Certifying the intrinsic character of a constitutive law for semicrystalline polymers: a probation test

Abstract

A study of methodological nature demonstrates the efficiency of a probation test allowing for the intrinsic character of a rheological constitutive law to be assessed. Such a law is considered here for semicrystalline polymers exhibiting necking and for large deformation. In the framework of a law of behavior of $( \dot{\sigma },\sigma , \dot{\varepsilon },\varepsilon )$, tensile experiments conducted at an imposed constant strain rate $\dot{\varepsilon }_{0}$ bring true stress responses, from which the constitutive (material) parameters can be identified from model-based metrology concepts. The same experiment repeated at various strain rates gives an access to the dependence of the nonelastic parameters on the strain rate. Then the intrinsic law is tested severely by considering a new set of experiments carried out for constant displacement rates of the grips. In that case the specimens show local strain rates that evolve strongly during the test (by a factor of 5–10). The parameter identification process requires the introduction of the exact realized input strain and strain-rate command into the model. Accounting for strain rate dependency additionally requires the knowledge of the preliminary identified strain rate dependence of the nonelastic constitutive parameters for good predictions of the experimental response directly. This is what is proven here. The conclusion speaks in favor of a possible upgrade of international standards for the mechanical characterization of polymers based on constant strain-rate tensile tests and properly applied model-based metrology.