Uniaxial and in-plane biaxial testing were conducted on cross-ply graphite/epoxy laminates with different orientations. Results from these investigations are presented here only for the [±45]12s samples under different stress states. The stress-strain behavior is related to microscopic events through micromechanics-based modeling. It is shown that crack nucleation starts in the matrix when the local shear stress exceeds the shear strength of the epoxy. Depending on the stress state in the plies, two types of cracks form: (i) transverse cracks and (ii) angled out-of-plane cracks. While transverse cracks slide and allow the sample to accommodate large strains, angled cracks lead to massive delamination and an overall brittle behavior.

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