AN IMPROVED METHOD FOR THE DETERMINATION OF PHOTOELASTIC STRESS INTENSITY FACTORS USING THE WESTERGAARD STRESS FUNCTION

Methods for photoelastic prediction of stress intensity factors traditionally compare the maximum in-plane shear stress obtained from the observed isochromatic fringe pattern ahead of a manufactured crack with the crack-tip stresses predicted by a truncated series derived from the Westergaard stress function. These truncated equations can significantly misrepresent the stress field, even close to the crack tip, when an appreciable shear, or far-field stress component is present. For practical three-dimensional photoelastic models where the crack-tip position is ill-defined it is shown that an optimum position exists where the origin of the experimental stress field coincides with the origin of the Westergaard stress function. The conditions, and the change of optimum position with varying mode mixity are demonstrated for practical crack-like flaws. A new method, based on Sanford and Dally's Multiple Points Over-deterministic method and incorporating a fuller expansion of the crack-tip stress series due to Westergarrd and an optimum crack-tip location routine is introduced. Accuracies for practical notch type geometries are presented. Stress intensity factors for semicircular surface cracks in plates and blocks under tension and bending are calculated using the new method. © 1990.