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International Journal of Fatigue

Archives Papers: 1,201
Elsevier
Abstracts:The following contribution deals with the growth of cracks in low-cycle fatigue (LCF) and thermomechanical fatigue (TMF) tested specimens of Inconel 718 measured by using the replica method. The specimens are loaded with different strain rates. The material shows a significantly higher crack growth rate if the strain rate is decreased. Electron backscatter diffraction (EBSD) is adopted to identify the failure mechanism and the misorientation relationship of failed grain boundaries in secondary cracks. The analyzed cracks propagated mainly transgranular but also intergranular failure can be observed in some areas. It is found that grain boundaries with coincidence site lattice (CSL) boundary structure are generally less susceptible for intergranular failure than grain boundaries with random misorientation. For modeling the experimentally identified crack behavior an existing model for fatigue crack growth based on the mechanism of time dependent elastic–plastic crack tip blunting is enhanced to describe environmental effects based on the mechanism of oxygen diffusion at the crack tip. For the diffusion process the temperature dependent parabolic diffusion law is assumed. As a result, the time dependent cyclic crack tip opening displacement ($Δ CTOD$) is used as representative value to describe both mechanisms. Thus, most of the included model parameters characterize the deformation behavior of the material and can be determined by independent material tests. With the determined material properties, the proposed model describes the experimentally measured crack growth curves very well. The model is validated based on predictions of the number of cycles to failure of LCF as well as in-phase and out-of-phase TMF tests in the temperature range between room temperature and $650 ° C$.
Abstracts:In this paper, the influence of different phase angles, load ratios and dwell times on the thermomechanical fatigue (TMF) crack growth of the nickel-based superalloys MAR-M247 CC (HIP) and CM-247 LC is studied. The thermomechanical fatigue crack growth tests are performed under in-phase (IP) and out-of-phase (OP) TMF loading between 300 $° C$ and 950 $° C$. It is shown, that the applied load ratio plays a crucial role for the resulting fatigue crack growth rates and whether IP or OP TMF loading is more detrimental. A digital optical microscope is installed, which allows in situ observation of the fatigue crack growth, the active damage mechanisms and the damage evolution even at high temperatures. The in situ observations are accompanied by fractographic investigations, which confirm, that IP TMF loading preferentially leads to interdentritic fracture. Finally, first steps towards a quantification of the active damage mechanisms are taken using digital image processing.