Dissertation Gorash
The structural analysis under in-service conditions at various temperatures requires a reliable creep constitutive model which reflects time-dependent creep deformations and processes accompanying creep like hardening and damage in a wide stress range. The objective of this work is to develop a comprehensive non-isothermal creep-damage model based on transitions of creep and long-term strength behavior in a wide stress range. The important features of the proposed creep and damage equations are the response functions of the applied stress which should extrapolate the laboratory creep and rupture data usually obtained in tests under increased stress and temperature to the in-service loading conditions relevant for industrial applications. The study deals with four principal topics including the basic assumptions of creep constitutive modeling, the conventional isotropic and anisotropic creep-damage models, the comprehensive non-isothermal creep-damage models for a wide stress range. Finally, the application to structural analysis of benchmark problems and engineering components is demonstrated.
The proposed non-isothermal creep-damage model for a wide stress range is based on several assumptions derived from creep experiments and microstructural observations for various advanced heat resistant steels. The constitutive equation affects the stress range dependent behavior demonstrating the power-law to linear creep transition with a decreasing stress. To take into account the primary creep behavior a strain hardening function is introduced. To characterize the creep-rupture behavior the constitutive equation is generalized by introduction of two damage internal state variables and appropriate evolution equations.
The examples of long-term strength analysis for various industrial components are highlighted to illustrate the effective features and importance of the continuum damage mechanics approach for the life-time assessments in structural analysis. Finally, an example of long-term strength analysis for the housing of a quick stop valve usually installed on steam turbines is presented.