High-temperature rupture of microstructurally unstable 304 stainless steel under uniaxial and triaxial stress states
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I.
INTRODUCTION
I N T E R G R A N U L A R fracture by cavity growth and coalescence has long been known to be an important failure mechanism in high-temperature components. Most investigations of this mode of failure have been performed with specimens tested under uniaxial tension. Although uniaxial stress experiments have led to a better understanding of the physical processes involved, they do not provide sufficient information to predict cavity growth and creep rupture under multiaxial stress states. Bending and torsion are examples of loading conditions that can cause multiaxial stresses in smooth components. Notches and other geometric irregularities typical of engineering components can also produce multiaxial stresses when the remote loading condition is purely uniaxial. Thus, multiaxial stresses must be addressed for accurate predictions of high-temperature rupture in many engineering structures. Hayhurst and his colleagues 11-41 proposed that for a smooth cylindrical specimen subjected to uniaxial tension, the rupture lifetime at a given temperature can be expressed as tf = M6r -x
[11
where ~r is the applied uniaxial stress and M and X are constants that characterize the evolution of damage at the temperature in question. The motivation for considering multiaxial stresses is demonstrated by the fact that Eq. [ 1] does not correctly predict creep rupture of notched bars even if the nominal stress in the notch is used in the expression. [2] Consequently, representative stress parameters are substituted for or in Eq. [1 ] to predict creep life data under'multiaxial stress states with rupture data obtained with specimens under uniaxial stresses. Several parameters which contain adjustable "constants" have
H O - K Y U N G KIM, Graduate Research Assistant, F A R G H A L L I A. M O H A M E D , Professor, and JAMES C. E A R T H M A N , Assistant Professor, are with the Materials Section, Department of Mechanical and Aerospace Engineering, University of California, Irvine, CA 92717. Manuscript submitted December 7, 1990. METALLURGICAL TRANSACTIONS A
been proposed to correlate rapture times for different stress states, p,5-71 However, the present work only considers multiaxial stress parameters that are rigid in the sense that their determination does not involve adjustable terms. These parameters were chosen because each is linked to a particular set of physical mechanisms that could control the rupture process. Evaluation of the validity of these nonadjustable stress parameters has been performed in the present investigation to assist in the development of a better understanding of the mechanisms that control high-temperature rupture. These stress parameters are briefly discussed in the following. A. The M a x i m u m Principal Stress
It is well founded that the diffusive growth of intergranular cavities is driven by the tensile stresses acting normal to the grain boundaries. This, combined with the general observation that intergranular fracture usually occurs first on grain boundaries that are perpendicul
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