MSA  Vol.9 No.13 , December 2018
Study on Fatigue Lifetimes and Their Variation of Mg Alloy AZ61 at Various Stress Ratios
Abstract: In this study, fatigue tests under different R ratios were conducted on the AZ61 Mg alloy to investigate its fatigue lifetimes and fatigue crack growth (FCG) behavior. The fracture surface of the failed specimens was investigated using a scanning electron microscope to study the size of the intermetallic compounds from which the pioneer fatigue crack initiated and led to the final failure of the specimen. To determine the maximum size of the intermetallic compounds existing within the cross section of the specimen at higher risk, Gumbel’s extreme-value statistics were utilized. In the present study, the intermetallic compounds contained within the specimen were assumed to be the initial cracks existing in the material before the fatigue tests. A modified linear elastic fracture-mechanics parameter, M, proposed by McEvily et al., was used to analyze the short FCG behavior under different stress ratios, R. The relation between the rate of FCG and M parameter was found to be useful and appropriate for predicting the fatigue lifetimes under different R ratios. Moreover, the probabilistic stress-fatigue life (P-S-N) curve of the material under different R ratios could be predicted with this method, which utilizes both the FCG law and a statistical distribution of sizes of the most dangerous intermetallic compounds. The evaluated results were in good agreement with the experimental ones. This correspondence indicates that the estimation method proposed in the present study is effective for evaluation of the probabilistic stress-fatigue life (P-S-N) curve of the material under different R ratios.
Cite this paper: Masuda, K. , Ishihara, S. , Ishiguro, M. and Shibata, H. (2018) Study on Fatigue Lifetimes and Their Variation of Mg Alloy AZ61 at Various Stress Ratios. Materials Sciences and Applications, 9, 993-1007. doi: 10.4236/msa.2018.913072.

[1]   Zheng, S.L., Yu, Q. and Jiang, Y.Y. (2013) An Experimental Study of Fatigue Crack Propagation in Extruded AZ31B Magnesium Alloy. International Journal of Fatigue, 47, 174-183.

[2]   Bernard, J.D., Jordon, J.B., Lugo, M., Hughes, J.M., Raybom, D.C. and Horstemeyer, M.F. (2013) Observations and Modeling of the Small Fatigue Crack Behavior of an Extruded AZ61 Magnesium Alloy. International Journal of Fatigue, 52, 20-29.

[3]   Castro, F. and Jiang, Y.Y. (2016) Fatigue Life and Early Cracking Predictions of Extruded AZ31B Magnesium Alloy Using Critical Plane Approaches. International Journal of Fatigue, 88, 236-246.

[4]   Wang, C., Luo, T.J., Zhou, J.X. and Tang, Y.S. (2017) Anisotropic Cyclic Deformation Behavior of Extruded ZA81M Magnesium Alloy. International Journal of Fatigue, 96, 178-184.

[5]   Jordon, J.B., Brown, H.R., Kadiri, H.E., Kistler, H.M., Lett, R.L., Baird, J.C. and Luo, A.A. (2013) Investigation of Fatigue Anisotropy in an Extruded Magnesium Alloy. International Journal of Fatigue, 51, 8-14.

[6]   Gumbel, E.J. (1958) Statistics of Extremes. Columbia University Press, New York.

[7]   McEvily, A.J., Eifler, D. and Macherauch, E. (1991) An Analysis of the Growth of Short Fatigue Cracks. Engineering Fracture Mechanics, 40, 571-584.

[8]   Irwin, G.R. (1958) Fracture, Elasticity and Plasticity. Springer, Berlin, 551-590.

[9]   Dugdale, D.S. (1960) Yielding of Steel Sheets Containing Slits. Journal of the Mechanics and Physics of Solids, 8, 100-104.

[10]   McEvily, A.J. and Minakawa, K. (1984) Crack Closure and the Growth of Short and Long Fatigue Cracks. Department of Metallurgy, 18, 215-233.

[11]   Kitagawa, H. (1976) Applicability of Fracture Mechanics to Very Small Cracks or the Cracks in the Early Stage. Proceedings of 2nd ICM, Cleveland, 627-631.

[12]   McEvily, A.J., Ishihara, S., Endo, M., Sakai, H. and Matsunaga, H. (2007) On One- and Two-Parameter Analyses of Short Fatigue crack Growth. International Journal of Fatigue, 29, 2237-2245.

[13]   McEvily, A.J., Ishihara, S. and Endo, M. (2005) An Analysis of Multiple Two-Step Fatigue Loading. International Journal of Fatigue, 27, 862-866.

[14]   Ishihara, S., McEvily, A.J., Sato, M., Taniguchi, K. and Goshima, T. (2009) The Effect of Load Ratio on Fatigue Life and Crack Propagation Behavior of an Extruded Magnesium Alloy. International Journal of Fatigue, 31, 1788-1794.

[15]   Murakami, Y. (1993) Metal Fatigue: Effects of Small Defects and Nonmetallic Inclusions. Yokendo, Tokyo, 17.

[16]   El Haddad, M.H., Topper, T.H. and Smith, K.N. (1979) Prediction of Non Propagating Cracks. Engineering Fracture Mechanics, 11, 573-584.

[17]   ASTM (1990) Standard Test Method for Plane-Strain Fracture Toughness of Metallic Materials. E 399-90, Annual Book of ASTM Standards, Part 10.

[18]   Kikukawa, M., Jono, M., Tanaka, K. and Takatani, M. (1976) Measurement of Fatigue Crack Propagation and Crack Closure at Low Stress Intensity Level by Unloading Elastic Compliance Method. Journal of the Society of Materials Science, Japan, 25, 899-903. (In Japanese)