Assessment of the fatigue crack closure phenomenon in damage-tolerant aluminium alloy byin-situhigh-resolution synchrotron X-ray microtomography

Abstract

Synchrotron X-ray microtomography has been utilized for the in-situ observation of steady state plane strain fatigue crack growth. A high resolution experimental configuration and phase contrast imaging technique have enabled the reconstruction of crack images with an isotropic voxel with a 0.7 µm edge. The details of a crack are readily observed, along with evidence of the incidence and mechanical influence of closure. After preliminary investigations of the achievable accuracy and reproducibility, a variety of measurement methods are used to quantify crack opening displacement (COD) and closure from the tomography data. Utilization of the physical displacements of microstructural features is proposed to obtain detailed COD data, and its feasibility is confirmed. Loss of fracture surface contact occurs gradually up to the maximum load. This is significantly different from tendencies reported where a single definable opening level is essentially assumed to exist. The closure behaviour is found to be attributable mainly to remarkable generation of mode III displacement which may be caused by local crack topology. Many small points of closure still remain near the crack-tip, suggesting that the near-tip contact induces crack growth resistance. The effects of overloading are also discussed.

Digital image correlation and fracture: an advanced technique for estimating stress intensity factors of 2D and 3D cracks

“…In particular, cracks can be observed [47,48], their closure [49], ‡ The latter is sometimes referred to as Volumetric DIC or V-DIC and CODs measured by manually tracking particles [50]. When the crack morphology is determined, X-FEM techniques allow for the evaluation of surface vs. bulk propagation features [51].…”

Three dimensional experimental and numerical multiscale analysis of a fatigue crack

“…Experimentally determined values of SIF along a crack front had been obtained in the past from the analysis of high resolution (voxel size of 0.7 µm) 3D tomographic images [68,69]. In that case, small porosities close to the tip of a crack were used as markers for "manual" tracking of the displacement field at the crack tip.…”

Visualising complex morphology of fatigue cracks in voxel based 3D datasets

“…Because of the restricted numbers of photons, however, the noise level might be higher for the tomography using second-generation synchrotron radiation sources. Toda et al 2 pointed out that noise and artefacts, which are directly reflected in the quantitative results such as crack-opening, are not specifically described in the literature. Consequently, the presented quantitative data are somehow questionable, especially if sub-voxel interpolation is integrated.…”