Fatigue Life Enhancement of Welded Aluminium Joints
Manipulations of the residual stress distribution in welds have an influence on the fatigue life of components by either retarding or accelerating fatigue crack growth. Established residual stress methods to enhance the fatigue life of welded structures are, however, highly time-consuming, labour-intensive, and very expensive. As a part of the NorLight project, SINTEF has introduced a simple, inexpensive, and flexible method to be integrated in the manufacturing process.
Scientific Basis: The method is based on changing the direction of the residual stresses at fatigue exposed regions from tension to compression during welding, as illustrated in Figure 1 where the chord has been elastically pre-deformed during welding.
Project Description: The welded components assessed in the study consist of two identical T-joints of rectangular hollow sections (RHS) of aluminium alloy 6082-T6 (Figure 1). Both batches of test specimens are produced using automatic GMA-welding, however, the chord of Batch 1 is unrestrained during welding while for Batch 2 the chord is pre-deformed elastically during welding. The joints were tested in 4-point bending with a sinusoidal, constant amplitude loading at R = 0.1 in laboratory at ambient air and temperature.
Project Results: Based on a statistical evaluation of the S-N data of the RHS T-joints (Figure 2), the introduction of residual stresses by elastic pre-deformation of the RHS profiles, in combination with different boundary conditions, improve the fatigue life of the welded RHS T-joints by a factor of 1.30 (on applied mean stress range).
Adjusting the manufacturing process is not straightforward, as the method requires exact information of the effect of the external manipulations, which is not known a priori. Thus, the method requires an accurate finite element analysis [2] for the prediction of residual stresses resulting from the welding process.
Industrial Implementation: The principles of the method presented above can be used as an effective and inexpensive tool in the design process in order to cope with fatigue problems caused by geometry and weld quality constraints.
References:
- Tveiten, B. W. “The Fatigue Strength of RHS T-joints - NFR Kompetanseprosjekt – KMB 144004/213 – Fatigue and Durability”. SINTEF report STF24 A03220, ISBN 82-14-02655-5, February 2003.
- Fjær, H. G., Myhr, O. R. Klokkehaug, S. Holm, E. “Advances in Aluminium Weld Simulations Applying Weldsim”. 11th International Conference on Computer Technology in Welding. Columbus, Ohio, USA, December 2001.
