Modeling viscoelasticity and cyclic creep of PEEK by parallel rheological framework (PRF)

Abstract

Although creep is usually regarded as a quasistatic phenomenon, in multiple engineering applications the load is not constant, hence creep under cyclic loading conditions should be considered. The current work presents the theoretical and experimental study on the viscoelasticity and cyclic creep of PEEK (Polyether ether ketone) – the polymer with good chemical and thermal stability. Among other demanding applications, PEEK is used in bearings industry for the fabrication of cages, which main function is to isolate and to guide rolling elements. In this application the polymer cage is subjected to the long-term load alternating in time, meaning that classical creep (under constant load) cannot be presumed. In the current study, the non-linear viscosity is included into the generalized Maxwell model, employing the Eyring equations which postulate that the viscosity coefficient is not constant but is dependent on stress. The model is implemented with the Parallel Rheological Framework (PRF) (the numerical technique recently implemented in the commercial software ABAQUS), used to construct the generalized Maxwell model with hyper-elastic springs and dashpots of non-linear viscosity. Initially, the viscoelastic material parameters are identified from the uniaxial test, and later these parameters are used to simulate the PEEK response under cyclic loading, to predict the time to creep/fatigue failure. Eventually, the numerical predictions are compared to the test results. The comparison indicates that the developed method and its numerical implementation by PRF can be successfully used to model the non-linear viscoelastic behavior of PEEK under uniaxial load. It is also found, that the simulation of cyclic creep can be reasonably performed by using the current material model and PRF.