| Popis: |
For efficient design of pressure components, and cost effective extension of lives of existing components, an analyst should allow for complex behaviour of components when they are subjected to plasticity and creep deformation. Strictly, there are not many models that can accurately and pragmatically be used to predict the elastic-plastic-creep lives of components. The existing models are mainly applicable to elastic-creep cases, but even these models are either too complex, not practical or they are not sufficiently accurate. This thesis proposes an accurate paradigm for predicting the lives of such components. The application of the proposed paradigm to an internally pressurized vessel shows that the elastic-plastic-creep life of the component can be predicted with an error of less than 10%. This thesis concentrates on creep and plasticity damage. Currently, there is no generally accepted expression for the damage functionD=f(D_p,D_cr), and failure models that combine plasticity and creep. The model proposes that the damage induced in the material is directly related to the internal energy density absorbed at critically loaded regions in the material. As a result, the proposed model takes into account both internal forces (stresses) and deformation (strains) yet it is simple to use in practice owing to its scalar nature. The proposed model assumes that the component is defect free and contains stress concentrators. Numerical analysis has been conducted until the failure point, using the FEA code developed by the author, to include the tertiary creep stage. Both elastic-creep and elastic-plastic-creep analysis has been performed. The stress redistribution has been studied within a component for each case, to assess a suitable safety factor addressing the failure mechanism of concern. Serious consideration is recognized to an appropriate weld strength factor which would be applied to the welded pressurised vessel, in order to improve the design against creep. The proposed model does not require material parameters whose evaluation is cumbersome and uneconomic. It is obvious that any uncertainties in the pertinent material properties, loading and the geometry of the component can substantially increase the error in the predicted lives of the components. To ease this problem, appropriate sensitivity analysis was conducted to cover all of these uncertainties. |
