Local and structural multiaxial stress states in welded joints under fatigue loading

A cylindrical stiffener on a plate and a tube through a hollowed plate, both circularly welded, were tested under uniaxial fatigue loading. Their fatigue cracking behaviour was seen to be really complex due to the fact that crack initiation sites changed their position as the number of cycles to failure increased. To investigate this anomalous behaviour, an accurate numerical investigation was carried out to study the distribution of both local and structural linear elastic stresses along the weld toe circumferences. The numerical results proved that the weld beads were subjected to complex stress states, even though the applied nominal load was uniaxial. By the light of this evidence, the fatigue behaviour of the investigated welded joints was then re-interpreted from a multiaxial fatigue point of view by applying the Modified Wöhler Curve Method in terms of hot-spot stresses. The proposed approach was seen to be successful allowing us to estimate both crack initiation sites and fatigue lifetime with a high precision level. This fact is very interesting because it strongly supports the idea that our method can be used to assess real welded components subjected to multiaxial fatigue loading by simply post-processing linear-elastic finite-element results.