Theoretical development and experimental validation of the Peak Stress Method for the fatigue design of steel welded structures

In the context of fatigue design of welded structures by using local approaches, the Peak Stress Method (PSM) is a rapid technique to estimate the Notch Stress Intensity Factors (NSIFs) at the weld toe and weld root, which are idealised and modelled as sharp notches having null tip radius. Essentially, the PSM takes advantage of the singular, linear elastic, opening, sliding, and tearing peak stresses evaluated at the notch tip using coarse free mesh patterns to estimate the mode I, II, and III NSIF-terms, respectively. By adopting the averaged Strain Energy Density (SED) as a fatigue strength criterion, a PSM-based design stress, i.e. the so-called equivalent peak stress, can be defined as a function of the relevant peak stresses. In present manuscript the Peak Stress Method has been theoretically developed and the proposed modifications have been verified against experimental data. Chapter 1 introduce the problem and gives the theoretical background on the local approaches used in the rest of the manuscript. In particular, the concepts of Stress Intensity Factor (SIF), the Notch Stress Intensity Factor approach (NSIF), the averaged Strain Energy Density (SED) criterion and the Peak Stress Method (PSM) will be introduced along with their theoretical background, and their application to fatigue lifetime assessment of welded joints. The Chapter 2 deals with the first extension of the PSM which allows to account for variable amplitude (VA) uniaxial as well as in-phase and out-of-phase multiaxial fatigue loadings applied to steel arc-welded joints. The extension to VA loading situations has been based on Palmgren-Miner’s linear damage rule (LDR) to account for cumulative damage. The proposed method has been validated against a large bulk of VA fatigue data taken from the literature proving the PSM as an extremely valid technique to design welded joints against CA or VA uniaxial as well as multiaxial fatigue local stresses. The proposed method has also been checked against new experimental data generated by fatigue testing non-load-carrying (nlc) fillet-welded double transverse or inclined attachments made of S355 structural steel under pure axial loading. In Chapter 3, another extension of the PSM will be presented, to estimate the constant amplitude uniaxial fatigue limit of welded structures in the stress-relieved state. A fracture mechanics criterion based on the cyclic R-curve analysis has been considered. The application of the method required the definition of an initial crack size, which value has been accurately calibrated by means of dedicate experimental tests. After the calibration, the method based on the cyclic R-curve analysis has been combined with the PSM obtaining a new procedure that allows to a rapid and effective design of weld toe failures in the infinite life region, without the need of complex and time-consuming fracture mechanics-based calculations. Finally, in Chapter 4, the possibility of experimentally determining the cyclic R-curve on complex specimens’ geometries has been investigated. In particular, the real-time crack length measurement is the most critical aspect, therefore the Direct Current Potential Drop (DCPD) method has been applied to single-edge-crack round bars subject to axial fatigue. Firstly, 3D electrical FE analyses have been performed to investigate the effect of the current and the potential probes position on the performances of the DCPD method in terms of measurability, sensitivity and reproducibility. Secondly, the accuracy of the numerical analyses has been checked against experimental results.