Matrix dominated, time-dependent failure of continuous fiber-reinforced thermoplastic composites

Abstract

Thermoplastic composites exhibit strong time-dependent behavior in the matrix-dominated transverse and off-axis loading: strength depends on the applied strain rate and they are highly prone to creep and fatigue failure. Hence, it is of utmost importance to predict their time-dependent behavior to use them effectively in engineering applications. Accurate prediction of the time-dependent behavior requires the identification of the failure mechanisms. Neat thermoplastics, which play a significant role in the transverse and off-axis failure of composites, exhibit plasticity-controlled failure at high stresses and low failure times and crack growth-controlled failure at low stresses and long times-to-failure. Although crack growth controlled nature of the failure is widely mentioned for the continuous fiber-reinforced composites, plasticity-controlled failure had not yet been identified. Hence, this thesis aims to identify the role of plasticity-controlled failure in the matrix-dominated, time-dependent failure of continuous fiber-reinforced composites, enabling accurate prediction of their time-dependent behavior. Firstly, we identify the role of plasticity-controlled failure mechanism in several unidirectional (UD) material systems, glass/iPP, carbon/PEEK, and carbon/PEKK, loaded in the transverse direction at room temperature. In further studies, we investigate the time-dependent behavior of glass/iPP in more detail by studying the effect of elevated temperatures and processing. Later, the relation between the time-dependent behavior of the neat matrix and the transversely loaded unidirectional composite is studied both experimentally and numerically with a micromechanical finite element model. In the last scientific chapter, the effect of the off-axis angle on the time-dependent behavior is identified and an anisotropic, viscoelastic-viscoplastic constitutive model is applied to predict the off-axis UD stress-strain response. Plasticity is found to play a crucial role for glass/iPP, being the only mechanism observed over the whole experimental time range, temperatures, and off-axis angles investigated. The time-dependent behavior of the neat matrix and the composite are found to be related: similar to neat iPP, glass/iPP is shown to exhibit multi-process deformation. Lifetime is predicted well by an analytical method using the Ree-Eyring approach and the concept of critical strain. It is found that the constitutive model applied successfully describes the off-axis UD stress-strain response for a large range of off-axis angles.