**1. Introduction**

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Components in power plant, chemical plant, manufacturing processes, aero-engines, etc. may operate at temperatures which are high enough for creep to occur [1]. Such compo‐ nents may contain cracks or must be assumed to contain cracks as part of design life or remaining life analyses which are required [2]. In order to perform these analyses a number of approaches have been used, based on, for example, a fracture mechanics approach [3], or a continuum damage mechanics approach [4, 5, 6]. This paper is related to the use of the damage mechanics approach. In particular the methods used to obtain the material constants in the multiaxial form of the creep damage and creep strain equations are described. Most of the constants are obtained by fitting to uniaxial creep data; this is a wellestablished method [7]. However, in this paper, the determination of the multiaxial stress state parameter, α [8], is based on results from compact tension (CT) tests; this approach is novel and results in properties which are particularly suited for predicting creep crack growth in components, where the crack growth is defined by a damage parameter, ω. When this damage parameter reaches a critical value (0.99 chosen for the presented work) the material is regarded as 'completely damaged' and hence a void or crack growth is assumed to be present. A previously used technique for obtaining the multiaxial stress state parameter, based on the notch strengthening which usually occurs in Bridgman notch [9] creep rupture tests, relative to corresponding uniaxial tests, does not closely represent the stress states and constraint which occur at crack tips. The validity of the method pro‐ posed has been established by comparing finite element predictions of creep crack growth

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in thumbnail cracked specimens with experimental data [7] using the material constants obtained from uniaxial creep and CT creep test results.

The material chosen for the investigation is a 316 stainless steel and a P91 steel because of the ready availability of uniaxial creep, uniaxial creep rupture, compact tension creep crack growth and thumbnail creep crack growth data at temperatures of 600°C and 650°C, respectively. The particular form of damage equation chosen for the investigation is that proposed by Liu and Murakami [6]. By comparison with the more commonly used Kachanov damage equations [4], it was found that the Liu and Murakami equations do not cause the time steps in the finite element analyses to become impractically small [10] and unlike the Kachanov equations, they produce results which are relatively insensitive to element size near the crack tip. These aspects are covered further in the paper.
