and 3-direction over a strain-rate range of 10⁻³ to 10³/s. It is also shown that the ultimate compressive strain (or failure strain) decreases marginally with strain rate in all three directions. Dominant failure mechanisms are found to significantly vary with strain rate and loading directions along the three principal material axes.