Effect of multiaxial stresses on creep damage of 316 stainless steel weldments
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INTRODUCTION AND BACKGROUND
IT is well known that during plastic deformation, variations in material strength can produce strong variations in the local state of stress. One of the best known examples o f this was shown in the study of silver-brazed joints of mild steel by Moffat and Wulff.tll They showed that a thin layer of silver could be bonded to a much stronger material and then loaded without failure with remote forces that correspond to several times the uniaxial ultimate strength of the material. The constraint afforded by the stronger material kept the braze material from yielding or fracturing. This was only possible because a strongly hydrostatic state of tension develops in the weak silver material. Very similar effects take place in almost all welded structures. Typically the weld, heat-affected zone (HAZ), and base metal all have quite different properties. If the structure is deformed, a complicated multiaxial state of stress invariably results.t2] For example, in the applications that motivate this work, stainless steel base materials are significantly stronger in creep than the weld materials used to join them. As a result, the weldment can withstand greater stresses (or show longer lifetimes to failure) than the unconstrained weld metal. However, failure usually takes place in the weld metal. In such applications, even if the remote loading is quite simple, variations in material properties can produce complicated multiaxial states of stress with strong spatial variationsY] Numerical techniques such as the finite element method (FEM) allow one to readily approximate these stress distributions. However, once the YU-HSIAN HSIAO, Postdoctoral Researcher, and GLENN S. DAEHN, Associate Professor, are with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210. HONGYAN ZHANG performed this work at Ohio State and is now a Research Fellow with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48106. This article is based on a presentation made at the "High Temperature Fracture Mechanisms in Advanced Materials" sympsosium as a part of the 1994 Fall meeting of TMS, October 2~5, 1994, in Rosemont, Illinois, under the auspices of the ASM/SMD Flow and Fracture Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
spatially varying stress distributions are known, it is not straightforward to predict how quickly damage will develop and what the component lifetime will ultimately be. The vast majority of creep rupture tests have been performed on smooth samples of uniform composition and structure loaded in uniaxial tension. While this test configuration is similar to many important practical cases and permits simple and accurate determination of constitutive behavior, it does not permit the evaluation of the effects of multiaxial stress state on creep rupture. There also has been a significant amount of work done on creep rupture of metals under multiaxial states of stress. This work has great relevance to the creep ruptu
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