Proportional and Non-proportional Biaxial Fatigue of Inconel 718
by
Justin Lee Koch
Abstract
A multiaxial LCF (low cycle fatigue) parameter is proposed for materials that fail in a shear strain dominated mode. Three bulk loading parameters have been identified to assess fatigue damage—maximum shear strain amplitude, and both the strain amplitude and mean stress normal to the plane of maximum shear strain amplitude.
Biaxial strain controlled tests were conducted on thin walled tubular specimens of Inconel 718 at room temperature. Forty-six specimens were tested at fatigue lives of less than 105 cycles. Fifteen loading paths were employed to systematically identify the critical fatigue damage parameters. Proportional, non-proportional, and out-of-phase loading cases were considered. Selection of strain paths and damage parameters were dictated by extensive observations of the cracking behavior. Shear crack initiation and growth dominated the fatigue damage process. Fatigue lives were correlated by the proposed damage parameter to within a factor of two.
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