Plasticity-controlled failure of fibre-reinforced thermoplastics

Flow-induced fibre orientation generally causes anisotropy in the mechanical response of short fibre reinforced thermoplastics. In this manuscript, this anisotropy is studied for a wide selection of fibre-reinforced thermoplastics, focusing on the strain-rate dependence of the strength, and its relation to the stress-dependence of the lifespan under static load (creep rupture). It is demonstrated that, for short- as well as long-fibre reinforced thermoplastics, the influence of fibre-orientation and applied strain-rate on the tensile strength can be multiplicatively decomposed; the response is factorizable in load-angle and strain-rate. This factorizability appears to be generic to fibre-reinforced systems, since it is observed regardless of fibre type, fibre length, fibre weight fraction, matrix type and level of interfacial fibre–matrix interaction. The apparent factorisation of fibre-orientation and strain-rate dependence of the fracture stress opens up a possibility to considerably reduce experimental efforts required for composite characterisation. Subsequently, an anisotropic viscoplastic model previously developed by van Erp et al. (2009), is analysed for its capability describe the observed factorizability. This model is expressed in a form of an associated flow rule, which combines the Eyring flow equation, required to describe the strain rate dependence at a reference orientation, with the Hill equivalent stress formulation to capture the load-angle dependence. The model not only describes the load-angle and strain-rate dependence of the tensile strength accurately, but, in combination with a critical strain concept, it also provides accurate predictions of the creep lifetime for different loading angles.