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.

The linear viscoelastic limit of poly(methyl methacrylate) (PMMA) is established on the basis of the Reiner‐Weissenberg criterion of strength. The obtained limit is not simply a “strainlimit” or a “stress‐limit” but a function of both strain and stress, an energetical limit. In stress‐strain representation the limit is independent of time and temperature, which means that the linear or non‐linear behavior of the material depends only on the actual values of strain and stress. Some experimental data obtained on PMMA confirm the theory.

Creep experiments carried out on transparent and translucent polymers permit the observation of irreversible material damage in the form of crazes or microcracks. The formation of such damage is strongly dependent on the applied stress, temperature and environmental conditions. The first appearance of observable material damage seems to be explicable by an energy criterion. The energy supplied by the external load can be divided into conserved and dissipated parts, each of them causing volume (isotropic) and shape (deviatoric) changes. The different parts of the energy can be computed if the creep‐compliance is approximated by a Prony‐Dirichlet series with a finite number of terms. Computations carried out for experiments with air as the environmental medium under isothermal conditions show the dependence between the appearance of first visible material damage (crazes or microcracks) and the conserved energy.

Three aspects of the failure of thermoplastics, having a special importance in engineering, are investigated. They are: (a) Transition from linear to nonlinear viscoelasticity; (b) Crazing; (c) Fracture. Energy related criteria, developed from the Reiner‐Weissenberg thermodynamical theory of strength, are used for the characterization and prediction of failure under its different forms, for simple uniaxial loading histories such as creep, stress relaxation and constant rate of strain. The computation of the stored and dissipated parts of the specific stress energy becomes possible in a relatively simple way, if the relaxation modulus and the creep compliance are approximated by Prony‐Dirichlet‐type series with a finite number of terms. Published experimental data, as well as experiments carried out by the author on different thermoplastics are in very good agreement with theoretical results. Further, based on experimental data, the equations obtained can be reduced to very simple and useful relations. The influence of elevated temperatures (below the glassy‐transition point) on failure is also considered.

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