Description of deformation and failure behavior of a nut-bolt-assembly by means of a viscoplastic constitutive equation

摘要

Nut bolt assemblies are used to fasten the flanges in casings of steam turbines. They are subjected to both creep loading during service under stationary conditions as well as fatigue load during start up and shut down of the turbine. The life assessment of such components is an important aspect to ensure reliable service of the steam turbine. The state of the art design of nut bolt assembly does not consider effectively the interaction between creep and fatigue in terms of cyclic hardening or recovery effects. In the paper calculations to characterize the time dependent stress-strain distribution in creep-fatigue loaded nut bolt assemblies are described. A unified model was used to describe viscoplasticity and cyclic plasticity of the materials. The model based on the constitutive equations of Chaboche and Nouailhas was implemented in the Finite-Element-Code ABAQUS. In addition a damage hypothesis was developed to describe creep damage evolution. The hypothesis has been included in the formulation of the constitutive equations. The numerical results have been verified by means of component like tests. A good agreement of calculated and measured strains have been observed. The damage hypothesis proved to be suitable to detect failure location and creep damage development.