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Advances in Materials Science and Engineering
Volume 2017, Article ID 7189369, 10 pages
Research Article

Very High Cycle Fatigue Crack Initiation Mechanism in Nugget Zone of AA 7075 Friction Stir Welded Joint

1School of Architecture and Civil Engineering, Chengdu University, Chengdu 610106, China
2Micromechanics Laboratory, Faculty of Engineering, Kyushu University, Fukuoka 8190395, Japan
3Department of Mechanical Engineering, Kochi National College of Technology, Nankoku 783-8508, Japan
4Key Laboratory of Energy Engineering Safety and Disaster Mechanics, Ministry of Education, Sichuan University, Chengdu 610065, China

Correspondence should be addressed to Qing-yuan Wang; nc.ude.ucs@yqgnaw

Received 9 June 2017; Accepted 30 August 2017; Published 27 November 2017

Academic Editor: Pavel Lejcek

Copyright © 2017 Chao He et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


Very high cycle fatigue behavior of nugget zone in AA 7075 friction stir welded joint was experimentally investigated using ultrasonic fatigue testing system (20 kHz) to clarify the crack initiation mechanism. It was found that the fatigue strength of nugget zone decreased continuously even beyond 107 cycles with no traditional fatigue limits. Fatigue cracks initiated from the welding defects located at the bottom side of the friction stir weld. Moreover, a special semicircular zone could be characterized around the crack initiation site, of which the stress intensity factor approximately equaled the threshold of fatigue crack propagation rate. Finally, a simplified model was proposed to estimate the fatigue life by correlating the welding defect size and applied stress. The predicted results are in good agreement with the experimental results.