Damage Accumulation During Stress Relaxation of Polymer Films in Bending

A nonlinear viscoelastic damage accumulation model with a physical approach is presented with a view to describing stress relaxation in amorphous and partially crystalline polymers. The deformation is represented as stress-aided transition through a series of symmetrical energy barriers. There is also a stress threshold, introduced by the authors and representing an asymptotic residual stress below which relaxation does not occur. The model comprises three parameters: an activation volume, V; a product of several parameters with physical meaning, A (only the combination of A and V in the form of the relaxation time constant can be reconstructed from the present set of experiments); and the relaxation threshold. It allows a practically unlimited number of energy barriers. The damage depends on the stress history. Experimental data were collected on stress relaxation of three polymers at different temperatures. The experiments involved bending of a polymer strip within a metal cylinder and measurement of the radius of curvature after removal of the strip from the cylinder, using a specially-developed method. The measured radii ranged from infinity to 1.5 cm, and the obtained data were fitted to the model. The results show that the residual stress and other model parameters are temperature-dependent.