A damage based model for crack initiation in unidirectional composites under multiaxial cyclic loading

In the present work a criterion for the non-fibre (i.e. matrix or fibre–matrix interface) – controlled fatigue behaviour of composite unidirectional laminae subjected to multiaxial loading is proposed. The criterion is based on experimental observations of the damage modes occurring at the microscopic scale leading to the fatigue failure of a lamina. In particular two parameters have been identified as representative of the driving forces for the damage initiation, and they have to be used for design purposes depending on the local multiaxial condition. These parameters, i.e. the local maximum principal stress and local hydrostatic stress, are calculated from the local stresses in the matrix, obtained from Finite Element analyses of a fibre/matrix unit cell subjected to periodic boundary conditions. The application of the proposed criterion to fatigue data taken from the literature shows that only two scatter bands can be used to completely describe the fatigue response of unidirectional laminae under generic multiaxial stress states.