Predictive Modeling of Arbitrary Loaded TMF Crack Growth in Superalloys

Abstract

Abstract. Hot sections in turbine engines are subjected to large variations in temperatures and mechanical/thermal loadings. As such, accurate predictions of fatigue crack growth must account for many physical phenomena: temperature dependent crack growth behavior, load interaction history effects, time at temperature effects, temperature history effects, and the effects of stress/temperature/time on the materials. Through extensive experimental work on superalloys, a very definite "temperature history effect" on the resulting crack growth behavior has been identified and modeled. This work also identified a Temperature Affected Zone (TAZ) that occurs ahead of the crack tip and affects subsequent crack growth rates. The size of the TAZ is dependent on temperature, hold time, and stress state. Measurements of the TAZ were made under various conditions. The changes that occur in this TAZ are a combination of oxidation and material microstructure evolution. Various simplified "hot section" engine spectra (changing temperatures and stress levels) were tested to determine resulting crack growth behavior. Correlation between the experiments and model predictions were good and generally conservative.

Time‐ and cycle‐dependent growth of surface cracks in alloy 718

“…28 Under sustained loading, either creep or tensile dwell, the propagating intergranular crack front leaves behind unbroken islands or ligaments that eventually fail by ductile rupture, when the local stress state exceeds the tensile strength of the material. 23,26,33,34 During long dwells after the incubation stage, the crack growth rate during the dwell tends to be independent of their duration. 24 If fatigue occurs after the tensile dwell, striation or other features of transgranular crack growth are observed in these ligament regions.…”

“…11 The length of the damaged zone depends on the history of hold times and interactions from load reversals. 26,34,38 Various phenomenological models have been used to describe the rate of increase in the TAZ. 8,11,22,23,26,33,34,38 Intergranular crack growth is observed when time dependent damage is extending faster than the fatigue crack.…”

“…26,34,38 Various phenomenological models have been used to describe the rate of increase in the TAZ. 8,11,22,23,26,33,34,38 Intergranular crack growth is observed when time dependent damage is extending faster than the fatigue crack. FCGR increases immediately after tensile holds that generated damage (i.e., TAZ) ahead of the crack tip.…”

“…Therefore, to model crack growth, both the rate of damage of the TAZ development and the acceleration of the crack growth rate through the TAZ needs to be modeled. 22,23,26,34,38 For example, one relationship for TAZ size is given by, 26,34…”

Transition modelling of surface flaws to through cracks

“…The model was modified by Barker et al to account for changes in fracture properties at different temperatures where it became MPYZ‐TMF. Further modifications were made by Radzikci in which the model was adjusted to better capture the effects of changing temperature and the effects of material degradation in Inconel 718.…”