The upper shelf fracture toughness and tearing resistance of two structural steels, HY-100 and ASTM A533, Gr. B, were determined over a wide range of applied constraint. The constraint conditions were varied by changes in specimen geometry and loading mode. Bend specimens with shallow and deep cracks, compact specimens, and single and double edge notched tension specimens were used in this study. A rotation correction was developed for the single edge notch tension specimen which greatly improved the behavior of the J-R curves determined using this specimen. The experimental results were used to investigate the applicability of the Q and T stress parameters to the correlation of upper shelf initiation toughness, JIc, and tearing resistance, Tmat. The J-Q and J-T stress loci, and corresponding plots of material tearing resistance plotted against Q and T, were developed and compared with the expectations of the O'Dowd and Shih and the Betégon and Hancock analyses. The principle conclusions of this work are that JIc does not appear to be dependent on T stress or Q while the material tearing resistance, Tmat, is dependent on T stress and Q, with the tearing modulus increasing as constraint decreases.