L.P. Borrego

Evaluation of overload effects on fatigue crack growth and closure

Abstract Fatigue crack propagation tests with single tensile peak overloads have been performed in 6082-T6 aluminium alloy at several baseline Δ K levels and stress ratios of 0.05 and 0.25. The tests were carried out at constant Δ K conditions. Crack closure was monitored in all tests by the compliance technique using a pin microgauge. The observed transient post-overload behaviour is discussed in terms of overload ratio, baseline Δ K level and stress ratio. The crack closure parameter U was obtained and compared with the crack growth transients. Experimental support is given for the hypothesis that plasticity-induced closure is the main cause of overload retardation for plane stress conditions. Predictions based on crack closure measurements show good correlation with the observed crack growth rates for all the post-overload transients when discontinuous closure is properly taken into account.

Fatigue life prediction in AlMgSi1 lap joint weldments

The fatigue behaviour of an AlMgSi1 alloy lap welded joint and the improvement in fatigue strength due to post-weld heat treatment were investigated. Two series of fatigue tests were performed: one in as-welded specimens and another in heat-treated specimens. The prediction of crack initiation life under a plane strain condition was made based on the local strain approach using Morrows modified equation. The local inelastic stresses and strains at the weld toe were calculated for the heat-treated specimens using three methods: Neuber, Glinka and finite element analysis. The accuracy of the different methods were evaluated and discussed.

Influence of spectrum loading on fatigue resistance of AA6082 friction stir welds

Purpose – Welded components are often subjected to variable amplitude service loads, increasing the uncertainty of fatigue life due to material strength, notch geometries, defect content and residual stresses. In the case of friction stir welding (FSW) of aluminium alloys no data were found available concerning fatigue behaviour under variable amplitude loading. The purpose of this paper is to determine the fatigue strength of friction stir welds in AA6082‐T6 under constant and variable amplitude loading and analyse the validity of Miners rule for these specific welding conditions.Design/methodology/approach – Fatigue tests were carried out in a servo‐hydraulic testing machine using a stress ratio of R=0. Typified Gassner amplitude spectra were considered, using four shape exponent values. Microhardness tests were performed to characterize the Vickers hardness profile in the vicinity of the weld area. Relatively to the base material (BM), the FSW process leads to a decrease of the static mechanical prope...

Fatigue analysis of thin AlMgSi welded joints under constant and variable amplitude block loadings

Abstract This paper reports the fatigue behaviour of thin AlMgSi1 aluminium alloy weldments and the improvement in fatigue strength due to post-weld treatments. Several fatigue tests were performed using two distinct types of thin welded joints, T and single lap, manufactured using a 6xxx series aluminium alloy, and the efficiency of fatigue life improvement techniques, such as post-weld heat treatment and weld toe’s burr dressing, was studied. Both the post-weld heat treatment and the burr dressing produced a fatigue strength improvement. This fact is discussed, for both geometries, in the context of the initiation and propagation phases contribution to total fatigue life. Finally, several fatigue tests were performed under variable amplitude block loading using T6 post-weld heat-treated single lap joints, and the correspondent fatigue lives were compared with the predictions of Miner’s rule.