Residual Stress Effects on Near-Threshold Fatigue Crack Growth in Friction Stir Welds

摘要

Friction stir welding is being explored as a potential tool for manufacturing aluminum aerospace structures. Several joint configurations, butt, lap and fillet joints have been produced in the production of such exploratory structures. Research work on S-N fatigue and fatigue crack growth in the weld zone is required to provide an understanding and tools to assess the damage tolerance issues in friction stir welded joints and structures. In this work, results of a study conducted on near-threshold fatigue crack growth in friction stir welded aluminum alloy 7050-T7451 are presented. Tests were conducted on weld coupons as a function of specimen geometry (compact tension and center-crack tension) and stress ratio to understand the effects of residual stresses in the heat affected zone (HAZ) of the alloy. Residual stresses were measured on a friction stir welded plate prior to testing. The crack growth results show that residual stresses play a key role in the crack growth parallel to the weld-path in the HAZ. Although friction Stir welding process induced low residual stresses in the welds, they are large enough to affect the near-threshold fatigue crack growth. At low stress ratio, the fatigue threshold in the HAZ was either Higher or lower than that of the baseline material depending on the geometry. At high stress ratio, the differences due to the geometry vanish. Center-crack tension geometry is less sensitive to the residual stress effects compared to the compact tension geometry. Crack growth analysis using equivalent residual stresses was used to determine the residual stress intensity factor and predict stress ratio effects.