Critical Strain: The Effect of Processing History and Associated Factors

Abstract

Since the mechanical properties of plastics components are greatly influenced by the processing history it is scarcely surprising that the lower limit of irreversible damage which is caused by a stress just sufficient to produce a critical strain is also highly dependent upon the previous processing history. The two examples given in Chapter 17 show the results obtained when testing in air, as well as those obtained after immersion in a specific aggressive medium. Tests in aggressive media provide an instantaneous indication of the changes induced during melt processing, and these changes may be expressed in terms of critical strain. This implies that the critical strain is not a unique value, but one which depends on a number of factors. Seen in this light a test in an aggressive medium is not merely the visible consequence of the presence of internal stresses, but also a strength determination in its own right. Having once obtained standard reference data on the behaviour of an isotropic specimen in an aggressive medium it immediately becomes possible to assess the degree of anisotropy and the presence of inhomogeneities generally. No longer do we look upon critical strain as a material-specific parameter based upon some hypothetical intrinsic structure; instead we begin to analyse defects and localised weaknesses in Griffiths’ sense. In fact, we start to determine flaws in the bulk structure as a function of the variables involved in melt processing and as a function of the variables involved in the afterprocessing of glassy polymers.