Relation Between Crack Velocity and the Stress Intensity Factor in Birefringent Polymers

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Abstract
An experimental investigation of the dynamic behavior of crack propagation in two birefringent polymers, Homalite-100 and KTE epoxy, is described. Photoelastic isochromatic fringe patterns associated with cracks propagating in center-pin-loaded, eccentric-pin-loaded, and crack-line-loaded single-edged-notched (SEN) specimens were recorded with a high-speed multiple spark camera. Experimental data were obtained in several tests for each polymer to cover cracks propagating from arrest to terminal velocity. The size and shape of the isochromatic fringe loops were used to determine the instantaneous values of the stress intensity function K by matching analytical and experimental results. Analytical results for the fringe loop were obtained by employing a Westergaard stress function of the form Z(z)=K2πz[1+β′(z∕a)] together with a superimposed σox=α′∕2πa. Approximately 14 000 computer generated fringe loops were compared to the experimentally generated results, and values of the fitting parameters K, α′, and β′ were obtained. Results obtained for ˙a versus K show an inverted L relationship for both polymers. Crack propagation behavior with respect to the shape of ˙a versus K curve is discussed.