Browsing by Author "Tu, Haoyun"

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    Numerical simulation and experimental investigation of the fracture behaviour of an electron beam welded steel joint
    (2016) Tu, Haoyun; Schmauder, Siegfried (Prof. Dr. rer. nat.)
    Welding techniques are widely applied in many industry fields. As the damage behavior of the weldment influences the service life of the component, strong attention is drawn to the weldment. This thesis focuses on the fracture behavior of an S355 electron beam welded joint. Three different models are adopted to describe the damage behavior of the welded joints, namely the Rousselier model, the Gurson-Tvergaard-Needleman (GTN) model and the Cohesive zone model (CZM). Although differences exist between these models, they successfully describe the damage behavior of the weldment and are able to predict the crack propagation of C(T)-specimens obtained from S355 electron beam welded joints. Simulation results are shown in the form of force vs. crack opening displacement (COD) and fracture resistance JR-curves. The cohesive zone model is considered to be the best model for the investigation of the fracture behavior of S355 electron beam welded joints as it can simulate both ductile and brittle fracture. Compared to the GTN model, the Rousselier model for ductile fracture simulations shows its superiority because of simplicity and reduced model parameters. In order to visualize the crack propagation at the surface of the material, C(T)-specimens extracted from the S355 base material are tensile tested together with the ARAMIS system monitoring the material deformation and crack growth behavior in the notched area. Images in the notched region and the equivalent strain distribution calculated from the ARAMIS system are shown. 2D and 3D GTN models are used to investigate the fracture behavior of a C(T)-specimen under tensile test process monitored with the ARAMIS system. To understand the damage mechanisms of the S355 base material and to show the real crack propagation within the material during the deformation process, Synchrotron radiation- computed laminography (SRCL) is performed on a thin sheet specimen from S355 base material for the first time. Reconstructed 2D laminography images from the middle section and from the section where the main crack is observed in the sheet specimen are shown in this work. Additionally, 2D cross-sections at the through thickness plane at two positions located ahead of the initial notch are shown. A shear band is observed between two neighbouring cracks before crack advancement at CMOD=1.25 mm. 3D reconstruction of the laminography scanning data confirms the damage evolution through void initiation, growth and coalescence originating from non-metallic inclusions being the main reason for a flat fracture happening before the slant fracture. Shear fracture connecting two neighbouring flat cracks to form the main crack is observed in 3D laminography images. The 3D Rousselier model is adopted to predict the flat fracture of the thin sheet specimen. The material in front of the initial notch is divided into many partitions of which the true f_0-values are obtained. According to the positions of the partitions, the Rousselier elements in front of the initial notch are divided into many sets where the corresponding true f_0-values are used in the simulations. With the true f_0-values, the longest 2D cracks (T-L) in an analyzed specimen are located at a cross section which is around 200 µm apart from the middle section of the sheet specimen which coincides with the laminographic image. The Rousselier model is able to predict the fracture surface of the sheet specimen before the occurrence of the shear fracture.