STUDY ON LOW CYCLE FATIGUE OF SINGLE CRYSTAL NI-BASED SUPERALLOY UNDER MULTIAXIAL NON-PROPORTIONAL LOADING

Strain-controlled tension/torsion low cycle fatigue (LCF) tests of single crystal Ni-based superalloy,DD3,were carried out under non-proportional loadings based on orthogonal design at elevated temperatures, using thin-walled tube specimens with 001 orientation. Experimental variable factors were chosen to be strain range, strain path angle, tension/torsion phase angle, strain ratio and temperature. Results show that strain path angle, tension/torsion phase angle and equivalent strain range, are the main affected factors on multiaxial LCF life. Dividing diamond strain loading path into proportional loading and non-proportional loading segments, the equivalent strain range parameter to characterize the effects of non-proportional loading was proposed, and a strain triaxiality factor for single crystal superalloy was introduced to reflect the tension/torsion strain path angle on multiaxial fatigue life. A formula of cyclic plastic strain energy, which is composed of the equivalent strain range taking account of the effects of non-proportional loading path and the strain triaxiality factor, is put forward as failure parameter. Multiple linear regression analysis show that a power law of the failure parameter has a good correlation with the failure cycle, and all test data fall into a scatter band of the factor of 2.0.