A new multiaxial creep damage model based on the exhaustion of internal energy.

The creep of materials is a research topic of major significance in the life assessment and design of many modern engineering components of advance technology such as: power generation plant, chemical plant, gas turbines, jet engines, spacecraft, components made of plastics and polymers, etc. To predict the creep lives of such components, one necessary ingredient is a creep damage model. The current creep damage models are either too cumbersome to be readily employed and/or not sufficiently accurate for practical applications.This thesis describes a new creep damage model to overcome some of the major shortcomings in current creep damage/life prediction methods. The proposed model is relatively simple and readily applicable to industrial cases yet it is sufficiently accurate. The proposed model assumes that, on a macroscopic level, the energy dissipated in the material may be taken as a measure of creep damage induced in the material. In another words, creep damage is directly proportional to the absorbed internal energy density (IED), i.e., the internal energy per unit of volume. In this way, the model takes into account both multi-axial loading and deformation. The model is formulated when the creep constitutive relationships may be expressed by primary plus steady-state or steady-state alone (IED-SS) as well as for the cases when the material behaviour includes the creep tertiary region (IED-T).The proposed model has been verified by applying it to various components for which the experimental creep lives are available from literature including thick/thin cylindrical vessels, notch bars with various notch-root radii and materials, multi-material cross welds bars, and perforated biaxial plates. The predicted creep lives of these components by the proposed model (IED-SS and IED-T) are compared with the experimental results and those obtained by the Reference Stress Method (RSM). It is shown that the maximum errors in relation to the creep lives of the above-mentioned components are: 18% when IED-SS is applied, 38% when IED-T is applied, and 301% when RSM is applied.To estimate the effects of uncertainties in material data on the predicted creep life, a sensitivity analysis has been conducted. To this end and in relation to Norton creep law, material parameters such as creep stress coefficient and stress exponent are considered. In addition, the sensitivity analysis included the uncertainties related to the uniaxial creep rupture data. As might be expected, the results suggest that the predicted creep life is most sensitive to the creep stress.Finally, the present research reveals that the proposed model is simple, practical and can be used in conjunction with any commercial finite element code with creep analysis capabilities