Issue 45

L.M. Viespoli et alii, Frattura ed Integrità Strutturale, 45 (2018) 121-134; DOI: 10.3221/IGF-ESIS.45.10 132 Figure 5 : One of the six specimens tested, showing the fracture in the predicted location for this class of welded joints. All the specimens showed the same kind of failure. Figure 6 : Detail of the fracture surface. The same type of surface characterizes all the failures in the experimental campaign. Fig. 4 shows how the Mean Strain Energy Method is, among the implemented methods, the most precise to predict the lower extreme of the fatigue band. All the assessment are executed at a survival probability of 97.7%, according to the respective fatigue bands. All the samples in the experimental campaign have shown, as expected [2], analogous kind of failure, with crack initiation and propagation from the weld toe of the main plate-longitudinal attachment conjunction. Fig. 7 represents the site of crack initiation for one of the samples. The crack starts in correspondence of the fusion line, which is not contained in a plane, but curved, thus originating the so called river pattern. The fatigue crack growths reaching the opposite side pf the main plate and propagates towards the external part of the same, being in practice a centered crack in a finite plate subjected to mode I loading. When the crack has reached a sufficient amplitude, a plastic final failure occurs. It is to be stressed how the engineering approaches followed confuse, committing an acceptable error in high cycle fatigue, the crack initiation life with the total life of the specimen. Comparing the fatigue life estimate provided by the three methods analyzed, it is clear how the SED approach predicts more accurately the lower band of the fatigue life, under the assumption of negligible propagation life, compared to the Nominal Stress and the Structural Hot Spot Stress approach. The assessment given by the Nominal Stress method appears to be conservative, but fit for the case tested, whilst the Hot Spot Stress approach has failed to safely predict the lower extreme of the fatigue band.