Fatigue life enhancement of aluminium joints through mechanical and thermal prestressing

This paper presents some simple and flexible methods to enhance the fatigue life of welded aluminium components. Besides enhancing the fatigue life, the proposed methods can easily be implemented into manufacturing processes. The key element of the methods is to change residual stresses from tension to compression at locations vulnerable to fatigue. This is accomplished by mechanical prestressing using elastic pre-deformation or by thermal prestressing using induction heating. The specimens tested are welded aluminium rectangular hollow section T-joints. Prior to fatigue testing, welding FE-simulations were carried out to verify the magnitude and pattern of the residual stress fields (through process modeling). Fatigue testing was later carried out on four different batches. One batch was produced using elastically pre-deformed chords, two batches were treated by means of thermal prestressing (induction heating), and one batch was “as welded” representing a “reference case”. Based on statistical evaluation of S–N data, the introduction of superimposed compressive stress fields results in a significantly improved fatigue life. Among the different batches, induction heating turned out to be the most promising method with a fatigue strength improvement factor of 1.5 on stress, compared to “as welded” components.