A micromechanical approach to time-dependent failure in composite systems

Models for the prediction of strength of composite materials are generally directed to short-term failure using classical lamination theory, coupled with common failure theories such as maximum stress and strain criteria. The input data for these macromechanical models is derived from standardized tests on unidirectional laminates and are typically low strain rate or quasi-static experiments. It is well-known, however, that the deformation and failure behaviour of polymers is strongly influenced by the time-scale of the experiment, and it is therefore eminent that composites based on these materials will also display a large influence of the time-scale, especially in off-axis loading situations where the properties are strongly governed by the viscoelastic polymer matrix. With respect to durability this implies that the mere fact that a composite material is loaded well below the critical stress, as determined in a short-term test, will not ensure that the material will sustain this load for an infinite period of time. It is clear that there is a need for the development of new approaches for the prediction of failure of composites which enable us to include time-variable circumstances. In case of matrix dominated failure-modes, such as transverse and shear failure, it seems therefore obvious to include the time- and stress-dependent failure of the polymer matrix. In the present investigation the time-dependent fracture behaviour of transversely loaded E-glass/epoxy laminates is studied using a micromechanical approach. To account for the complex stress and strain situation in a composite, micromechanical simulations are performed with the Finite Element Method (FEM), using the so-called compressible Leonov model (Tervoort, 1996), a constitutive equation that is able to capture the rate-dependent yield characteristics of the epoxy matrix. The micromechanical model is subsequently employed to evaluate local failure in unidirectional composites in 10, 45 and 90 degrees off-axis loadings in constant strain rate and constant stress (creep) experiments.