Constitutive model for thermoplastics with structural applications

A constitutive model for thermoplastics is described, which allows for hyperelastic-viscoplastic response due to intermolecular resistance and entropic hyperelastic response due to re-orientation of molecular chains. The Raghava yield function is introduced to capture the pressure-dependent behaviour, and a non-associative viscoplastic flow potential is assumed for volumetric plastic strain control. The strain-rate effects are formulated in a format suitable for structural applications. Assuming isothermal conditions, the material model requires 10 parameters which are easy to identify with uniaxial tensile and compressive tests. The constitutive model developed herein captures important features observed in polymers’ behaviour, e.g. pressure dependency, volumetric plastic strain and strain-rate sensitivity. A detailed description of the constitutive model with numerical verification and parameter identification procedures is provided. Numerical simulations of three-point bending of beams and centrally loaded plates, all made of a polypropylene material, indicate that the constitutive model is able to predict reasonably well the load-carrying capacity observed in the tests.