Creep rupture mechanisms in annealed and overheated 7075 Al under multiaxial stress states
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I. INTRODUCTION
UNIAXIAL stress conditions have commonly been employed in comprehensive studies concerning failure of engineering materials at high temperatures. These investigations have provided substantial information regarding the factors governing creep rupture. Different failure modes have been classified, and systematic descriptions for the mechanics and mechanisms of creep rupture have been well established.[1,2,3] It has been observed[4] that, for uniaxial creep conditions, a fundamental power law exists, which simply relates the time to rupture, tf , and the applied stress, s, as follows: tf 5 Ms2x
[1]
where M and x are stress-independent constants for a given material and testing condition. In high-temperature applications, however, the majority of the components are subject to stress states varying in both time and position. Under such complex loading conditions, the stress used in Eq. [1] must be modified to correctly predict rupture time. Due to the difficulties in establishing stress distributions for creep deformation of multiaxially stressed components, the general effort has been geared toward obtaining a representative stress parameter, srep, which is substituted for s in Eq. [1] and is used to forecast
AHMADALI YOUSEFIANI, formerly Graduate Student Researcher with the Department of Chemical and Biochemical Engineering and Materials Science, University of California, Irvine, is now Senior Engineer/Scientist with the Space and Communications Group, The Boeing Company, Huntington Beach, CA 92647-2099. FARGHALLI A. MOHAMED and JAMES C. EARTHMAN, Professors, are with the Department of Chemical and Biochemical Engineering and Materials Science, University of California, Irvine, Ca 92697-2575. Manuscript submitted December 15, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
tf utilizing data from conventional uniaxial creep rupture tests. Table I describes several examples of such parameters expressed in terms of principal stresses, s1 . s2 . s3. The majority of stress parameters developed for predicting creep life under multiaxial stress states are based on continuum mechanics approaches.[4–8] Multiaxial stress terms are combined in a weighted formula, and the relative contribution of each term is described by an adjustable factor, which is determined from data generated at different stress states (Eqs. [2] and [3]). The terms M and x 5 2 log tf / log s are obtained from uniaxial creep rupture tests (Eq. [1]), and through analysis of experimental data, coefficients a or n are adjusted to obtain the optimum correlation between the rupture time data for different stress states. Multiaxial stress parameters advanced by the continuum mechanics approach may be utilized satisfactorily to predict the creep life of components. However, it should be considered that (1) determination of these parameters will require extensive multiaxial creep and rupture data, and (2) limited information is provided regarding the physical mechanisms involved in the creep rupture process. Such limitations can be overcom
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