Shear Modulus Degradation in Fiber Reinforced Laminates

Matrix damage, involving transverse and shear cracks, is a common failure mode for composite structures, yet little is known concerning their interaction. A modified Iosipescu coupon is proposed to study the evolution of the shear and transverse damage and their mutual effects. The layup and coupon geometry were selected in a way that controls the severity of the damage and allows the measurement of shear and transverse stiffness degradation directly. The results were compared to material degradation models where damage was dominated by matrix failure. While positive agreement was generally observed in the transverse direction, no model was able to predict the observed shear damage. A new elasticity solution was, therefore, proposed for the shear stress-strain field of a transversely cracked laminate. The approach used a classical shear lag theory with friction applied to the crack surfaces. Using the constitutive relations, the shear modulus reduction was found as a function of crack density, and showed good agreement with experimental measures.