To indicate the effect of mean compressive stresses on the structural durability, fatigue tests have been carried out under constant-amplitude load. As a particularly suitable design detail, the longitudinal stiffener with non-load-carrying fillet welds was selected. This detail represents the permissible limit of notch condition in highly stressed ship structures. The fillet weld was made with two layers, so that a fully developed residual stress field could be expected.In order to gain more insight into the technical background under consideration of the weldinduced residual stresses and their redistribution during cyclic loading, finite element analyses as well as measurements were carried out. On uncracked specimens the weld-induced residual stresses were measured using the X-ray method. The redistribution of the residual stresses during crack propagation was measured using the neutron diffraction method.Different models were used for the numerical simulation using the finite element (FE) method. In the first phase the FE calculation could be limited on a linear analysis, because the semielliptical crack propagating through the plate thickness remained open over the full cycles, which is due to the high residual stresses. After the crack became a through-thickness crack, closure effects became significant, requiring a non-linear crack propagation analysis with a shell model. Therefore the weld-induced residual stresses must be reproduced with a solid model and transmitted to the shell model.Taking into account the complete crack length during the simulation, it was possible to estimate the fatigue life under constant amplitude loads in a satisfactory way. Downloaded from Fig. 7 Analysis of the weld-induced residual stresses in the longitudinal direction 638 J Rörup

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KurzfassungZur Untersuchung des Einflusses der Mittelspannungen auf das Schwingfestigkeitsverhalten wurden Proben einer axial beanspruchten Platte mit aufgeschweißter Längssteife untersucht. Es zeigte sich, dass das Niveau der schweißinduzierten Eigenspannungen einen entscheidenden Einfluss auf die erreichte Lebensdauer hat. Ein Zugeigenspannungsfeld, welches die Rissspitze vor sich her schiebt und mit zunehmender Risslänge an Größe und Intensität verliert, ermöglicht ein Fortschreiten des Risses in Bereiche, in denen Druckeigenspannungen aus dem Schweißprozess vorliegen. Mittels nicht-linearer Strukturanalysen mit der Methode der Finiten Elemente wurden Schließeffekte für einen Durchriss erfasst. Dabei konnte die in den Versuchen ermittelte Rissverzögerung durch die berechnete Rissfortschrittskurve in zufrieden stellender Weise abgebildet werden.

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