Study of Mixed Mode stress intensity factors by two- and three-dimensional photoelasticity

An experimental study was performed to investigate the stress intensity factors from two-dimensional inclined edge cracked models and three-dimensional inclined semi-circular surface cracks that penetrate part-way through a thick plate;To improve the accuracy of photoelastic data collection, fringe multiplication and fringe sharpening technique with a digital image processing system were used. Fracture coefficients, together with the exact origin of the crack tip, were extracted from data sets that produced an overdeterministic system of equations solved by an iterative least squares method. Power series type Williams equations were used in the analysis. The accuracy of the experimental results were evaluated qualitatively and quantitatively. For qualitative visual assessment, regenerated fringes were plotted by using the fracture equations and coefficients estimated from the data sets. For quantitative accuracy evaluation, statistical parameters, such as standard deviation and correlation coefficients between theoretical and observed fringes at the data points were introduced;The accuracy evaluation indicated that the first 4 terms of Williams equations are sufficient to describe the stress field in the vicinity of the crack tip for both Mode I and Mixed Mode cases. It was found that the modified Westergaard equations or the first two terms of Williams equations cannot be used to extract Mixed Mode fracture parameters;The average values of the new experimental results, K(,I)/K(,Io) and K(,II)/K(,I), for the two-dimensional inclined edge cracked plates lie within the range of numerical solutions and previous experimental values. Variation of stress intensity factor along a circumference of a semi-circular surface crack in pure Mode I loading is similar to that predicted by a finite element solution, but at higher numerical values. New experimental solutions for K(,I)/K(,Io) and K(,II)/K(,I) at the maximum depth of the three-dimensional inclined semi-circular cracks are presented