Fatigue crack initiation and growth in AlMg4.5Mn butt weldments

The fatigue life of full-penetration and partial-penetration 5 and 25 mm thickness AlMg4.5Mn (AA5083) aluminium alloy butt weldments was investigated under (R = 0 and R= -1) constant amplitude loading. The fatigue lives of the tested specimens were predicted using an analytical model which estimated both the crack initiation and crack growth portions of the total fatigue life. The fatigue life of partial-penetration weldments was found to be substantially less than that of full-penetration weldments because of the greater stress concentrations of the incomplete joint: penetration and the consequent absence of a substantial crack-initiation life period. Tensile mean stresses (R = 0 versus R = -1-test conditions) markedly reduced the fatigue life of the weldments studied and greatly diminished the duration of the fatigue crack growth period. The extra material provided by the weld reinforcement noticeably increased the fatigue life of the partial-penetration weldments. Weld angular distortion-induced bending stresses greatly affected the smaller thickness (5 mm) full-penetration weldments offsetting the Fatigue strength bonus anticipated for small-size weldments. Except for the predictions for R = -1 full-penetration weldments at long life, which the analytical model underestimated, the agreement between experiment and analytical prediction was within a factor of 2, that is, as good as can be generally expected.