In situ analysis of cracks in structural materials using synchrotron X-ray tomography and diffraction

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Abstract

The structural integrity and performance of many components and structures are dominated by cracks and hence the study of cracked bodies study is of major economical and social importance. Despite nearly 30 years of study, there is still no detailed consensus regarding either the fundamental parameters that drive cracks or the precise mechanisms of their growth in most materials. Thus, virtually all crack life prediction models currently in engineering use are largely phenomenological rather than physically based. Historically, a major hindrance to our understanding of crack initiation and propagation has been the inability to measure either the crack tip stresses or the crack morphology deep within materials. The development of very high-resolution strain and tomography mapping on third generation synchrotron sources such as the ESRF has opened up the possibility of developing complementary techniques to monitor the entire plastic/process zone growth mechanisms and the accompanying crack tip field and crack wake field around growing cracks. If realized, such techniques would produce unique information that would be invaluable both in validating present finite element simulations of fatigue crack growth and in developing the future high accuracy simulations necessary for the development of physically realistic fatigue life-prediction models. Recent technique developments at the ESRF, Grenoble, opens up the possibility of imaging cracks and crack tip stress/strain fields, and the ability to study the extend of crack closure and overload effects, even under in situ loading. In this paper, first results from synchrotron X-ray diffraction and tomography experiments performed on ID11 and ID19 (respectively) at the ESRF, Grenoble, are presented and discussed in comparison with predictions from finite element modeling.

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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