A micromechanical approach to time-dependent failure in composite systems

The time-dependent failure behaviour of off-axis loaded composites is investigated, assuming that fracture is matrix dominated. Since the stress and strain state of the matrix in composite structures is complex, the yield and fracture behaviour of a neat epoxy system is investigated under various multi-axial loading conditions. A good description of the multi-axial yielding behaviour of the matrix material is obtained with the 3-dimensional pressure modified Eyring equation. The parameters of this 3-dimensional yield expression are implemented into a constitutive model, which has been shown to describe the deformation behaviour of polymers under complex loadings correctly. By means of a micromechanical approach, the matrix dominated transverse strength of a unidirectional composite material was investigated. Numerical simulations show that a failure criterion based on maximum strain provides a good description for the rate dependent transverse strength of unidirectional glass/epoxy composites. Furthermore, such a strain criterion is also able to describe the durability (creep) of off-axis loaded unidirectional composites.