Predicting fatigue life of welded joints under variable amplitude loading spectra

Abstract

This study aims to improve the understanding of fatigue damage in welded joints under variable amplitude (VA) loading spectra and provide recommendations for future revision of fatigue design standard BS 7608 with respect to the guidance on the assessment of fatigue damage under VA loading. Fatigue tests and analyses were conducted in the present study, aiming to address concerns about the fatigue performance of welded joints under VA loading. Welded joints with transverse fillet welded attachments, manufactured using S355 steel plates with three thicknesses, were tested under cycling down (CD) loading sequences with various constant maximum stresses under either uniaxial or bending loading. In addition to as-welded joints, life-improved welded joints treated by ultrasonic impact treatment (UIT), were also fabricated, and tested. Test results suggest that the CD sequence with a high maximum stress can significantly degrade the fatigue performance of welded joints, with the Miner’s sum 𝐷 being only of around 0.5 at fatigue failure. However, the severity of such a loading sequence decreases when the maximum stress is below a certain level, with a 𝐷 value much higher than unity when fatigue failure occurs. Besides, the 𝐷 value is also found to be dependent on the plate thickness proportionally. UIT can improve the fatigue life of the welded joints under VA loading, and the extent of the improvement depends on the maximum stress applied under CD loading sequence. Residual stresses at the weld toes of several specimens with different thicknesses were measured using either the X-ray diffraction or the Centre-hole drilling method in both as-welded and fatigue-tested conditions. Measurement results show the magnitudes of the residual stress in the as-welded specimens are in proportion to the plate thickness. residual stress relaxation occurs in both tensile and compressive loading conditions. The residual stress decreases significantly within the first two loading cycles and then becomes almost constant – only minor reductions were seen in further loading cycles. The effect of the applied mean stress on the fatigue performance of welded joints was investigated by considering the actual maximum stress, which is the combination of the applied maximum stress and the residual stress, and new mean stress correction models were developed accordingly on the basis of the conventional correction models. In addition to the mean stress effect, the models developed also can consider the effect of different VA loading sequences through a newly introduced factor. Experimental data obtained from the present study and reported in the published literature were adopted for validation. Results show that the proposed new models can more accurately predict the fatigue life of welded joints under given VA loading spectra, with the Miner’s sum 𝐷 at fatigue failures mainly being in the range between 0.5 and 1.5. The fatigue crack growth (FCG) under VA loading was also studied. A new sequence factor which can take into account the effect of a VA loading sequence was introduced by considering the local residual mean stress as the primary mechanism for the interaction between stresses in a VA loading sequence. FCG predicted by the new model shows a good agreement with the experimental data. Fatigue lives can also be predicted reasonably well using the model developed, with the values of 𝐷 at fatigue failures ranging between 1 and 2.