This publication, Resistance to Plane-Stress Fracture (R-Curve Behavior) of A572 Structural Steel, is one of the papers presented at the Eighth National Symposium on Fracture Mechanics which was held at Brown University, Providence, R. I., 26—28 Aug. 1974. A summary only appears in the proceedings of this symposium (Mechanics of Crack Growth, ASTM STP 590). The symposium was sponsored by Committee E24 on Fracture Testing of Metals of the American Society for Testing and Materials. J. R. Rice and P. C. Paris, Brown University, presided as symposium co-chairmen.