In situ analysis of cracks in structural materials using synchrotron X-ray tomography and diffraction
Introduction
High-energy synchrotron X-ray radiation is establishing itself as a unique tool for the characterisation of materials on very small length scales in bulk materials [1], [2], [3]. It is now possible to study features on length scales on a par with the grain size in many engineering materials. In this paper, we report on recent progress in the high-resolution visualisation of fatigue cracks and measurements of strains in the immediate vicinity of a fatigue crack tip in a 1 mm thick, ultra-fine-grained aluminium alloy 5091 (Al–Li–Mg–C–O). The objective was to study local geometry of fatigue crack growth (via tomography) and measure associated crack-tip strains/stresses, in particular with respect to crack closure. The measured data can then be used for comparison with and validation of the results of finite element prediction and correlated with the visualisation of the crack geometry through high-resolution tomography. Very high spatial resolution of approximately 1 μm and 20 μm for tomography and diffraction, respectively, was achieved in a thin specimen for which the plane-stress condition can be assumed. The validation of experimental models with existing data has important implications for the life-prediction of safety critical engineering components. The measurements were undertaken on beam lines ID11 and ID19 at the European Synchrotron Radiation Facility (ESRF) in Grenoble as part of a long-term proposal studying crack behaviour and growth. This paper is intended to give an overview on the recent work with a description of the techniques used, based on the experimental results of a specimen with fatigue crack that was subjected to a 100% overload. A complete discussion of the materials science or engineering aspects of the findings is beyond the scope of this paper and will be reported elsewhere.
Section snippets
Background
Despite nearly thirty years of study, a comprehensive understanding of the fundamental mechanism of fatigue crack propagation has yet to be established, especially for overload/underload interaction under variable amplitude loading [4], [5], [6], [7]. One of the main reasons for this has been the lack of direct quantitative data describing the actual complete crack-tip stress/strain field accompanying fatigue crack growth. It is now widely accepted that for fatigue crack propagation the linear
The material
The material used was the aluminium–lithium alloy 5091 (Al–Li–Mg–C–O). This material [14], [15] which is prepared by a mechanical alloying, powder metallurgy route, has an ultra-fine-grained microstructure with relatively equiaxed grains of less than 1 μm size, stabilised by dispersions of 20–50 nm aluminium oxides (Al2O3) and carbides (Al4C). The very small grain size makes it an ideal material for high-resolution investigation by X-ray diffraction since it allows very narrow slits to be used
Results
In this section, the different results are presented and compared with the modelling predictions, followed by a discussion.
Conclusions
In this paper, we report on the direct measurement of the residual strains accompanying a fatigue crack in a 1 mm thick metallic specimen and how they are affected by a 100% overload. The results reveal a compressive enclave in the crack wake and a significant compressive zone at and behind the crack tip following a 100% overload. This work is part of an ongoing program to develop direct, in situ measurements of the effects of crack closure on local stresses ahead of the crack tip and in the
Acknowledgements
The authors would like to thank the ESRF for providing access to beam time under the proposals HS-2252 and the staff of ID11 as well as ID19 for their support.
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