PSI - Issue 7

Y. Nadot et al. / Procedia Structural Integrity 7 (2017) 530–535

535

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NADOT Yves / Structural Integrity Procedia 00 (2017) 000–000

Using Stromeyer’s interpolation curves, maximum stresses at five million of cycles were determined for the two microstructures and for each defect size. These fatigue limits at five million of cycles, noted σ D , were traced in function of defect diameter in Fig. 3, for the two grain sizes. The two Kitagawa diagrams obtained are for uniaxial tension/compression loading (R = 1). Several observations can be noted: For a defect size fixed, the fatigue strength for fine microstructure is more sensitive to defect influence than for coarse microstructure. Indeed, in a case of defect diameter D = 1000 µ m, for D g = 30 µ m, there is a 32% reduction in fatigue limit; whereas for D g = 340 µ m it is 11%. So, when the grain diameter D g grows, the fatigue limit σ D becomes less sensitive to the defect: the reduction is divided by three. The finer the grain, the larger is the decrease observed. When increasing the size of microstructure, the presence of the defect becomes less significant. When defect diameter D grows, the fatigue limit σ D becomes less sensitive to the grain diameter; fatigue limit σ D is more governed by defect diameter. For D close to zero, σ D gap between two microstructures is 85 MPa and for D = 1000 µ m, σ D gap is 35 MPa. So, when defect diameter D grows (1000), the fatigue limit gap between two microstructures is divided by 2.4. 4. Conclusions Through this experimental work, we have obtained data on the fatigue behavior (fatigue limit at five million cycles) of Armco iron with hemispherical surface defect (manufactured by EDM) under uniaxial tension loading (R = 1, 10 Hz) for various defect diameters and two grain diameters in order to get a wide range of defect size to grain size ratios, including the case where the defect size is smaller than the grain size. - When grain size increases, the fatigue limit of sound material decreases notably. For grain size fixed, an increase of the defect size induces, as expected, a marked decrease in fatigue limit. - While keeping defect size fixed, the defect sensitivity regarding fatigue limit is even greater when the microstructure is fine. - When Kitagawa diagrams are presented in relative values (respected to grain size), there is a single curve that combines the two microstructures. This dimensionless Kitagawa diagram allows to analyse the reduction of fatigue limit caused by defect. It makes clear that the actual physical size of the defect does not seem to be as important as the relative size of the defect with respect to the characteristic microstructural dimension. [1] Murakami Y. Metal fatigue: effects of small defects and nonmetallic inclusions. Oxford: Elsevier Science Ltd; 2002. [2] Murakami Y, Endo M. Effects of defects, inclusions and inhomogeneities on fatigue strength. Int J Fatigue 1994;16:163–82. [3] Li P, Maijer D, Lindley T, Lee P. A through process model of the impact of inservice loading, residual stress, and microstructure on the final fatigue life of an A356 automotive wheel. Mater Sci Eng: A 2007;460–461:20–30. [4] Vallellano C, Mariscal MR, Navarro A, Dominguez J. A micromechanical approach to fatigue in small notches. Fatigue Fract Eng Mater Struct 2005;28:1035–45. [5] Buffiere J-Y, Savelli S, Jouneau P, Maire E, Fougeres R. Experimental study of porosity and its relation to fatigue mechanisms of model Al– Si7–Mg0.3 cast Al alloys. Mater Sci Eng: A 2001;316:115–26. [6] Nicoletto G, Konecna R, Fintova S. Characterization of microshrinkage casting defects of Al–Si alloys by X-ray computed tomography and metallography. Int J Fatigue 2012;41:39–46. [7] Pippan R. Threshold and effective threshold of fatigue crack propagation in ARMCO iron I: the influence of grain size and cold working. Mater Sci Eng: A 1991;138:1–13. [8] Bellows RS, Muju S, Nicholas T. Validation of the step test method for generating Haigh diagrams for Ti6al4v. Int J Fatigue References