Investigation of fatigue damage to welded joints under variable amplitude loading spectra

The paper presents the results of the investigation on the effect of loading spectra with different mean stresses on the validity of Miner’s rule, and the effect of stresses below the constant amplitude fatigue limit (CAFL) on the fatigue performance of two types of weld joint. In support of understanding the mechanism for any deficiency to Miner’s rule, fracture mechanics analysis was carried out by measuring and predicting the crack growth in specimens tested under both constant amplitude and variable amplitude loading. The experimental results showed that, although Miner’s rule would predict the same fatigue life for each type of specimens tested under the same spectrum, in fact the actual value of Σ(n/N) at failure strongly depended on the sequence applied. The influence of the loading sequence on Σ(n/N) was in agreement with that on crack growth rates. The deficiency in Miner’s rule was attributed primarily to the stress interaction effects resulting from the type of loading sequence used. The experimental results also showed that, under certain circumstances, stress ranges well below the fatigue limit were found to be as damaging as implied by the S–N curve extrapolated beyond the CAFL without changing the slope. The value of the minimum fully damaging stress range was found to depend on the basic fatigue strength of the weld joint.