Failure Criteria for Polymeric Solids
MONSANTO RESEARCH CORP ST LOUIS MO
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A theory for predicting the stress-strain characteristics of polymeric solids developed in terms of a description of microdefect formation. The process of irreversible change in these solids is assumed to be a combination of nucleation of submicroscopic defects at stress inhomogeneities and their subsequent growth to macroscopic dimensions. Straining results in the simultaneous generation of free volume and macrodefects so that catastrophic failure can occur through either a general yielding of the material or by brittle fracture. It is assumed that nucleation of sub-microscopic defects is an activated process and that defect growth is one dimensional and linear. The total strain is expressed as the sum of an elastic recoverable strain and a nonlinear, nonrecoverable strain and expressions are obtained for the stress as a function of time, temperature and loading history. The criterion for yielding is defined in terms of free volume concepts and a criterion for brittle failure is defined in terms of a critical defect size. The parameters of the resulting model are calculated for polyphenylene oxide polymers based on constant rate of loading experiments and then the general creep behavior,including the time required under constant load for cold flow, is predicted. Experimental data are shown to agree with these predictions.