Dependence of the tensile strength of pitch-based carbon and para-aramid fibres on the rate of strain

Understanding the rate dependencies of the tensile strength of reinforcing fibres is a key for the understanding of the rate dependencies of the properties of the corresponding composite materials. Hence, in this study it is attempted to clarify the mechanical responses of aramid and carbon fibres at different rates of strain in the light of our previous observations of strain rate dependence of the corresponding hybrid composites under both static and fatigue flexural conditions. In addition, it is attempted to correlate the rate sensitivity with the degree of structural order in the fibres. The study is carried out with low-, medium- and high-modulus pitch based carbon fibres and with Kevlar 29, 49 and 149 para-aramid fibres, whose strengths were tested at strain rates ranging between 0.004 to 2.0% s-1. It is shown that the strength results of the two fibre families follow the Weibull distribution at all strain rates studied. In the case of the carbon fibres two different régimes are observed for the scale parameter as a function of strain rate. At low strain rates the scale parameter increases slowly with the rate, whereas a strong decrease is observed at higher strain rates. This trend becomes more evident as the crystallinity of the fibre increases. The low strain rate behaviour is governed by the power law breakdown rule model, whereas the high strain rate behaviour is accounted for by the rate of growth of a sharp inter-crystallite flaw. In the case of the aramid fibres the scale parameter is insensitive to the strain rate, which supposedly results from a situation where fracture in these fibres does not necessarily involve an activation volume controlled mechanism.