On the Effect of Temperature on the Creep Behavior of Neat and Carbon Fiber Reinforced PEEK and Epoxy Resin

Abstract

Schapery's nonlinear formulation for viscoelastic materials has been successfully used by many investigators at room temperature [1-3]. Little work has been presented in the literature where the above approach is applied to viscoelastic materials at elevated temperature. In the present work, Schapery's constitutive equation is used to study the nonlinear viscoelastic creep response of neat and carbon fiber-reinforced Polyether-etherketone (PEEK) and epoxy resin at different temperatures. As reinforced materials, the laminates [904] s and [±454] s were investigated. Series of 10-hour isothermal tensile creep tests were conducted on each laminate at four temperatures (up to 140°C for the epoxy system and up to 120°C for the PEEK system) and different stress levels. For comparison reasons the same type of experiments was conducted on the respective neat polymers. Schapery's approach was used to characterize the nonlinear viscoelastic response of the above materials. The stress and temperature dependence of the nonlinearity factors was evaluated using a numerical procedure based on least squares techniques. The results show that the linear viscoelastic limit is shifted to lower values with increasing temperature. This was observed for both neat polymers as well as for the [±454] s laminates investigated. On the other hand, for the [904] s laminates the influence of the temperature on the linear viscoelastic limit seems to be relatively restricted. Moreover, for all resins and laminates studied it is shown that the influence of temperature on the nonlinearity of the instantaneous material response is significantly lower than that on the transient nonlinearity. For the investigated temperature range it can therefore be assumed that the instantaneous creep response is linear and independent of temperature over a stress range relevant in practical applications. On the other hand, the influence of temperature on the transient creep response was found to be nonlinear. The transient creep response of the composite materials subjected to intralaminar shear stress showed higher temperature sensitivity than that under normal stress.