Issue34

C. Fischer et alii, Frattura ed Integrità Strutturale, 34 (2015) 99-108; DOI: 10.3221/IGF-ESIS.34.10 99 Focussed on Crack Paths Influence of local stress concentrations on the crack propagation in complex welded components C. Fischer, W. Fricke Hamburg University of Technology, Institute of Ship Structural Design and Analysis, Hamburg, Germany w.fricke@tuhh.de A BSTRACT . While a long stable crack propagation phase was observed during experiments of complex welded components, very conservative assessments of the fatigue life were achieved in the past. The difference was explained by the stress gradient occurring over the plate thickness. This paper deals with numerical crack propagation simulations which were performed for different geometrical variants. The variants differ related to global geometry, boundary conditions and weld shape. The analyses aim to investigate how the crack propagation is altered if the structural configuration gets more complex. In conclusion, the stress gradient over the plate thickness, the apparent plate thickness and the notch effect slows down the crack propagation rate if the same stress value being effective for fatigue appears at the weld toe. Thereby, the load-carrying grade of the weld, the weld flank angle and the geometrical configuration have an impact on both the notch effect and the local stress concentration. K EYWORDS . Crack propagation simulation; Fatigue life; Notch effect; Stress gradient. I NTRODUCTION omplex welded components can have hot-spots showing high local stresses with steep stress gradients. These could be critical with respect to fatigue. An example is the three-dimensional intersection of plates shown as variant 5 in Fig. 1. Different variants of this detail were experimentally investigated by means of both large-scale and small-scale fatigue tests over the last decades, see e.g. [2, 11-15]. Dijkstra et al . [2] observed an early crack initiation, but a long period of stable crack propagation until the crack had grown through the loaded plate. However, very conservative fatigue assessments were discovered applying the common fatigue approaches to complex structures, see e.g. [6, 7]. Lotsberg and Sigurdsson [14] explained the disagreement by the stress gradient over the plate thickness which was not covered by the fatigue approach they had selected. Fischer and Fricke [5] established three additional reasons: the apparent plate thickness, the load-carrying grade of the weld and bending constraints of the detail. They supposed correction factors to account for these influences in the fatigue assessment. Generally, the approaches are based on the analysis of tests of small-scale specimens with more or less simple welded joints. Here, no three-dimensional complex stress distribution is on hand. This paper deals with different crack propagation analyses at both simple joints and complex welded structures in order to investigate further effects, besides the stress gradient, which affects the crack propagation, shape and fatigue life. At first, a comparison of a transverse attachment and its geometrical modifications with the intersection of plates is presented. Four effects which are identified on the basis of different stress distributions and geometry are validated by numerical and analytical crack propagation analyses. In the next step, the geometry is modified to vary the notch effect and the stress C