Inconel 718 강재에 대한 고온 다축 반복 하중 하에서의 피로수명 예측 및 라체팅에 관한 연구

Abstract

Ratcheting and fatigue behavior of Inconel 718 has been investigated at 649°C under various proportional and nonproportional loading with load control. The effects of loading path, magnitude of stress amplitude and mean stress on the development of ratcheting strain have been studied over the lifetime. Ratcheting strain was measured to near failure cycles. Ratcheting occurred even under fully reversed uniaxial loading in tensile direction and torsional loading in initial loading direction. The equivalent ratcheting strain is examined for different loading paths. It is found to increase with cycles in the manner similar to the creep curve under static load. The steady-state is dominant under the loading conditions used. High ratcheting rates are found for circular paths and when the cyclic stress is in the same direction as the mean stress. The ratcheting strain at failure varies widely with test conditions and is therefore deemed not a primary life-determining factor. Both normal and shear fracture modes are observed in the range of test conditions. The examination of fracture surface and crack pattern shows that the test material tends to fail in normal mode at smaller stress amplitudes and in shear mode at larger stress amplitudes. The existing multiaxial stress-life criteria are found to be unable to correlate life data. A new fatigue criterion has been proposed based on the maximum value of the parameter given in terms of the shear stress amplitude, the normal stress excursion between the minimum and maximum shear stress points, and the maximum hydrostatic stress.