The influence of multiaxial states of stress on the hydrogen embrittlement of zirconium alloy sheet
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I., I N T R O D U C T I O N
THE hydrogen
embrittlement of zirconium and its alloys has been studied extensively because of their technological importance in water-cooled water-moderated nuclear power reactors. Hydride precipitates, which form in Zr-base alloys during operating conditions, are known to cause substantial embrittlement, especially in the presence of notches ~'2 or at low temperatures in smooth specimens. 3 In these alloys, it is well established that the precipitated hydride phase cracks during straining 3-6 and that the cracked hydrides accelerate a ductile fracture process. ~-5.7The criteria for fracture initiation at hydrides have recently been studied on a microstructural scale by Simpson, and the importance of normal stresses, especially stress triaxiality, has been clearly identified. 5 Despite the general knowledge that hydrogen embrittlement of Zr and its alloys is enhanced by increasing the normal stresses, such as in the presence of notches or by testing at low temperatures, there has been no systematic study of the influence of stress state on the mechanism of hydrogen embrittlement in these alloys. This situation exists despite the fact that dilute Zr alloys in the form of sheet or tubing are used as fuel element cladding or pressure tubes in applications in which the states of stress are multiaxial. Hydrogen embrittlement studies on pressurized tubes have been performed, 8'9'~~but while these are valuable in predicting failure stresses under operating conditions, the studies do not attempt to identify influence of stress state on the embrittlement mechanism (this may be due in part to the susceptibility of pressurized tubes to instabilities which usually govern their fracture behavior). Utilizing the techniques of punch-stretch testing, n this study examines the hydrogen embrittlement of Zr alloy, ZIRCALOY-2 sheets containing 21, 150, 350, or 615 wt ppm H which have been deformed to failure over a range of stress states from uniaxial to equibiaxial tension. The resulting fracture behavior will be cor*ZIRCALOY is a trademark of General Electric Company. FAN YUNCHANG, Visiting Scholar, and D. A. KOSS, Professor, are with the Department of Metallurgical Engineering, Michigan Technological University, Houghton, M1 49931. Manuscript submitted June 12, 1984. METALLURGICAL TRANSACTIONS A
related with quantitative metallography and fractographic observations in order to determine the influence of stress state on (a) the strain to fracture hydrides and to nucleate voids at fractured hydrides, (b) the void growth rates, and (c) the critical density of voids at the initiation of void link-up and subsequent material failure. The observed dependence of hydrogen embrittlement on multiaxial stress state will then be examined within the framework of the existing theories of ductile fracture. Correlations will also be established with a companion study on the hydrogen embrittlement of commercially pure titanium sheet under multiaxial states of stress. ~'~2
II.
EXPERIMENTAL PROCEDURE
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