An investigation into methods to increase the fatigue life of friction stir lap welds
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Recent advancements in friction stir welding (FSW) technology have potential for applications in aerospace structures. Friction stir spot welds have been found to be much stronger than rivets in the same material thickness, while maintaining the discontinuous crack growth path preferred by aircraft designers. In this study, the test coupons have been investigated in fatigue with the weld aligned with the loading direction. The purpose of this study was to better understand crack initiation at friction stir weld exit holes in no-load transfer coupons representative of aircraft fuselage applications. The goal was to document the effects of weld exit location on fatigue life in discontinuous friction stir welded panels and to determine possible solutions in order to reduce the stress concentration around the exit hole location, thereby increasing the panel’s fatigue life. Aluminum alloys 7075-T6 and 2024-T3, which are commonly used in conventional airframe construction, were chosen for the FSW lap welds in this thesis. The methodology of this research was to weld the coupons with discontinuous friction stir lap welding using different exit hole configurations. The weld parameters such as rotation speed, travel speed, lead angle and load force had already been evaluated prior to this investigation in an earlier study during the first year of this project by Josh Merry. Once all of the coupons were welded, the next step was to fatigue test them with constant amplitude in order to determine the number of fatigue cycles and then compare all the different coupon results with the baseline coupon result that were determined in the previous study. This project investigated a number of weld exit strategies with conventional one-piece weld tools. This study also included welds produced with a two-piece weld tool called Retractable Pin Tool (RPT) in which the length of the tool probe can be adjusted during welding. The RPT weld tool was used to eliminate the exit hole of the weld in order to reduce or eliminate viii the stress concentration around the weld exit. The ultimate goal of this research was to achieve equivalent or better fatigue life in discontinuous FSW joints as compared to riveted coupons.
Thesis (M.S.)--Wichita State University, College of Engineering, Dept. of Mechanical Engineering.