Parts and structures are often welded together in different ways, as it is cost and weight effective in comparison to conventional bolted and riveted joints. Steel followed by aluminum alloys, are the most frequently welded metal. Welding results in inhomogeneous and different materials near the joint which may lead to defects. These defects may be the cause of initiation and development of cracks as a result of cyclic loading. In the present work fatigue crack growth rate of a mild steel plate welded by friction stir welding (FSW) has been studied under constant amplitude load with different values of R-ratio. Hardness in the base metal was found to be low in comparison to thermo-mechanically affected and weld nugget zone. Grain size of weld zone was much smaller to base metal and it was the same to heat affected zone and base metal. A C-T specimen with notch at welded and non welded region was tested to get the behavior of Fatigue Crack Growth (FCG) at different zones. It has been found that the fatigue crack growth rate in welded material is lower as compared to base material.

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