TWI Industrial Member Report Summary 123/1980
By S J Maddox
Background
Fatigue tests were carried out on fillet welded joints in four steels, with yield strengths ranging from 332 to 727N/mm2 under various applied load ratios. Some specimens were stress-relieved but the remainder were spot-heated to ensure that high tensile residual stresses, as would be present in welds in real structures, were present in the specimens. The aim was to investigate the effect of tensile residual stresses on the fatigue behaviour of fillet welded joints under different load ratios and the relevance of the tensile strength of the steel.
The results showed that the tensile strength of the steel had no effect on the fatigue strength of the joints in both the as-welded and the stress-relieved conditions. Furthermore, it was found that for the range of R values used (R = 0 (compression) to R = 0.67) applied load ratio had no effect on the fatigue strength of the as-welded joints provided that 'failure' under compressive loading was taken to be a crack length less than or equal to that at which the rate of crack growth began to decrease. Stress relief was only partially effective 'with the result that applied compressive stresses were still damaging. Thus, under partly compressive loading the fatigue strength of the joint increased but not greatly. However, stress relief had no effect on the fatigue strength of the joint when it was subjected to tension loading.