1. Key features

1.1 Plastics and viscoelastic materials

It follows that the relationships that can be established between the stresses σ imposed on them and the resulting deformations ε involve time t. To see this for yourself, simply apply a constant uniaxial tensile load to a plastic bar, and you'll see that the deformation measured immediately after the load is applied continues to increase. Clearly, a simple, purely elastic model cannot describe this behavior.

If a rigid PVC profile is bent on itself at room temperature, its appearance changes at the point of bending: it becomes lighter in the stretched section and darker in the compressed section, but no breakage is observed.

If the end of this profile had been struck against a hard surface, part...