Multi axial fatigue IN welded components

In order to study the fatigue behaviour of 6000 aluminium alloy welded structures and to take into account the small size of the affected zone, a specific heat treatment was developed to get a structure equivalent to that of the heat affected zone (simulated HAZ). Such an equivalent structure was determined by comparing hardness, microstructure and tensile properties of both the ZAT and the equivalent material. A new cumulative damage law has been developed taking in account the loading history. Several existing multiaxial fatigue criteria (Dang Van K.[1], Findley[2], Matake [3], McDiarmid [4]) have been evaluated under Castem 2000. The misalignment, the local geometry and the residual stresses have been considered in the numerical model. Specimen were tested in the High Cycle Fatigue regime, under uniaxial and in-phase and out-of-phase biaxial loadings. The results of all simulations were compared to the test data. The position of the critical plane given by the simulations was also compared to the test results. Some criteria (Dang Van, K. & McDiarmid) were found to be conservative. The criterion of McDiarmid was found to be the best whatever the loading, while the criteria of Dang Van and Matake were found to give out life predictions not enough precise, the proportion between the test results and the life assessments being sometimes larger than 10. The criterion of Findley did not agree well with the test results. A general discussion in the light of the multi axial criterion in biaxial loading (in phase and out phase) is made and compared to the IIW recommendations.