Fatigue endurance of welded joints subjected to different blocks of bending and torsion loading

The fatigue strength of pipe-to-plate welded joints under bending, torsion and combined (in-phase and out-of-phase) bending and torsion has been already investigated in previous works by the authors. The specimen consisted of a pipe joined by seam welding to a plate. Both the pipe and the plate were made of S355JR steel. The test apparatus allows to apply any combination of proportional and non-proportional bending and torsion loads to the specimen. For the analysed specimens failure originated mainly from the weld root, where a severe notch is present, even if some failure from the weld toe was observed in case of bending loading. However, the crack propagation and fracture surface under bending and under torsion were significantly different. For this reason, in order to investigate any possible influence of the loading order on the fatigue endurance, the effect of different loading blocks was analysed in this work. This subject has not been widely investigated in the technical literature about welded joints. In a first series of tests, specimens were loaded in bending for a given fraction of the estimated endurance and then were loaded in torsion till failure. A similar series of tests was then conducted by varying the loading order: specimens were loaded in torsion for a given fraction of the estimated endurance, followed by a block of bending loading till failure. The whole test campaign was repeated for two different fractions of the estimated life, i.e. 0.3 and 0.45, respectively. The failure was intended as the presence of a through the thickness crack, whose presence was monitored by a drop in the internal pressure of the pipe. Results are discussed in terms of the Miner’s rule based on nominal stresses and to the cumulative damage suggested by Eurocode and IIW.