In order to analyze stress intensity factors for surface cracks with arbitrary shapes in plates and shells, the authors have developed a new finite element scheme based upon the line spring model. In this scheme Ahmad's thick shell elements are combined with the newly developed line spring elements with quadratic shape functions. This line spring method has more potential of application than a previously reported method where flat shell elements were combined with the line spring elements with linear shape functions.
In the analysis of K-values for a single semielliptical surface crack in tension, it has been found that the method has nearly the same accuracy as the three-dimensional finite element method, while the computing time is one order of magnitude less. The method has been applied to the problem of interaction of twin semielliptical cracks in tension. The estimation scheme for K-values by ASME Boiler and Pressure Vessel Code has been found to give conservative K-values for most of the separated twin cracks, except for very deep cracks. For overlapping twin cracks, K-values at the intersecting points become very large; for example, the highest value is about three times larger than the maximum value for a single crack with the same size as one of the twin cracks. Therefore caution must be used in dealing with these cracks.
Single rectangular and triangular cracks have been analyzed. It has been found that the ASME Code gives conservative K-values for triangular cracks but underestimates values for rectangular cracks.