Mechanism-based failure laws for biaxially compressed IM7/8551-7 graphite-epoxy laminates

Failure mechanisms and stress-strain behaviors have been investigated for [+/- 15](6s), [+/- 30](12s), and [+/- 45](12s) graphite-epoxy (IM7/8551-7) laminates under in-plane biaxial compression using a cruciform biaxial test frame and microscopic examination of the load-interrupted samples. The loading confinement ratio R is varied from 0 to nearly 1.0 to measure the sensitivity of sample failure mechanisms and stress-strain behaviors to different stress states. The failure modes for each orientation are found to be strongly confinement-ratio-dependent. For R=0, all the samples failed by in-plane shearing on a plane aligned with the fiber direction. As the confinement ratio is increased, the in-plane failure mode in [ 30]12s, and [+/- 45](12s) samples transitioned to out-of-plane shearing at R=0.25 and 0.5, and multiple delamination at R = 0.75 and 1.0. For the [+/- 15](6s) samples, the failure modes transition to fiber buckling at R=0.25 and then out-of-plane shearing at R=0.5, 0.75, and 1.0. Microscopic examination of the load-interrupted samples confirms the source of the individual delaminations to be the wing cracks associated with the angled shear cracks in the out-of-plane direction. The classical laminate theory in combination with the standard rule of mixture is used to calculate the maximum shear stress in the matrix and the fibers. The failure data conform well with a Mohr-Coulomb-type shear failure law when applied to the fibers but not to the matrix. Thus, fiber shearing appears to be the failure-setting mechanism in this material under all material orientations and biaxiality ratios.