Determination of Mode I, Mode II and mixed mode stress intensity factors using automated polariscopes

Two automated polariscopes have been developed and are briefly described. One uses the method of phase stepping while the other uses the technique of spectral contents analysis. The use of either instrument results in full field images of the isochromatic parameter comprising of 256 by 256 pixels.

The images from both automated photoelastic systems were modified for the analysis of cracks by removing data in the region directly around the crack tip and around the edge of the crack. This was necessary to minimise interference from the crack during processing of the data and to ensure that data is not processed from the near crack tip region which is susceptible to plasticity and notch effects. Data from the singularity dominated zone were input to an algorithm, developed separately, to calculate the stress intensity factors. The method uses an adapted form of Sanford and Dally's Multiple-Point Over Deterministic Method in which Fourier analysis is used to solve the stress field equations. Various crack geometries involving a range of Mode I, Mode II and mixed-mode stress intensity factors were analysed using the polariscopes. Comparison of the resulting stress intensity factors with analytical values showed good agreement.