PSI - Issue 7

Jean-Yves Buffiere / Procedia Structural Integrity 7 (2017) 27 – 32

31

Jean-Yves Bu ffi ere / Structural Integrity Procedia 00 (2017) 000–000

5

regular crack font. For the reasons evoked in section 2, however, this transition from a short to a long crack regime where the crack is crossing several tens of grains, has never been imaged in 3D yet.

5. Conclusion

Imaging fatigue cracks in 3D can be performed using X ray tomography. Because such cracks have a small opening, the best images are obtained with synchrotron tomography which permits to use phase contrast in order to detect sub pixel features. The current limits of the method in terms of specimen size (resulting in small fatigue cracks and small defects) have been underlined. Crack initiation and propagation from natural defects has been performed mainly in cast Al alloys and have confirmed that internal defects are generally less critical than internal ones unless the level of plasticity in the sample is high. Artificial notches are an e ffi cient way of initiating cracks and the use of di ff raction contrast tomography o ff ers the possibility to analyse local crack propagation from those defects in relation with local crystallographic orientations. The spatial resolution reached by 3D observations can give access to the crack font with a reasonable confidence but the details of the crack deviations are still missing at a scale relevant for an e ffi cient physical simulation. A mesoscopic approach based on K t (resp. K ) calculation might be more appropriate to predict crack initiation (resp. propagation), respectively but the extraction of K values along a complex curved crack front and a comparison with experimental data has not been achieved yet.

Acknowledgements

The author is sincerely grateful to many students, colleagues and friends who have contributed to the studies which have been very partially and imperfectly mentioned here and whose names appear in the list of references.

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