J. M. Larsen

The Role of Local Microstructure on Small Fatigue Crack Propagation in an α + β Titanium Alloy, Ti-6Al-2Sn-4Zr-6Mo

Microstructural origins of the variability in fatigue lifetime observed in the high- and very-high-cycle fatigue regimes in titanium alloys were explored by examining the role of microstructural heterogeneity (neighborhoods of grains with similar crystallographic orientations or microtexture) on the initiation and early growth of fatigue cracks in Ti-6246. Ultrasonic fatigue of focused ion beam (FIB) micronotched samples was used to investigate long lifetime (107 to 109) behavior for two microstructural conditions: one with microtexture and one without microtexture. For specimens containing notches of nominally 20xa0μm in length, fatigue crack initiation in the microtextured material was most likely to occur from notches placed in neighborhoods with a microtexture favorably oriented for easy basal slip. Initiation lifetimes in the untextured material with similar sized notches were, on average, slightly greater than those for the microtextured condition. In both materials, the crack-initiation lifetime from micronotches of length 2cxa0>xa020xa0μm was a very small fraction (<1xa0pct) of the measured fatigue lifetime for unnotched specimens. Furthermore, in the microtextured condition, small fatigue crack propagation rates did not correlate with the microtextured regions and did not statistically differ from average small crack growth rates in the untextured material. As the micronotch size was reduced below 20xa0μm, fatigue crack initiation was controlled by microstructure rather than by FIB-machined defects. Finally, predictions of the fraction of life consumed in small and long fatigue crack growth from preexisting cracks nominally equivalent in size to the micronotches was compared with the measured fatigue life of unnotched specimens. The predicted range of lifetimes when factoring in the experimentally observed variability in small fatigue crack growth, only accounted for 0.1xa0pct of the observed fatigue lifetime variability. These findings indicate that in the high-and very-high-cycle fatigue regimes, fatigue life is dominated by crack initiation and that the variation in the initiation lifetime is responsible for the observed variation in total fatigue life.

Vacuum Levels Needed to Simulate Internal Fatigue Crack Growth in Titanium Alloys and Nickel-Base Superalloys: Thermodynamic Considerations

Prior studies have examined fatigue growth of surface cracks in vacuum to simulate subsurface growth in Ti alloys and Ni-base superalloys. Even with the highest vacuum level attained using “state-of-the-art” pumps, it is unclear if conditions of internal crack growth are truly simulated. We consider thermodynamics of the oxidation process to help answer this question. This consideration helps explain a previously reported anomalous behavior of longer life in air than in vacuum under certain material/test conditions.

Demonstration of advanced life-prediction and state-awareness technologies necessary for prognosis of turbine engine disks

This paper summarizes a spin test of an IN100 minidisk that demonstrated advanced fatigue crack growth predictive tools under dwell fatigue and the ability to infer the damage state from state-awareness sensed data. The test was performed at elevated temperature in a partial vacuum and was a major success. The advantages of employing 3D fracture mechanics tools were clearly demonstrated when the prediction using the advanced analysis technique produced a crack growth lifetime 10 times greater than the standard 2D tools and well within a factor of two of the test result. However, when the effect of the partial vacuum was also taken into account, the predictions more closely resembled the test results. Another success was demonstrated when the blade-tip time-of-arrival sensors detected deflections of 50 microns and greater at temperature. Using the 3D analysis tools, a transfer function was created that related blade-tip deflection to crack size. The crack size was tracked in near real time for the final 150 cycles. This test represented significant demonstrations of both 3D fracture mechanics tools and state-awareness sensing. Both are advanced technologies that will eventually impact the life management of turbine engines.

Correlating Scatter in Fatigue Life with Fracture Mechanisms in Forged Ti-6242Si Alloy

AbstractUnlike the quasi-static mechanical properties, such as strength and ductility, fatigue life can vary significantly (by an order of magnitude or more) for nominally identical material and test conditions in many materials, including Ti-alloys. This makes life prediction and management more challenging for components that are subjected to cyclic loading in service. The differences in fracture mechanisms can cause the scatter in fatigue life. In this study, the fatigue fracture mechanisms were investigated in a forged near-α titanium alloy, Ti-6Al-2Sn-4Zr-2Mo-0.1Si, which had been tested under a condition that resulted in life variations by more than an order of magnitude. The crack-initiation and small crack growth processes, including their contributions to fatigue life variability, were elucidated via quantitative characterization of fatigue fracture surfaces. Combining the results from quantitative tilt fractography and electron backscatter diffraction, crystallography of crack-initiating and neighboring facets on the fracture surface was determined. Cracks initiated on the surface for both the shortest and the longest life specimens. The facet plane in the crack-initiating grain was aligned with the basal plane of a primary α grain for both the specimens. The facet planes in grains neighboring the crack-initiating grain were also closely aligned with the basal plane for the shortest life specimen, whereas the facet planes in the neighboring grains were significantly misoriented from the basal plane for the longest life specimen. The difference in the extent of cracking along the basal plane can explain the difference in fatigue life of specimens at the opposite ends of scatter band.n

Fatigue crack propagation and load interaction effects in a titanium alloy

An experimental investigation of load interaction effects in Ti-6Al-2Sn-4Zn-2Mo is presented. Simple variable amplitude loading spectra were applied to test samples, and the crack propagation stress intensity factor, K P R , was subsequently measured. The data were condensed to two equations (master curves) that describe the influence of an unloading cycle after constant amplitude loading and a single overload cycle, respectively. The results suggest that load interaction effects are caused by residual compressive stresses ahead of the crack front, whereas the influence of crack closure is minor.

Quantitative Characterization of Microscale Fracture Features in Titanium Alloys

Quantification of the fracture mechanisms is important for design and sustainment of fatigue–critical components and for improved life prediction models. The traditional fractography in a scanning electron microscope (SEM) provides important, but qualitative, information on fracture mechanisms. In this study, the fatigue fracture features at the crack-initiation site in a near-α titanium alloy were characterized quantitatively in an SEM. The crack initiated in a surface-located primary α grain, with the resultant formation of a facet. A serrated feature was observed on the fatigue crack-initiating facet within an α grain for the first time in this study. The spatial orientations of the crack-initiating facet and the fracture lines forming the serration were determined in 3D with the quantitative tilt fractography technique, which consists of analyzing an SEM image pair acquired at different stage tilt angles. The electron backscatter diffraction (EBSD) data were collected nondestructively from the grain associated with faceted crack-initiation, without recourse to cross-sectional polishing. Combining the spatial orientation and the EBSD data established that the facet plane was basal and the lines forming the serrated feature were along 〈a〉

A new paradigm of fatigue variability behavior and implications for life prediction

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Nominal vs Local Shot-Peening Effects on Fatigue Lifetime in Ti-6Al-2Sn-4Zr-6Mo at Elevated Temperature

i.e.,21¯1¯0-type slip directions. The methodology presented in this study is applicable to quantification of crystallography for other planar and linear features in crystalline materials.