In this paper, crack growth tests on 19501 aluminum alloy friction stir welded butt joints after a single overload cycle in tension or in compression on compact tension (CT) specimens, along with constant amplitude tests, are investigated. The effect of the tensile overload or compressive overload applied just after 20,000 load cycles on the crack growth rate, crack growth path, and growth profile are investigated. The overload cycle exhibits a significant influence on fatigue life endurance. Increases of 39.14 % and 131.58 % in fatigue life with delayed cycles of 13,874 and 46,646 are seen because of the application of 1.78 times tension or compression overload. The crack propagation after overload cycles is investigated by inspection of the fatigue crack propagation rate. Results are discussed with reference to the retardation characteristic parameters, such as retardation ratio, plastic zone size, crack deviation, etc. Compressive overload is found to be more effective in retarding the growth rates compared to tensile overload of the same magnitude in friction stir welded joints. Secondary crack formation is seen after the tensile overload, whereas no such secondary crack formation is observed with compressive overload. The possible reasons for the crack growth behavior due to tensile or compressive overload are further analyzed in terms of scanning electron micrographs and plastic zone size.
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