Effects of crack tip stress states and hydride-matrix interaction stresses on delayed hydride cracking
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I.
INTRODUCTION
PRESSUREtubes of cold-worrked Zr-2.5Nb alloy are the primary containment for the coolant of CANDU nuclear generating stations (NGS's). Under certain conditions, zirconium and its alloys are susceptible to a localized embrittlement process called delayed hydride cracking (DHC). tx,2,41 Delayed hydride cracking is caused by the preferential and repeated accumulation and fracture of hydrides at stress misers such as cracks, causing the cracks to slowly increase in length over a period of time. tl-51 Because understanding and controlling DHC initiation and growth is of great importance in ensuring the safe and economical operation of CANDU NGS's, the DHC mechanism continues to be the subject of extensive experimental and theoretical investigations. A theoretical model developed by Dutton and Puls m gave an expression for the DHC velocity as a function of stress intensity factor, material yield stress, temperature, hydrogen concentration, and bulk hydride distribution. This model provided an adequate explanation for many significant features of the cracking process, the most important being the dependence of the crack velocity on temperature and stress intensity factor, /(1.t21 Later changes in the model involved modifying the relationship that is used for translating the flux of hydrogen coming into the crack tip region to a hydride/crack growth rate ~ and refining the expressions used for defining the hydrogen solubilities at the hydride source and sink locations, t4] The refined DHC model was able to M.P. PULS, Research Scientist, is with the Materials and Mechanics Branch, AECL Research, Whiteshell Laboratories, Pinawa, MB R O E 1L0, Canada. Manuscript submitted November 1, 1989. METALLURGICAL TRANSACTIONS A
provide a rationale for the observed dependence of the DHC velocity on the direction of approach to the test temperature. [4.5] No provision was made in the model to allow for a detailed description of the fracture process occurring at the crack tip. It was assumed that hydride growth occurs at the position of maximum hydrostatic stress in the plastic zone at the crack tip. To translate this to crack growth, it was postulated that the hydride cracks as soon as it is formed. However, in plane strain, the maximum hydrostatic stress is some distance in front of the crack tip. Thus, in order for microcrack formation in the hydride to result in extension of the main crack, the microcrack must be able to link up immediately with the main crack. The fractographic and metallographic evidence is that most of the region up to the crack tip is hydrided, implying that there must also be some hydride growth between the crack tip and the point of maximum hydrostatic stress. This means that the hydride length must be proportional to the length of the plastic zone boundary along the fracture face and must increase with K/z in the same way as does the plastic zone boundary. The experimental evidence from acoustic emission (AE) measurements during D H C I6'7'8] and from postmortem fractographic observations of
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