Voiding mechanisms in semi-crystalline polyamide 6 during creep tests assessed by damage based constitutive relationships and finite elements calculations

Notched round bars made of a semi-crystalline polymer, polyamide 6, were submitted to creep tests. In order to study the microstructural evolution of the material during creep deformation, these tests were stopped at two characteristic creep times: i) the end of the secondary creep stage, ii) the onset of the failure during the tertiary creep stage, allowing 3D through thickness Synchrotron Radiation Tomography (SRT)-inspections. This SRT-technique allowed features of the damage to be assessed at a micrometre resolution within the notched region, such as spatial distributions of void volume fraction and void heights and diameters. These deformation mechanisms have already been observed and studied during steady strain rate loading and a multi-mechanism model coupled with damage formulation proposed to simulate notched specimens submitted to tensile loads. This model, based on porous visco-plasticity, was used here to simulate the creep behaviour of this PA6 material. The optimization procedure has led to a set of material coefficients capable of reproducing both macroscopic behaviour (creep curves) and damage micro-mechanisms. The level of maximum damage within the material and the spatial distributions of void volume fraction obtained numerically were compared successfully to experiments and the Cauchy stress tensor has been proved to be related to the void deformation mechanisms.