Origins and measurements of internal stress in plastics

The term ‘internal stress’ is given various meanings in the literature and these are listed and discussed. The origin of deformation-induced internal stress is considered, and the development of residual (moulding) stresses in a number of polymer processing operations is described. The deformation-induced internal stress can be investigated by stress relaxation procedures, and has been found to be sensitive to the state of ageing or annealing, although no quantitative interpretation has been developed to date. The layer removal process for determining residual stress distribution is described in some detail and examples of applications are presented, including a full biaxial treatment. The results of this technique are quite revealing and some of the practical implications are indicated.