Study on small fatigue crack initiation and growth for friction stir welded joints

Small fatigue crack initiation and growth behavior for 7075 aluminum alloy friction stir welded joints are investigated with the replica technique. The effects of microstructure on the initiation and propagation of small cracks are studied under different loading stress levels. The test results show that the initiation and propagation of small cracks are mainly affected by strengthening particles, grain boundaries, rough pits and hardness distribution. Small cracks begin to grow when the cycle numbers are about less than 20% of the fatigue life at the stress ratio of 0.1. With the decrease of stress ratio to - 0.3, the small cracks grow until the cycle numbers are about 30-45% of the fatigue life. The phase of small crack initiation and propagation accounts for about 50-80% of the fatigue life. The growth rate of the major crack is similar to that of other small cracks in the stage of small cracks growth. In the stage of long cracks growth, the growth rate of the major crack is far greater than that of other small cracks. The microstructures of different regions of the joint have an effect on the fracture mode.