| Popis: |
In this study, a biaxial testing machine is used to stretch polypropylene sheet at elevated temperatures (150–160° C) in a number of modes, including simultaneous equibiaxial and a sequential mode that consists of planar extension followed by a second perpendicular extension to achieve a final equibiaxial state. On completion of the stretching process the applied forces continue to be monitored to give further data on stress relaxation. A constitutive model is devised that combines the Eyring process and large strain elastic theory. A Guiu‐Pratt model, including a single Eyring process, is fitted to the stress relaxation curves and produces values of Eyring parameters. Fits are improved if the initial part of the curve is excluded, indicating that the activation volume derived relates to a process in the relatively long term. In support of this, the strain rate dependence of the loading curves indicates a quite different value of activation volume. A two‐process model is derived with parameter values dedu... |
