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Fatigue cracks increase the ductile-brittle transition temperature (dbtt) by about 20 C, and an evaluation was made of the contribution of the fine-radius fatigue crack tip to this increase by annealing specimens, partially cracked by fatigue, at 650 C for 3 h and comparing the dbtt of fatigued and annealed specimens. Transition temperatures (tt) were determined from impact tests, and the fatigue cracks were introduced by plane bending. Precracked specimens were aged (232 C for 1 h), and V-notched specimens were nitrided in ways that eliminated or retained shear lips. Precracked test specimens were nitrided only at the crack tip. Aging does not alter the tt appreciably, and V- notched and fatigue-cracked specimens that were similarly nitrided, possess the same energy absorbing properties with a higher tt than fatigued ones. Thus, the nitrided zone overrides the effect of notch sharpness and structure at the roots of notches. The elimination of shear lips causes a further rise in tt. Electron and optical microscopes were used, and the fractographic examination revealed how the ductile initiation zones (diz) at the roots of stress concentrators decreased in thickness with decreasing temperature and decreasing notch radius. Macro- and microfractographic features as a function of temperature are described. The results indicate semiquantitatively the energy absorbed in producing the diz and shear lips and thereby demonstrate their protective influence.
structural steel, notch tests, fatigue materials, aging, nitriding, impact, transition temperature, shear tests, energy absorption, electron microscopy, fractography, optical measurement, evaluation, tests
Fegredo, D. M.
Research scientist, Mines and Resources, Ottawa, Ont.