Numerical modeling of uneven thermoplastic polymers behaviour using experimental stress-strain data and pressure dependent von Mises yield criteria to improve design practices

Classical plasticity theories and yield criteria for ductile materials, such as Tresca and von Mises original formulations predict that yielding is independent on the hydrostatic stress state (pressure), which means that tensile and compressive stress-strain behaviours are considered equal and are equally treated. This approach is reasonable for ductile metallic materials but sometimes inaccurate for polymers, which commonly present larger compressive yield strength, therefore being characterized as uneven. Polymer unevenness can be of great interest for mechanical structural design since components that present regions operating under compression can be optimized taking this phenomenon into account. Compressive stress-strain data for polymers are very scarce in the literature, and as a step in this direction this work presents three key-activities: i) four selected polymers were tested under tension and compression to identify unevenness and assess its levels; ii) pressure dependent yield criteria applicable for polymers were briefly reviewed; iii) experimental results were incorporated in adapted design practices using modified yield criteria implemented in optimization and finite element computations. Results show that taking unevenness into account and implementing modified criteria in the numerical techniques for structural calculation can provide mass reductions up to similar to 38% even with simple geometric changes, while keeping original safety and stiffness levels. (C) 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of ICM11