Entwicklung und Anwendung des gekoppelten Schädigungsmodells im Falle der Ultra-Low Cycle Fatigue Belastungen

In this work, the topic of Ultra Low Cycle Fatigue (ULCF) is investigated with regard to experimental investigations on notched small-scale samples and components (L-bow of a pipeline) as well as in terms of ULCF damage modeling with the help of a damage mechanics model. Two different failure mechanisms are identified under ULCF loading of notched specimens: failure type I - disperse continuum damage due to formation and growth of cavities, failure type II - localized damage in the form of crack growth beginning from the surface with a pronounced crack closure effect. The experimental results, in particular the number of cycles until fracture as a function of the strain amplitude, are characterized using the extended Manson-Coffin equation. The locally resolved strains under ULCF loading are determined by means of a FEM analysis and take into account the variation of the most important influencing factors such as the selection of the plasticity model, the definition of the calibration strategy based on the nominal material properties and the definition of the FEM platform. For the description of the ULCF failure processes, a coupled damage mechanics material model (ULCF-CDM) was developed. The model was implemented for the Abaqus/Standard Solver in the form of a UMAT subroutine. For the description of damage, a failure criterion was used together with an optimized law of damage evolution. The model was successfully applied on small-scale samples as well as on components. The formulation of the calculation scheme of the ULCF-CDM model plays an important role. The development of damage is taken into account dynamically within an increment and directly influences both the calculated stresses and the material stiffness of the material model. In order to optimize the FEM modeling of the components in case of ULCF loading, a reference component strategy was developed. With the help of this strategy, the effort for the description of the behavior of a component under ULCF loading can be reduced considerably with the aid of the FEM model. The determination of the material parameters is directly based on the experimental results of the components in form of an inverse analysis. Eventually, the proposed solutions can be transferred to other areas of damage modeling