Rate dependent mechanical behavior of high strength plastics: Experiment and modeling

The Viscoplasticity Theory based on Overstress (VBO) has been used successfully in modeling the rate dependent behavior of several metals and alloys. However, its modeling capabilities with plastics has not been as widely explored. The intent of the work here has been to study the homogeneous, inelastic deformation behavior of polymers and to investigate the possibility of using VBO to model the observed behavior. As a result, tests were conducted on polymers specifically in order to compare various aspects of the real material behavior to the predicted behavior of the model. Nylon 66 was selected as the primary material to study, with additional experiments being conducted on Poly (ether ether ketone) (PEEK) and Polyetherimide mainly for purposes of comparison. Tests were conducted in both stress and strain control. Strain rates ranged from ;One of the more interesting results that has been found for all three polymers is the increase in stress magnitude during repeated short term relaxation tests along the unloading curve. It has also been found that the unloading behavior appears to depend not only on the strain rate but also on the prior history. In addition, the strain recovered at zero stress increases with the unloading strain rate and with increasing strain at the start of the recovery period. Other interesting results for the Nylon 66 include overshoots or undershoots in the stress after relaxation periods or rate changes. These transients have also been found to be quite pronounced along the unloading curve.;Modeling of the observed behavior was conducted with the VBO model. The base theory was modified to include many of the aspects of mechanical behavior observed in the polymers. Comparisons of the simulations with experiments show a good correlation for strain control. However, the model develops some instabilities when simulations are conducted in stress control.