INVESTIGATION OF MECHANISM OF FAILURE OF HIGH-STRENGTH MATERIALS. PART III. A COMPARISON OF FRACTURE TOUGHNESS OF HIGH-STRENGTH SHEET STEEL AS DETERMINED BY COMPLIANCE-GAGE AND FRACTURE-APPEARANCE METHODS.
A compliance gage of modified design was conceived and constructed. Using the modified gage, an experimental curve relating compliance and crack extension factor was obtained. The rather excellent agreement between the experimental curve and the Irwin-Westergaard theoretical curve was the basis for considering the modified gage to be reliable. A comparison of K sub Ic and K sub c3 values revealed that the former were of lower magnitude, consistently, than the latter for the yield strength level of 157 ksi. The reason for this behavior, apparently contradictory to the observations of others, was not established. The dependence of K sub c3 on thickness, although not of primary interest in this investigation, was observed to reflect a changing trend with varying strength level. The slopes of K sub c3 versus thickness curves changed from positive, to essentially zero, to negative as the yield strength increased from 157, to 214, to 235 ksi, respectively. Author