Fatigue life improvement for cruciform welded joint by mechanical surface treatment using hammer peening and ultrasonic nanocrystal surface modification*

For the improvement of the fatigue strength for welded structures, mechanical posttreatments have been applied in various industrial fields and in most cases have been found to give substantial increases in their fatigue lives. These methods, generally, consist of the modification of weld toe geometry and the introduction of compressive residual stresses. In mechanical surface treatments, for example, PHP (pneumatic hammer peening) and UNSM (ultrasonic nanocrystal surface modification), the weld profile is modified due to removed or reduced minute crack-like flaws, and compressive residual stresses are also induced. In this study, a PHP procedure and a UNSM device were introduced, and a quantitative measure of fatigue strength improvement was performed. The fatigue strength at 2 x 10(6) cycles of hammer-peened and UNSM treated on a non-load-carrying cruciform welded joint shows 220 and 260 MPa, respectively, which are more than two times higher than that of as-welded specimen. Especially, the surface layer in the vicinity weld toe treated by the UNSM provides nanocrystal structure created by an ultrasonic cold forging and introduces very high welding residual stress in compression.