PSI - Issue 5

U. Zerbst et al. / Procedia Structural Integrity 5 (2017) 745–752 U. Zerbst et al./ Structural Integrity Procedia 00 (2017) 000 – 000

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notches, local plasticity may be introduced. It is known from weldments that stress relaxation occurs during the first loading cycle and that this continues for some ten loading cycles when the material experiences cyclic softening, i.e., the gradual decrease of the yield strength until a stable cyclic stress-strain curve is reached (Nitschke-Pagel & Wohlfahrt, 2001). Fig. 10 (a) shows results of simulations which indicate a difference of the effect of cyclic loading depending on the stress ratio R (Tchoffo Ngoula et al., 2017). Whilst the residual stresses at the notch root (in the present case the weld toe) are relaxed independent on whether the initial state was tensile or compression residual stresses when cyclically loaded at R = -1, compression residual stresses are generated or increased for R = 0. Note that this result still lacks experimental confirmation. It might, however, be an explanation for the conservative prediction of the finite life strength in Fig. 10 (b) which is also confirmed by the analysis of individual cracks (Madia et al., 2017). The compression residual stresses introduced by cycling at nominal R = 0 act as mean stresses with the consequence that the real R is lower than the nominal one that was used for the analyses.

Fig. 8: Example for the stochastic simu- lation of the finite life and endurance fatigue limits of a butt welded plate, (material: steel S355NL) using the model of Madia et al. (2017). Because the short crack mainly propa- gates through the heat affected zone (HAZ) the simula- tion using HAZ pro- perties yields better results than those using base metal properties.

Fig. 9: Simulation of the propagation of an indi- vidual crack) marked at the photo) (a) Crack depth, (b) Half-length at surface, (c) Aspect ratio a/c. The steps are due to crack coa- lescence; Steel S355NL butt weld, Loading at R = -1.

Summary The paper provided a discussion of the relation between fatigue life and fracture mechanics assessment. The key points addressed in this context were the simulation of short fatigue crack propagation based on a  J crack driving force parameter and a correction function for the gradual build-up of the crack closure effects, the definition and determination of an initial crack size which fulfills