Effect of temperature and strain rate on the formation of shear bands in polymers under quasi-static and dynamic compressive loadings: Proposed constitutive model and numerical validation

The mechanical behaviour of polymers is strongly influenced by temperature and strain rate conditions. Their effects are clearly observed in the Young modulus and Yield strength assessment. In some cases, a change in material failure mode can be noted, which can be both ductile and brittle. For polymers exhibiting shear bands during compression loading, test conditions may enhance or diminish this effect. In this study, isotactic Polypropylene has been considered due to its strong sensitivity to test conditions; with some combinations of strain rate and temperature, the material is able to undergo large compression with nearly uniform strain distribution, whereas significant shear bands can be observed with other strain rate and temperature combinations. This paper proposes a constitutive law for the mechanical characterization of polymers that show shear banding in compression loading. The proposed law extends a visco-plastic model present in literature, allowing to predict the onset or not of micro-shear banding compression instability. The dependence of constitutive parameters with test conditions is determined by applying the Ree-Eyring equation. The model parameters have been calibrated analytically according to experimental data obtained from compression tests performed in a wide range of strain rates (10−3 to 103 1/s) and temperatures (253–313 K). In addition, the constitutive model has been implemented in a commercial FEM code by means of a user-defined subroutine. Results show a good correspondence between experimental observations and numerical calculations, regarding both stress-strain relationship and shear band formation.