Initiation and Early-Stage Growth of Internal Fatigue Cracking Under Very-High-Cycle Fatigue Regime at High Temperature
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DUCTION
FATIGUE is the single largest failure reason based on the jet engine component distress mode statistics.[1] All engine parts should have a minimum fatigue life of 109 cycles[2] and this number is based on both the laboratory observations and lessons learned from the industry that a fatigue endurance limit (defined as the lower limiting stress amplitude at Nf = 107) does not exist for most metals.[3] An internal fatigue failure mode is particularly important for fatigue life in the very-high-cycle fatigue (VHCF) regime.[4,5] The most characteristic feature of this failure mode is that the fracture surface exhibits a ‘‘fish-eye.’’[6] In almost all cases, the fish-eye appears circular, with a dark area in the center, inside which the crack initiation site is Z. ZHAO is with the School of Materials Science and Engineering, Beihang University, Beijing 100191, China. Contact e-mail: [email protected] F. ZHANG, C. DONG, and X. YANG are with the School of Energy and Power Engineering, Beihang University, Beijing 100191, China. B. CHEN is with the School of Engineering, University of Leicester, Leicester, LE1 7RH, UK. Contact e-mail: [email protected] Manuscript submitted August 8, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
located. Controversy exists as to the presence of this dark area, and hence terms of, for example, optically dark area, fine granular area, and granular bright facet[6] reflect different crack initiation mechanisms and earlystage crack growth behavior that cause such a macroscopic feature. The origin of internal fatigue cracking can be attributed to the presence of the material discontinuity, casting including non-metallic inclusions,[7–11] [12,13] second-phase particles,[14] and some pores, microstructural inhomogeneities[15,16] for a wide range of alloys (e.g., steel,[16–20] titanium,[15] aluminum[21] and Ni-base superalloys in a form of single-crystal,[14,22] directionally solidified,[23] and polycrystal[9,24,25]). In VHCF regime, the cycles spent for the crack initiation can account for a very large fraction of the fatigue life (i.e., Ni/Nf being greater than 90 pct and up to 99 pct[6,7,20,26]). In principle, the fatigue life consumed for crack initiation under VHCF regime should involve both the initiation and early-stage growth process. Unfortunately, their underlying mechanisms have not been fully understood yet.[18,27] The primary limitation to study VHCF crack initiation and early-stage growth is the lack of experimental method to monitor the internal cracking, although there are many techniques for surface fatigue cracking.[28] As
a consequence, attempts to characterize the early-stage growth kinetics of an interior fatigue crack were based on modeling approach that often involves the calculation by subtraction together with the integration of the classic Paris law.[6] For example, Li et al.[5] claimed that the crack propagated at a slow rate of below 1010 m/cycle within the fish-eye. Similarly, the early-stage crack growth rate for Cr-Mo steels under VHCF loading was estimated a
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