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The adverse effects of stress concentrations on the high temperature strength appear to have been first noticed in the brittle failure of bolts and flanges used in steam generating and chemical processing equipment (Bailey, 1928; Buchmann, 1937; Houdremont, 1937; Kaissling, 1937; Reincke, 1937; Ruttmann, 1937; Wellinger and Keil, 1941, 1943; Schaub, 1946). Such parts were generally designed to operate at large factors of safety based on laboratory determined properties of smooth specimens. Specimens cut from failed bolts exhibited a loss in notched impact strength but no change in tensile strength or unnotched bend properties (Reincke, 1937). The first systematic investigations of this problem clearly revealed that the creep rupture strength of certain, otherwise ductile, steels was greatly reduced by notching a specimen, and by testing it under certain combinations of temperature and stress (Scherer and Kiessler, 1939; Siebel and Wellinger, 1940; Siegfried, 1943, 1945, 1947, 1951, 1952, 1953, 1955). Steels exhibiting such notch weakening, also had their room temperature impact properties greatly reduced, if first subjected to creep. This deterioration of the metal has been called creep damage (Thum and Richard, 1949; Brown and Sachs, 1951, 1952; Richard, 1952, 1953). The preceding phenomena led to an extensive program of creep rupture tests in Germany, before World War II. The tests conducted at 932 F (500 C), for durations up to 100,000 hr, included a variety of heat resisting boiler and bolting steels (Thum and Richard, 1941, 1942, 1943, 1949; summarized Thum and Richard, 1953). Since the war, interest in long time notch tests has resumed in Germany. Results have been reported on a series of heat resisting steels, tested at 1012 F (550 C) (Theis, 1951, 1953); and a large cooperative program is now under way to obtain data at temperatures between 932 and 1300 F (500 to 700 C) up to 100,000 hr (Richard, 1955).
Associate DirectorConsultant to NASA, Syracuse University, Syracuse, New York
Sessler, J. G.
Senior Project Engineer, Syracuse University, Syracuse, New York
Brown, W. F.
Chief, Lewis Research Center, NASA, Cleveland, Ohio