4339 - INVESTIGATIONS OF FAILURE BEHAVIOUR OF FLAWED STEEL SPECIMENS WITH ELECTRON BEAM WELDS

The failure behaviour of the steel specimens with electron beam welds is analysed in order to understand the mechanisms of crack path deviation, which occurs during stable crack growth. The results can be used as background information for the safety assessment of flawed steel components. The deviation of the crack from the weld into the base metal reduces the risk of sudden load drop occurrence (pop-in's), since the base metal hardly contains brittle zones in contrast to the heterogeneous weld seam. Therefore, it is assumed that the crack path deviation increases the component's resistance to fracture. The crack growth is numerically simulated in fracture mechanic specimens by applying the damage model of GTN (Gurson-TvergaardNeedleman), which requires the identification of the specific parameters both for the base and weld materials. The metallographical analysis provides the first parameter, the initial void volume fraction f0, which represents a micromechanical feature. The critical damage parameter fc is determined from the calculations on the axisymmetric cell model in dependence of the different stress triaxialities. Further parameters, such as element size and damage acceleration factor κ, result from the adaptation of the numerical to the experimental results. For this purpose tests are carried out on the notched round tensile and fracture mechanic specimens under quasistatic loading and compared to the numerical calculations. Once the parameters are determined, they can be used for damage process simulation in specimens with different configurations. The simulation of the crack path deviation is achieved by varying the distance between the fusion line and crack, which is located in the weld seam. The first crack is initiated at the fatigue crack tip in the weld seam. With increasing loading a second crack appears at the fusion line in the base metal due to the high stress triaxiality and the development of plastic zones. In the further process path deviations of both cracks lead to the their coalescence.