Issue 41

P. Lorenzino et alii, Frattura ed Integrità Strutturale, 41 (2017) 191-196; DOI: 10.3221/IGF-ESIS.41.26 196 Although this has rarely been reported for 3D part-through cracks [10], this is consistent with the experimental observation of a grain size gradient at the surface of material IV. A smaller grain size restricts the plastic zone size (Hall Petch effect) and therefore the tunneling effect is reduced or even suppressed. Experiments at larger R ratio on materials III and IV (not shown here) tend to support this interpretation. C ONCLUSION n situ fatigue tests monitored by synchrotron X-ray tomography have been carried out on four different forged materials (two different steels + two different forging processes). The residual stresses which have been measured in some of the bulk materials have been released in the fatigue samples because of their small size. For the experimental conditions investigated, it was found that there is no influence of the forging process on crack growth curves. Differences in crack front shapes have been observed for the material which has been shot blasted. Those differences are interpreted in terms of the modifications induced in the sub-surface microstructure by the forging processes: a reduction of sub- surface crack closure due to a local increase of the yield stress. A CKNOWLEDGEMENTS his project has been funded by the French Agence Nationale de la Recherche (Defisurf project). The authors want to thank Prof F.Morel and Dr. E.Pessard for fruitful discussions. R EFERENCES [1] Gerin, B., Pessard, E., Morel, F., Verdu, C.,Mary, A., Beneficial effect of prestrain due to cold extrusion on the multiaxial fatigue strength of a 27MnCr5 steel International Journal of Fatigue 92 (2016) 345–359. DOI: 10.1016/j.ijfatigue.2016.07.012 [2] Gerin, B., Pessard, E., Morel, F., Verdu, C., Influence of surface integrity on the fatigue behaviour of a hot-forged and shot-peened C70 steel component Materials Science & Engineering A 686 (2017) 121–133. DOI: 10.1016/j.msea.2017.01.041. [3] Buffiere, J.-Y., Maire, E., Adrien, J., ·Masse, J.-P., Boller, E., In Situ Experiments with X ray Tomography: An Attractive Tool for Experimental Mechanics Experimental Mechanics 50 (2010) 289–305. DOI: 10.1007/s11340-010-9333-7. [4] Lachambre, J.,Réthoré, J.,Weck, A., Buffiere, J.-Y., Extraction of stress intensity factors for 3D small fatigue cracks using digital volume correlation and X-ray tomography International Journal of Fatigue 71 (2015) 3–10. DOI: 10.1016/j.ijfatigue.2014.03.022. [5] P. Willmott, P., An Introduction to Synchrotron Radiation: Techniques and Applications, John Wiley & Sons, (2011). [6] Buffiere, J.-Y., Ferrie, E., Proudhon, H., Ludwig, W., Three dimensional visualisation of fatigue cracks in metals using high resolution synchrotron X-ray micro-tomography. Mater. Sc. Technol., 22(9)(2006) 1019–1024. [7] Murakami Y., Metal fatigue: effect of small defects and non-metallic inclusions. Elsevier, (2002). [8] Newman, J.C., Raju, I.S., An empirical stress-intensity factor equation for the surface crack Eng. Frac. Mech., 15 (1981) 185–192. [9] Dawicke, D.S., Grandt, A.F., Newman, J.C., Three-dimensional crack closure behavior Engineering Fracture Mechanics, 36(1) (1990) 111-121. [10] Ferrié, E., Buffiere, J.-Y., Ludwig, W., Gravouil, A., Edwards, L., Fatigue crack propagation: In situ visualization using X-ray microtomography and 3D simulation using the extended finite element method Acta Materialia, 54(4) (2006) 1111-1122. DOI: 10.1016/j.actamat.2005.10.053. I T