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

Safety assessment of steels under ULCF loading conditions with damage mechanics model

AbstractUltra-low cycle fatigue (ULCF) loading conditions raise a complex problem for design and safe exploitation of steel constructions in seismic active zones on our planet. The investigation of the damage and failure of steel constructions under loading with large strain amplitude can contribute to the development of safety assessments for earthquake-resistant steel structures.Damage mechanics provide a good approach for the description of damage and failure of materials and structures under ULCF loading. In the frame of the presented work a coupled continuum damage mechanic material model based on Yoshida-Uemori (YU) plasticity model (Yoshida and Uemori 2003) and effective strain concept (Ohata and Toyoda 2004) is presented. In comparison with the Armstrong-Frederik and Chaboche plasticity models (Armstrong and Frederick 1966, Chaboche 1989) YU-model provides a better description for plastic material properties. Combination of YU-model model with effective strain concept and coupled damage allows using the presented model as a powerful instrument for material characterization and optimization of structures under ULCF loading.The developed damage mechanics model is validated on cyclic tests of small scale samples under loading with large strain amplitudes. It is capable to predict the force-displacement response and number of cycles to fracture. Additionally, the presented model is applicable for the simulation of large scale tests.The use of the proposed models for development of steel safety assessments and transfer of the material model parameters for modeling of large steel structures are discussed