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

Abstract

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

Fatigue damage criteria classification, modelling developments and trends for welded joints in marine structures

“…Several two-stage two-parameter models have been proposed involving an assumed crack transition size a t (e.g. Brandt et al 2001;Lassen et al 2005;Darcis et al 2006;Lassen and Recho 2009;Chattopadhyay et al 2011) based on different arguments: theoretical (e.g. fracture mechanics modelling restrictions), practical (e.g.…”

“…The involved notch stress concentration factor K f (K t ) requires a notch radius ρ. Proposed values include either a worst case effective value ρ e = ρ f similar for weld toe and weld root notches (Brandt et al 2001) or an extreme real value EV(ρ) including the statistical component (Darcis et al 2006), respectively obtained using notched weld (root) material specimen fatigue test S − N data assuming N ∼ N i and weld geometry measurement results. In case of a ρ = 0 assumption the SED rather than K f (K t ) can be adopted as well (Section 2.5; Chattopadhyay et al 2011).…”

“…Cyclic mechanical loading turns K into a driving force ΔK, introducing the Paris equation (da/ dn) = C g '·(ΔK) mg in order to obtain the Basquin type of relation for the crack growth part: log(N g ) = log(C g ) − m g ·log{∫f (ΔK )da}. Typically standard crack growth specimens are used to acquire the intercept C g ' and the slope m g (Brandt et al 2001;Lassen et al 2005;Darcis et al 2006;Lassen and Recho 2009).…”

“…In case of a ρ = 0 assumption the SED rather than K f (K t ) can be adopted as well (Section 2.5; Chattopadhyay et al 2011). The material parameters including the intercept log(C i ) and slope m i are determined using smoot hour-glass shaped weld (root) material specimen in strain controlled tests (Brandt et al 2001) or material dependent empirical relations calibrated using welded joint S − N i fatigue test data covering the number of cycles up to the crack transition size a t (Lassen and Recho 2009;Chattopadhyay et al 2011). The mean stress σ m is obtained using cyclic stress-strain measurement data of the hourglass shaped specimens (Brandt et al 2001), welded joint fatigue resistance data with and without residual stress (Chattopadhyay et al 2011) or is simply assumed to be at yield magnitude because of the welding induced residual stress (Darcis et al 2006).…”

Fatigue life and crack growth behavior of post welded Aluminum 5183 alloy

“…Among them, FSW (a solidstate welding technique), is the most effective one. TIG and MIG welding often results in reduction of fatigue strength [3][4] due to welding defects such as porosity, hot cracking, tensile residual stresses, distortion, etc. Porosities are most commonly encountered defects and act as stress raisers thus lowering down the crack nucleation period significantly depending upon their size, shape, position and reduce the fatigue lives [5].…”