| Two nickel-base superalloys were studied for their multiaxial fatigue behavior under both uniaxial and biaxial loading conditions at room and elevated temperature. In particular, the study was focused on the relationship between the fatigue life prediction methods and the crack damage process, i.e., the crack nucleation and propagation model. The materials studied were PW Astroloy and Wrought Waspaloy. The materials were received from Allied-Signal Aerospace Company (Garrett Auxiliary Power Division). These materials are used widely in gas turbine engines.Six fatigue life prediction models for multiaxial fatigue behavior were examined with fatigue data. The results of the fatigue tests demonstrated that the fatigue life of the specimens is strain level, strain state, and temperature dependent. The effect of biaxial states on the fatigue life is dependent on the applied strain parameter and materials. The Socie strain parameter and the Brown-Miller strain parameter are good selections for correlating the uniaxial fatigue data to the biaxial loading condition. In most cases fatigue life of two materials decreased with the increase in temperature. It was believed that oxygen might play a role in both the nucleation and the growth of fatigue cracks.An extensive fractographic examination of all the specimens tested under different loading conditions at two different temperatures revealed that the damage processes followed the classic stages of shear nucleation, Stage I and Stage II crack growth. At a high strain level, multiple crack nucleation dominated the damage process. Rapid linking of cracks resulted in the reduction of fatigue life. Both nucleation and propagation processes were controlled by maximum shear strain and the normal strain on the plane of maximum shear strain. In long fatigue life, low strain level, nucleation occurs on planes that experience the maximum range of shear strain. Crack growth was controlled by the principal stress. The most appropriate fatigue damage model is dependent on the test conditions. At short fatigue life, shear strain based parameters relate best to the physical damage. At long fatigue lives, most of the life is Stage II growth, and consequently, principal strain parameter best relates to the physical damage. In a mixed model, a modified strain base parameter test relates to the physical damage.The specimens used in this study were smooth surface cylindrical specimens with 0.40" and 0.4" diameter and 1" gage length. All of the fatigue tests were conducted under strain control. A triangular wave form of strain amplitude was used, which keeps the strain rate constant. Frequency was 20 cpm and the strain ratio was 0.05. |
