Detection of small internal fatigue cracks in Ti‐6Al‐4V via synchrotron radiation nanocomputed tomography

Abstract

Two types of synchrotron radiation computed tomography (SR‐CT)—projection CT (micro‐CT) and phase‐contrast imaging CT (nano‐CT)—were used to observe internal fatigue cracks in (α + β) Ti‐6Al‐4V alloy. Micro‐CT detected cracks in the specimen at ~1 μm spatial resolution, and the nano‐CT provided magnified images at ~200 nm spatial resolution. The crack initiation sites were clarified as the α‐phase for both the surface and internal cracks; however, their opening behaviors differed. A sharp crack tip was observed in the surface crack, and the crack tip opening displacement (CTOD) increased with an increase in the applied load. By contrast, a blunted crack tip, similar to that of a crack in a vacuum, was observed for the internal crack, and its CTOD remained almost constant regardless of the applied load. These phenomena are likely to explain the different behaviors of surface and internal cracks, particularly the slower growth rate of internal cracks, which leads to a longer fatigue life in the very high cycle fatigue regime.

Observation of flaking process in rolling contact fatigue of bearing steel with spheroidal inclusions by laminography using ultrabright synchrotron radiation X‐rays

“…Therefore, it has been difficult to sequentially observe the RCF process. On the other hand, three‐dimensional imaging of cracks in steel with submicron resolution by computed tomography using ultrabright synchrotron radiation X‐rays (SR‐μCT) have succeeded 42,52–54 . We also observed microstructural alternations related to fatigue crack initiation by diffraction contrast tomography (DCT) using ultrabright synchrotron radiation X‐rays 55,56 …”

“…On the other hand, three-dimensional imaging of cracks in steel with submicron resolution by computed tomography using ultrabright synchrotron radiation X-rays (SR-μCT) have succeeded. 42,[52][53][54] We also observed microstructural alternations related to fatigue crack initiation by diffraction contrast tomography (DCT) using ultrabright synchrotron radiation X-rays. 55,56 We first performed RCF tests on specimens with EDM-machined circular holes simulating stringer-shaped inclusions of various diameters and depths, and we then observed the crack initiation morphology by SR-μCT of the specimens cut out including the holes.…”

Environment effect on internal fatigue crack propagation studied with in-situ X-ray microtomography

“…The influence of environment on 3D louis.hebrard@insa-lyon.fr (L. Hébrard) ORCID(s): 0000-0002-2380-0727 (T. Palin-Luc) crack propagation has been observed for example in presence of humidity [42] or in vacuum by imaging internal crack propagation. [7], [29] and [51] could detect many internal cracks in a single Ti alloy specimen with a synchrotron µCT approach. [41] have followed a crack that initiated from an internal casting defect in an Al alloy and [30] could detect an internal crack initiation from an inclusion in a Ni-based super-alloy also with this method.…”

Virtual special issue: Fatigue 2021: The properties, processing, and performance of structural materials

“…Once again, this research will support greater accuracy to the definition of appropriate stress intensity factors. In contrast, Xue et al 7 focus their attention on earliest stages of fatigue crack initiation, in particular the phenomenon of multiple subsurface cracking that can be a common feature of fatigue failures in titanium alloys. Planar slip damage leading to cracking in alpha phase grains was invariably detected as the source of cracking; however, differences in crack tip opening morphologies were noted for surface and subsurface cracks.…”