Probabilistic micromechanical modeling of fatigue-life variability in an α+β Ti alloy

Abstract

The fatigue-life variability in an ␣ ϩ ␤ Ti alloy (Ti-6Al-4V) has been examined through a probabilistic micromechanical model that treats the crack-initiation and growth processes at the grain-size level. First, a physics-based crack-initiation model is described. This is followed by a summary of a physicsbased fatigue-crack-growth model. The combined model is applied to predict the variability of crack initiation and growth lives due to microstructural variations in Ti-6Al-4V. Finally, possible fatigue mechanisms or scenarios that can lead to the worst-case fatigue life are elucidated via probabilistic modeling of the fatigue-crack-initiation process, the driving force of the grain-sized cracks, as well as the intrinsic (closure-free) threshold and the closure-affected threshold of the small cracks. In the absence of preexisting cracks, the worst-case total fatigue life is obtained when two conditions are met:(1) the crack size at initiation is on the order of 1 to 2 times the grain size, and (2) the driving force (applied ⌬K ) exceeds the intrinsic threshold of the small cracks. The probabilistic results are also used to elucidate the conditions for the occurrence of dual fatigue limits in high-cycle fatigue (HCF) or giga-cycle fatigue.

Estimating fatigue sensitivity to polycrystalline Ni‐base superalloy microstructures using a computational approach

“…The critical cracks formed in the subsurface of the specimens, mainly along slip bands inside grains or crystallographic facets interacting with inclusions, or at the surface at slip bands. They asserted that the variability in fatigue life is not due to crack growth mechanisms; it is dictated by number of cycles required for a crack to form in a slip band and propagate as a microstructurally small crack, which is consistent with other alloy systems 7 . The variability of fatigue life increased as the applied stress amplitude decreased.…”

Mitigating time-dependent crack growth in Ni-base superalloy components

“…Overall, the DARWIN simulations serve to demonstrate the potential benefits of improving disk life and reduced risk of disk fracture when locationspecific microstructure and properties are taken into account when performing life-prediction and risk assessment analyses of gas turbine disks. In general, a fine grain size is beneficial for crack initiation life, while a coarse grain size is beneficial for crack growth life[32,33]. A similar trend appears to be applicable to the tertiary c 0 size.…”

“…In this analysis, the computed disk life is based on crack growth alone without considering the crack initiation life as an appropriate microstructure-based fatigue crack initiation model is not currently available. Since the effects of grain size on crack initiation life is opposite to those for crack growth life [32,33], the conclusion reached in this paper is limited to the crack growth life only and should not be applied to situations where crack initiation life is significant. When dominated by crack initiation, the disk life may be improved by a microstructure of a fine grain size as a fine-grained microstructure is more resistant to fatigue crack initiation compared to a large-grained microstructure.…”

Durability of Fe-Co thin films on Ti-6Al-4V

“…[35] This leads to a G p of 2.4 3 10 4 J/m 2 or higher. The value of G i ranges from 2 to 4 J/m 2 for Fe-Co/Ti-6Al-4V.…”

“…Another possible measure of G p is the fatigue crack growth threshold DK th of small cracks or large cracks at a high R ratio. For Ti-6Al-4V, DK th 5 2 MPa ffiffiffiffi m p [35] for small cracks at R 5 0, and it leads to G p 5 21 J/m 2 and a G i /G p ratio of 0.095 to 0.19 (filled squares in Figure 15). More than ten data points each were used to obtain the average and the standard deviation for the experimental data in Figure 15.…”