A comparative study on fatigue indicator parameters for near‐α titanium alloys

Abstract

Nucleation of in‐service cracks leads to detrimental consequences for structural components of near‐α titanium alloys subjected to fatigue loads. Experimental observations show that the fatigue initiation facets usually form in certain crystallographic orientation ranges of “hard” primary α grains which differ between pure and dwell fatigue loads. In this manuscript, a comparative study has been performed using several fatigue indicator parameters (FIPs) to assess their ability to predict the location of fatigue crack nucleation in near‐α titanium alloy microstructures. All selected FIPs are implemented within the same polycrystalline plasticity finite element modeling framework to facilitate one‐to‐one comparisons. Comparison on predictability of critical initiation locations and their crystallographic orientations is studied for incorporated FIPs under pure and dwell fatigue. The critical locations predicted by some FIPs were found to be close to each other, and consistent with the crystallographic orientation ranges from fractography measurements, in addition to the range transition from pure to dwell fatigue loads. Critical locations from slip driven FIPs are obtained to be several grains away from that of the former ones and are inclined to capture orientations of slip traces from experiments.

Tool for automatic macrozone characterization from EBSD data sets of titanium alloys

“…Work on crystal plasticity models by Dunne et al (2007) has defined the 'rogue pair' as the worst combination of hard-soft orientation for quasi-cleavage facet nucleation, where a hard grain with its basal plane nearly perpendicular to the LD is neighbouring a soft grain with its prismatic plane normal at 70 to the load. Moreover, several researchers have established differences between crack initiation and propagation regions: for example, Bantounas et al (2010) highlighted a difference between grains orientated for crack initiation (c axis < 15 from LD) and those for crack propagation (c axis at 15-40 from LD) under high cycle fatigue, while probabilistic work on crack plane orientations has revealed that the critical crystallographic orientations for nucleation sites are found in grains with the c axis between 10 and 30 from LD preferentially (Ozturk et al, 2017;Liu et al, 2023). More recently, research by Harr et al (2021) suggests that a certain degree of connectivity between grains is required for crack growth in Ti6242 bimodal microstructures under dwell-fatigue loading conditions, and therefore both grain connectivity and orientation criteria must be considered when characterizing macrozones linked with dwell-fatigue failure.…”