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

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.