Residual stiffness of cracked cross-ply composite laminates under multi-axial in-plane loading

In this paper, the Equivalent Constraint Model (ECM) together with a 2-D shear
lag stress analysis approach is applied to predict residual stiffness properties of polymer and ceramic matrix [0/90n/0] cross-ply laminates subjected to in-plane biaxial loading and damaged by transverse and longitudinal matrix cracks. It is found that the longitudinal Young’s modulus, shear modulus and major Poisson’s ratio undergo large degradation as the matrix crack density increases, with Poisson’s ratio appearing to be the most affected by transverse cracking. In cross-ply laminates with thick 90° layer strip-shaped delaminations begin to initiate and grow from the tips of matrix cracks at the 0°/90° interface. These
delaminations contribute to further stiffness degradation of such laminates, and hence have to be taken into account in failure analysis models. The thickness of the 90° layer plays an important role; the thicker the 90° layer, the bigger stiffness reduction suggesting a size (volume) effect at ply level. In SiC/CAS cross-ply laminates reduction in the longitudinal modulus occurs mainly due to transverse cracks, while the shear modulus appears to be the most affected by the presence of longitudinal cracks. The shear modulus reduction ratio predicted previously by a semi-empirical formula is, in the most of cases, within 10% of the
current ECM/2-D shear lag approach value. In some cases, though, the error of the semiempirical finite element expression can be as big as 20% since it fails to capture damage mode interaction.