Numerical Simulation and Experimental Investigation of the Fracture Behaviour of an Electron Beam Welded Steel Joint

In this thesis, the author investigates experimentally and numerically the fracture behavior of an electron beam welded joint made from two butt S355 plates. The 2D Rousselier model, the Gurson-Tvergaard-Needleman (GTN) model and the cohesive zone model (CZM) were adopted to predict the crack propagation of thick compact tension (CT) specimens. Advantages and disadvantages of the three mentioned models are discussed. The cohesive zone model is suggested as it is easy to use for scientists & engineers because the CZM has less model parameters and can be used to simulate arbitrary crack propagation. The results shown in this thesis help to evaluate the fracture behavior of a metallic material. A 3D optical deformation measurement system ARAMIS and the synchrotron radiation-computed laminography (SRCT) technique reveal for the first time the damage evolution on the surface of the sample and inside a thin sheet specimen obtained from steel S355. Damage evolution by void initiation, growth and coalescence are visualized in 2D and 3D laminographic images. Two fracture types, i.e., a flat crack propagation originated from void initiation, growth and coalescence and a shear coalescence mechanism are visualized in 2D and 3D images of laminographic data, showing the complexity of real fracture. In the dissertation, the 3D Rousselier model is applied for the first time successfully to predict different microcrack shapes before shear cracks arise by defining the finite elements in front of the initial notch with inhomogeneous -values. The influence of the distribution of inclusions on the fracture shape is also discussed. For the analyzed material, a homogeneous distribution of particles in the material provides the highest resistance to fracture. This is a very valuable information for material designers and producers in industry.