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Fatigue tests were carried out on fillet welded joints in four steels, with yield strengths ranging from 332 to 727 N/mm2, under various applied load ratios. Some specimens were stress-relieved but most were spot-heated to ensure that high tensile residual stresses, as would be present in as-welded joints in real structures, were present in the specimens. The aim was to investigate the effect of tensile residual stresses on the fatigue behavior of fillet welded joints under different load ratios and the relevance of the tensile strength of the steel, particularly in relation to the magnitude of residual stresses developed. In the present specimens, the residual stresses were no higher in the high-strength steels than the mild steels, with the result 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 = −∞ (zero compression to R = 0.67) applied load ratio had little 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.

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Fatigue crack propagation tests have been carried out on three C-Mn structural steels at stress ratios between 0.67 and −4. Surface strain gage techniques have been used to determine crack-tip closure stress and an expression proposed to relate the proportion of the stress cycle for which the crack tip is actually open U to stress ratio. Satisfactory correlation of the crack propagation results for this range of stress ratios has been achieved by defining an effective stress intensity factor range as the proportion U of the total stress intensity range. In addition, reasonable correlation of the results was obtained on the basis of the tensile portion of the stress intensity factor range.

The fatigue behavior of aluminum-zinc-magnesium (Al-Zn-Mg) alloy fillet-welded joints was analyzed in fracture mechanics terms. Basic crack propagation data were obtained with −2 ⩽ R ⩽ + 0.5 and correlated using formulas in the literature and, more successfully, in terms of ΔKeff, based on the results of crack closure experiments. The form of the da/dN versus ΔK relationship was influenced by the specimen geometry. A fracture mechanics analysis of the fatigue life of Al-Zn-Mg alloy fillet welds based on the da/dN versus ΔKeff relationship indicated that the weld toe was less severe from the fatigue viewpoint than the same region in a steel fillet weld. This was compatible with the fact that metallurgical examination of Al-Zn-Mg alloy fillet welds has failed to reveal toe defects similar in magnitude to those which act as fatigue crack initiators at the toes of steel fillet welds. The analysis showed that the fatigue life obtained from the Al-Zn-Mg alloy weld could be predicted on the basis that defects only one tenth the size of those observed in steel were present. Fatigue failure from the weld root in a cruciform joint was also analyzed and the optimum weld design, which gives an equal chance of failure from the root and toe, was determined. The analysis was supported by fatigue test results. Comparison with results obtained for steel added confirmation to the finding that if toe defects are present in Al-Zn-Mg alloy welds, they are smaller than those in steel.