High Temperature Multiaxial Creep-Fatigue and Consitutive Equation for Ni-Base Single Crystal Superalloy

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Combined tension-torsion multiaxial low-cycle fatigue (LCF) tests were carried out using Mar-M247LC directionally solidified (DS) and CMSC-2 single crystal (SC) superalloys. Several multiaxial strain and stress parameters were applied to the experimental data. A new constitutive equation wad developed based on the slip theory, taking account of the anisotropy of elastic constants. The main conclusions obtained are as follows.(1)The conventional multiaxial low cycle fatigue strain parameters, such as Mises equivalent strain, maximum principal strain, maximum shear strain and equivalent strain based on crack opening displacement (COD), could not correlate the multiaxial LCF data of the DS and SC superalloys.(2)This research developed a new COD strain parameter taking account of the anisotropy of the elastic constant. The COD strain correlates the multiaxial low cycle fatigue data of DS and SC superalloys within a small scatter band.(3)LCF fatigue data were correlated by Miss equivalent stress, maximum principal stress, maximum shear strain and COD strain. Mises and COD strain correlated the multiaxial LCF parameter with a small scatter, but the other stress parameters give a large scatter of the data.(4)A new consititutive equation wad developed based on the slip theory and anisotropy of elastic constants. The developed constitutive equation accurately estimates the cyclic stress-strain relation of DS and SC superalloys at elevated temperature.