Issue 46

A. Deliou et alii, Frattura ed Integrità Strutturale, 46 (2018) 306-318; DOI: 10.3221/IGF-ESIS.46.28 315      ‘   2 2 Area of the curve P Q U da da B B dN dN                 (6) where B the specimen thickness and da dN is the fatigue crack growth. U decreases with K  at small K  values to achieve a constant value at high K  levels. These results indicated that the specific energy for the different zones of the weld joint studied is not a constant and depends the microstructure and on the decrease of crack closure phenomenon. These results are consistent with the work of [53], [57-58] where they consider that the hysterical work is essentially dispelled in the plasticized zone and that in the case where the closing phenomena are important, it is conceivable that a part of the energy U is dissipated in the zone situated in the wake plasticized along the crack front. Figure 13 : Evolution of specific energy as a function of ΔK C ONCLUSION atigue behavior of welded structures is a more complicated than that of base material due to the presence of heterogeneous metallurgical zones produced during the welding process. Fatigue crack propagation in base metal BM, weld metal WM and heat affected zone HAZ of welded joints of an API X70 pipeline steel was investigated. The most generally model used for the fatigue crack growth based is LEFM and the crack propagation is the dominant stage of the fatigue life. The fatigue crack growth behavior of welded joints API X70 pipeline steel affected by variation of microstructure, weld materials and the loading parameter. Fatigue crack propagation in heat affected zone of X70 steel is delayed compared to the propagation to the other zones. The hysteretic energy Q dissipated during one cycle increases when ΔK increases for the three zones studied and specific energy U reaches a constant value at high ΔK levels. R EFERENCES [1] Nykanen, T., Li, X., Bjork, T. and Marquis, G. (2005). A parametric fracture mechanics study of welded joints with toe cracks and lack of penetration, Engineering Fracture Mechanics, 72, pp.1580–1609. F