A micromechanical. approach to time-dependent failure in off-axis loaded polymer composites

The time-dependent failure behaviour of off-axis loaded composites is inves tigated, assuming that fracture is matrix dominated. Since the stress and s train state of the matrix in composite structures is complex, the yield and fracture behaviour of a neat epoxy system is investigated under various mu lti-axial loading conditions. A good description of the multi-axial yieldin g behaviour of the matrix material is obtained with the three-dimensional ( 3D) pressure-modified Eyring equation. The parameters of this 3D yield expr ession are implemented into a constitutive model, which has been shown to d escribe the deformation behaviour of polymers under complex loading correct ly. By means of a micromechanical approach, the matrix dominated off-axis s trength 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 off-axis strength of unidirection al glass/epoxy composites. Furthermore, such a strain criterion is also abl e to describe the durability (creep) of off-axis loaded unidirectional comp osites. (C) 2001 Elsevier Science Ltd. All rights reserved.