Tension-Torsion Multiaxial Creep-Fatigue Lifetime for CMSX-2 Nickel Base Single Crystal Superalloy

Abstract

In this paper, the tension-torsion multiaxial creep-fatigue lifetime for CMSX-2 nickel base single crystal superalloy at 1173K is described. Strain controlled tension-torsion low cycle fatigue tests with various hold times were carried out using hollow cylindrical specimens, the axis of which coincides with the [001] crystallographic direction. An introduction of the hold time into the torsion tests was more detrimental to the lifetime than that into the tension tests. The Von Mises' equivalent strain range gave a larger scatter for the correlation of creep-fatigue lifetimes. The relationship between elastic strain range and creep-fatigue lifetime had a strong dependency on the principal strain ratio. But those between plastic strain and the lifetime, and creep strain and the lifetime had almost no dependency on the principal strain ratio. A linear damage rule yielded an accurate lifetime estimation in the tension tests, but substantially overestimated the lifetimes in the torsion tests. A new simple lifetime prediction method, taking account of anisotropy of the elastic modulus, was proposed. The method predicted the experimental lifetime within a factor of 3 scatter band. Cracks in the tension tests propagated in the principal strain direction but those in the torsion tests propagated in the maximum shear direction. In the torsion tests, cracks were only observed in the soft zones predicted by numerical analyses.