Shear yielding threshold and viscoelasticity in an amorphous glassy polymer: a study on a styrene–acrylonitrile copolymer

The framework of the present research is the study of the effects of viscoelasticity on the yield behaviour of amorphous glassy polymers. The research regards the shear yielding mechanism in an amorphous glassy polymer and the identification of a yield criterion, which takes into account the effect of mechanical history on the yield point (defined as the onset of plastic strain), is set as its final goal. To this end, constant strain-rate, creep and stress relaxation tests were carried out at room temperature under uniaxial compression conditions on a styrene–acrylonitrile copolymer. The results obtained emphasise that yield onset cannot be identified by a critical value of the applied stress or strain. Referring to models of the plastic deformation mechanism previously proposed in the literature, the anelastic strain, the anelastically stored energy and the viscoelastic energy were suggested to reach a threshold value at the yield point. All three parameters were evaluated at the yield threshold and showed fairly constant values irrespective of the mechanical history. Nevertheless, the data dispersion and the simplicity of the analogical model used to estimate these quantities did not allow to identify which of them actually controls the yield process.