The fracture extension resistance curve (R curve), which delineates the increasing rate of plastic work needed to cause crack propagation in nonbrittle metals, is determined by using Dynamic Tear (DT) test procedures. The resistance parameter is the slope of the R curve. The effect of specimen thickness on the R-curve slope was investigated for three high-strength steels of high, intermediate, and low resistance to ductile fracture. R curves were determined for each steel in the full thickness (1 in.) and for thicknesses of 0.625 and 0.325 in.