PSI - Issue 7

A. Rotella et al. / Procedia Structural Integrity 7 (2017) 513–520 Antonio Rotella et al. / Structural Integrity Procedia 00 (2017) 000–000

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4. Conclusion High cycle fatigue tests have been conducted for a positive load ratio (R=0.1) on specimens affected by the presence of cavity and sponge shrinkages with a defect grade ranging from 2 to 4. The experimental results highlighted that the impact on the fatigue limit of a natural shrinkage with a grade ranging from 2 to 3 is notable and reduce the fatigue limit with respect to the reference material (grade < 1) of about 50%. This effect is higher for a defect grade of 4. Nevertheless any substantial difference have been detected between two distinct defect families (cavity and sponge shrinkages) on the fatigue limit. The effect of a local perturbation of the defect morphology have been investigated introducing artificial surface defects. A modified spherical defect (addition of a local morphological variation at the defect tip) have the same impact on the fatigue limit of a simple spherical defect. A Kitagawa-Takahashi diagram has been plotted for both natural and artificial defects. A reduction of the fatigue limit have been identified between a normalized defect size of 0.05 and 0.2, the diagram saturates for a higher defect size. Finite elements simulations have been conducted on the real defect geometry reconstructed after a µ-CT scan. The stress /strain field around the natural pore have been investigated, resulting into a higher K t pl when the defect is close to free surface of the specimen. The results have been compared with two equivalent geometries, the ellipsoid give a good approximation over the K t pl factor, resulting into an average absolute difference of about 3% with respect to the real shrinkage. The sphere gives a worse approximation resulting into an average absolute difference of about 11%. References ASTM standard E2422-11, 2011. Standard Digital Reference Images for Inspection of Aluminum Castings. Bellows, R.S., Muju, S., Nicholas, T., 1999. Validation of the step test method for generating Haigh diagrams for Ti–6Al–4V. Int. J. Fatigue 21, 687–697. Billaudeau, T., Nadot, Y., Bezine, G., 2004. Multiaxial fatigue limit for defective materials: mechanisms and experiments. Acta Mater. 52, 3911– 3920. Buffière, J.-Y., Savelli, S., Jouneau, P.-H., Maire, E., Fougeres, R., 2001. Experimental study of porosity and its relation to fatigue mechanisms of model Al–Si7–Mg0. 3 cast Al alloys. Mater. Sci. Eng. A 316, 115–126. Iben Houria, M., Nadot, Y., Fathallah, R., Roy, M., Maijer, D.M., 2015. Influence of casting defect and SDAS on the multiaxial fatigue behaviour of A356-T6 alloy including mean stress effect. Int. J. Fatigue 80, 90–102. Kitagawa, H., Takahashi, S., 1976. Applicability of fracture mechanics to very small cracks or the cracks in the early stage. In Second International Conference on Mechanical Behavior of Materials, Boston, 1976, 627-631, p. Lanning, D., 2005. On the use of critical distance theories for the prediction of the high cycle fatigue limit stress in notched Ti-6Al-4V. Int. J. Fatigue 27, 45–57. Le, V.-D., Morel, F., Bellett, D., Saintier, N., Osmond, P., 2016. Multiaxial high cycle fatigue damage mechanisms associated with the different microstructural heterogeneities of cast aluminium alloys. Mater. Sci. Eng. A 649, 426–440. Lemaitre, J., Chaboche, J.-L., Benallal, A., Desmorat, R., 2009. Mécanique des matériaux solides - 3ème édition (Dunod). Li, P., Lee, P.D., Maijer, D.M., Lindley, T.C., 2009. Quantification of the interaction within defect populations on fatigue behavior in an aluminum alloy. Acta Mater. 57, 3539–3548. Luo, Z., and Zhong, E., 2005. Derivation of Formula for Any Polygonal Area and Its Applications. Coll. Math. 21, 123–125. Mu, P., Nadot, Y., Nadot-Martin, C., Chabod, A., Serrano-Munoz, I., Verdu, C., 2014a. Influence of casting defects on the fatigue behavior of cast aluminum AS7G06-T6. Int. J. Fatigue 63, 97–109. Murakami Y., 2002. Effects of Small Defects and Nonmetallic Inclusions (Amsterdam: Elsevier). Nadot, Y., Ranganathan, N., Mendez, J., Béranger, A.S., 1997. A study of natural cracks initiated on casting defects by crack front marking. Scr. Mater. 37, 549–553. Nicoletto, G., Konečná, R., Fintova, S., 2012. Characterization of microshrinkage casting defects of Al–Si alloys by X-ray computed tomography and metallography. Int. J. Fatigue 41, 39–46. Roy, M., Nadot, Y., Maijer, D.M., Benoit, G., 2012. Multiaxial fatigue behaviour of A356-T6. Fatigue Fract. Eng. Mater. Struct. 35, 1148–1159. Serrano-Munoz, I., Buffiere, J.-Y., Mokso, R., Verdu, C., Nadot, Y., 2017. Location, location & size: defects close to surfaces dominate fatigue crack initiation. Sci. Rep. 7, 45239. Wang, Q.G., 2004. Plastic deformation behavior of Aluminum casting alloys A356/357. Metall. Mater. Trans. A 35, 2707–2718. Wang, Q.G., Apelian, D., Lados, D.A., 2001. Fatigue behavior of A356/357 aluminum cast alloys. Part II–Effect of microstructural constituents. J. Light Met. 1, 85–97. Wang, Q.G., Praud, M., Needleman, A., Kim, K.S., Griffiths, J.R., Davidson, C.J., Cáceres, C.H., Benzerga, A.A., 2010. Size e ffects in aluminium alloy castings. Acta Mater. 58, 3006–3013.