EXHAUSTION OF DUCTILITY UNDER NOTCH CONSTRAINT FOLLOWING UNIFORM PRESTRAINING.

The purpose of the work was to measure the amount of uniform precompression of ABS-B and Project E-steel resulting in brittle fracture under the stron constraint of a subsequently machined severe circumferential groove. The elongation at the shoulders, measured with a special extensometer, was found to be a far more sensitive measure of brittleness than the average fracture stress. Prestrains as low as 0.05 caused a reduction of the elongation at the shoulders from about 0.017 - 0.050 in. to about 0.003 - 0.006 in. At low prestrains average fracture stress equaled or exceeded the theoretical flow limit of 2.68 sigma 0.1, where sigma 0.1 is the 0.1 offset yield stress in simple tension at the same prestrain. At a prestrain of 0.20 the fracture stress fell below the flow limit and at 0.60 it was close to sigma 0.1. The conditions of fracture at a notch in a strain hardening material are discussed. The total plastic elongation of a region surrounding a sharp notch in prestrained steel determines whether or not fracture will be initiated in large structures, hence is a direct and realistic measure of the remaining ductility and provides an excellent test of the materials resistance to embrittlement. Author