Experiments and modelling of fracture initiation at impact

Abstract: The thesis addresses different questions, each necessary in the understanding of dynamic fracture initiation and crack propagation in steel. Emphasis lays on dynamic crack propagation experiments performed in the ductile to brittle transition region and possible conclusions, that can be drawn from them. One question regards the physical basis for for viscoplasticity. Dislocation slip by thermal activation provides means for suitable choices of the yield function and the flow rate function in the standard Perzyna formulation for metal viscoplasticity. Another concern is the choice of correct and efficient estimators for parameters in rate dependent material models. New estimators, based on analysis of non-linear time series are derived. A coherent set of dynamic fracture experiments, metallurgical characterisations and data for constitutive characterisation of two different steel qualities is presented. Crack propagation experiments were performed on large Single Edge Bend specimens in the ductile to brittle transition region. The material data mirrors both the properties at high strain rates as well as the conditions required for cleavage fracture. The dynamic fracture experiments are analysed through numerical modelling, and conditions for the Rice-Knott-Ritchie cleavage initiation model are found. In addition, efficient ways of modelling and application of boundary conditions at the impact of the Single Edge Bend specimens are established.