- Cracks in composite laminates are characterized by a set of vectors, each representing an individual cracking mode. The vector components are taken as internal variables in the elastic strain energy function and the elastic constitutive equations are derived for the inplane loading condition. For low concentration of cracks in laminates, the residual stiffness properties are related to the initial elastic constants and the magnitude of the damage vectors. These equations are then used to predict stiffness reductions of composite laminates from the observed crack densities. Very good agreement with the experimental values is found.