Cracked structural components fabricated from relatively thin sections (less than 26 mm thick) of common structural steels and weld metals (with minimum Charpy toughness requirements) will fail by ductile tearing at service temperatures. In order to estimate the residual ductility of cracked structural steel members; it is necessary to predict hundreds of millimeters of crack extension as a function of applied displacement. A number of problems with application of the J-R curve approach to cases of such extensive crack growth are described. A very simple approach, i.e. predicting crack extension using a constant crack-opening angle, is shown to be reasonably accurate. Over 30 large-scale welded steel members containing fatigue cracks were loaded under displacement control. These members, fabricated from HSLA-80 steel and EH-36 steel, exhibited a crack-opening angle of 0.24 radian for both steels in tension and in bending.