Fucong Lu

Viscoelastic–Viscoplastic Cyclic Deformation of Polycarbonate Polymer: Experiment and Constitutive Model

A series of uniaxial tests (including multilevel loading–unloading recovery, creep-recovery, and cyclic tension–compression/tension ones) were performed to investigate the monotonic and cyclic viscoelastic–viscoplastic deformations of polycarbonate (PC) polymer at room temperature. The results show that the PC exhibits strong nonlinearity and rate-dependence, and obvious ratchetting occurs during the stress-controlled cyclic tension–compression/tension tests with nonzero mean stress, which comes from both the viscoelasticity and viscoplasticity of the PC. Based on the experimental observation, a nonlinear viscoelastic–viscoplastic cyclic constitutive model is then constructed. The viscoelastic part of the proposed model is constructed by extending the Schaperys nonlinear viscoelastic model, and the viscoplastic one is established by adopting the Ohno–Abdel-Karims nonlinear kinematic hardening rule to describe the accumulation of irrecoverable viscoplastic strain produced during cyclic loading. Furthermore, the dependence of elastic compliance of the PC on the accumulated viscoplastic strain is considered. Finally, the capability of the proposed model is verified by comparing the predicted results with the corresponding experimental ones of the PC. It is shown that the proposed model provides reasonable predictions to the various deformation characteristics of the PC presented in the multilevel loading–unloading recovery, creep-recovery, and cyclic tension–compression/tension tests.

A new form of equivalent stress for combined axial–torsional loading considering the tension–compression asymmetry of polymeric materials

Although the von Mises criterion has been generally adopted to obtain the equivalent stress for materials under multiaxial loading, it does not consider the influence of the tension–compression asymmetry of pressure-sensitive materials. For polymeric materials, to include the non-negligible effect of the tension–compression asymmetry, this work proposes a new form of equivalent stress in which the tensile and compressive yield strengths are presented. This form is compared with the work of von Mises, Bai and Christensen with the support of experimental data. It was found that the proposed form is more suitable for polymeric materials under combined axial–torsional loading conditions. The implications of the present findings for designing stress-controlled combined axial–torsional loading experiments are also discussed.

Uniaxial cyclic deformation and internal heat production of ultra-high molecular weight polyethylene

The cyclic deformation and corresponding internal heat production of ultra-high molecular weight polyethylene (UHMWPE) polymer were investigated under the uniaxial strain-controlled and stress-controlled cyclic loading conditions. It is seen that the UHMWPE behaves basically a cyclic stabilizing feature since the responding stress amplitude does not remarkably change during the cyclic loading, except for that at high strain rate (where an obvious cyclic softening is caused partially by the thermal softening); an apparent mean stress relaxation occurs in the asymmetrical strain-controlled cyclic tests, and the degree of mean stress relaxation increases with the increasing mean strain; an obvious ratchetting takes place in the asymmetrical stress-controlled cyclic tests, and the ratchetting strain depends greatly upon the applied mean stress and stress amplitude, as well as the prescribed stress rate. Moreover, it is found that the temperature on the surface of specimen increases apparently in the uniaxial strain-controlled cyclic tests and the temperature variation becomes more remarkable when the prescribed strain rate is higher. However, the temperature variation is not so apparent in the uniaxial stress-controlled cyclic tests due to much smaller responding strain amplitude.