13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Fatigue Crack Growth Modelling in Welded Stiffened Panels under Cyclic Tension Željko Božić1,*, Siegfried Schmauder2, Marijo Mlikota2, Martin Hummel2 1 University of Zagreb, Faculty of Mech. Eng. And Nav. Arch., I. Lucica 5, 10000 Zagreb, Croatia 2 University of Stuttgart, IMWF, Pfaffenwaldring 32, D-70569 Stuttgart * Corresponding author: zeljko.bozic@fsb.hr Abstract The influence of welding residual stresses in stiffened panels on effective stress intensity factor values and fatigue crack growth rate was studied in this paper. Interpretation of relevant effects on different length scales such as dislocation appearance and microstructural crack nucleation and propagation are taken into account using Molecular Dynamics (MD) simulations as well as a Tanaka-Mura approach for the analysis of the problem. Mode I stress intensity factors (SIF), KI, were calculated by the ANSYS program using shell elements and assuming plane stress conditions. The SIFs were calculated from FE results using the crack tip displacement extrapolation method. A total SIF value, Ktot, is contributed by the part due to the applied load Kapp, and by the part due to weld residual stresses, Kres. In the FE software package ANSYS the command INISTATE is used for defining the initial stress conditions. The FE analysis for the stiffened panel specimens showed that high tensile residual stresses in the vicinity of a stiffener significantly increase Kres and Ktot. Correspondingly, the simulated crack growth rate was higher in this region, which is in good agreement with experimental results. Compressive weld residual stresses between two stiffeners decreased the effective SIF value, Keff, which was considered as a crack growth driving force in a power law model. Keywords Fatigue crack growth rate, Welding residual stresses, Stiffened panel, Dislocation, Microstructural crack 1. Introduction In stiffened panels of a ship deck structure, fatigue cracks may initiate under cyclic loading at sites of stress concentration and further propagate, which can eventually result in unstable fracture and structural failure. The crack growth rate in welded stiffened panels can be significantly affected by the residual stresses which are introduced by the welding process. The high heat input from the welding process causes tensile residual stresses in the vicinity of a stiffener. These tensile stresses are equilibrated by compressive stresses in the region between the stiffeners. Welding residual stresses should be taken into account for a proper fatigue life assessment of welded stiffened panels under cyclic tension loading. The complete process of fatigue failure of mechanical components may be divided into the following stages: (1) micro-crack nucleation; (2) small crack growth; (3) long crack growth; and (4) occurrence of final failure. In engineering applications, the first two stages are usually termed as the “crack initiation or small crack formation period” while long crack growth is termed as the ‘‘crack propagation period”. Dislocation development can be simulated by using the molecular dynamics (MD) simulation code IMD [1]. To analyze dislocation development atomistic scale simulation methods are implemented, [2, 3, 4, 5]. The crack initiation period generally accounts for most of the service life, especially in high-cycle and very high cycle fatigue [6]. In pure metals and some alloys without pores or inclusions, irreversible dislocations glide under cyclic loading. This leads to the development of persistent slip bands, extrusions and intrusions in surface grains that are optimally oriented for slip. With continued strain cycling, a fatigue crack can be nucleated at an extrusion or intrusion within a
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