Improvement mechanisms of fatigue strength in weld joints by hammer peening

The hammer peening process is well known as one of the methods to improve fatigue lives in welded joints. Peening induces a compressive residual stress field near the weld toe in which fatigue crack usually initiates. When this method is applied to large scale welded strictures, the effects of the induced compressive residual stress on fatigue crack initiation stage and propagation stage should be separately comprehended because the balance of both stages in total fatigue life will be changed for different structure scales. In this study, the improvement mechanisms in fatigue crack initiation and propagation lives due to a compressive residual stress field was clarified by experimental observations of crack propagation behavior. The morphologies of propagating surface crack front were quantified in the specimens of plate and gusset welded joint with and without the hammer peening process. The experimental results suggested that the deeper aspect ratio of an elliptical surface crack just after crack initiation and propagation in the peened specimens induced fatigue crack retardation. Fatigue crack retardation in the peened specimens was discussed to clarify the fatigue crack life improvement mechanisms from the viewpoint of the stress intensity factor range suppression due to the crack aspect ratio.