Crack growth resistance in plane-stress fracture testing

Abstract

New developments in crack growth resistance measurement using crack-line-wedge-loaded (CLWL) specimens include improved fixtures, and a double compliance technique for obtaining both load and effective crack length. The latter results in marked improvement in the accuracy of R curves for materials having large plastic zones. The CLWL specimen, developed for high strength sheet materials, is now being used tor light plate, at intermediate strength levels. Several investigations are reported in which center-cracked-tension (CCT) and CLWL specimens have been used in attempts to determine whether R curves are specimen independent, and whether K c instability stress intensity of CCT specimens can be predicted from CLWL determined R curves. Systems proposed by Feddersen and Alien for predicting the specimen width and crack length dependency of K c in CCT specimens are examined, and R curves are compared over the wide range of specimen sizes and crack lengths tested by T. W. Orange. Principal stresses along the crack path are determined by strain gages for CCT and CLWL specimens. The stress patterns are widely different, but the effective combined stresses near the elastic-plastic border are similar, which may account for similarity of R curve development for the two specimen types. It is concluded from available evidence that R curves appear to be characteristic of a material, in a given thickness, but the data are not conclusive and more verification is needed.