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, spacecrafts, 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 multiaxial loading and deformation. The model is formulated when the creep constitutive relationships may be expressed by primary plus steadystate 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.

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, spacecrafts, 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 multiaxial loading and deformation. The model is formulated when the creep constitutive relationships may be expressed by primary plus steadystate 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.

A wide range of experiments have been carried out on a 2CrMoNiWV low alloy steel to investigate the effect of various heat treatment conditions on microstructural change, alloy carbide transformation mechanism and mechanical properties. Two complete continuous cooling transformation (CCT) diagrams were constructed for this steel on the basis of experimental dilatometry thermal analysis, metallographic examination and current phase transformation theory. The significance of these two diagrams is in that they can be directly utilised in industrial practice as a reference during heat treatment for this material. Meanwhile it was confirmed that this 2CrMoNiWV steel can be transformed to a fully bainitic microstructure over a wide range of cooling rates and this feature proved this steel suitable for large diameter steam turbine rotor application. An innovative carbide extraction technique for the XRD identification of carbide phase has been developed. The detailed description of this new technique and its advantages are discussed in this thesis. The extensive work using TEM/EDX has set up essential "finger prints" for the quick examination of large amounts of individual carbide existing at various heat treated conditions. Simultaneous measurements and determinations were made on particle composition, morphological change, the type, amount and distribution of these carbide phases. Thus the sequence of carbide transformation for this 2CrMoNiWV steel during tempering has been established. The characteristic microstructures of various heat treated specimens were carefully examined and discussed. Theoretical thermodynamic equilibria predictions were calculated using MTDATA. A very good agreement was found between experimental results and theoretical predictions on those critical transformation temperatures and a good correlation of carbide evolution sequences was obtained. Based on experimental results and theoretical predictions, the role of tungsten in promoting creep resistance to the material is elucidated. The usefulness of equilibrium thermodynamic calculations using MTDATA in predicting the microstructural changes and carbide evolution has been demonstrated in this work, particularly the separate effect of composition on the stable carbide dispersion where a thermodynamic approach offers great benefits. A possibly optimised heat treatment route is suggested for the large diameter rotor forgings which involves austenitising at 980°C for 10 hours following by oil quenching and then tempering at 675°C for 20 hours following by air cooling. Some general conclusions are drawn from this study, especially with regard to the effect of heat treatment on the microstructure of this 2CrMoNiWV steel and suggestions for further work are made.

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published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

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Thesis (M.S.)--University of Wisconsin--Madison, 1969. / eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

An extensive creep study of a superplastic material has not previously been made. The present study was carried out to determine if there are any basic differences between the creep of coarse grained materials and fine grained super-plastic materials. The results give information about the mechanical properties of superplastic alloys and are relevent to an understanding of the mechanics of superplasticiy. At high strain rates the superplastic lead-tin eutectic deforms by recovery creep and a 3-stage creep curve is observed, similar to that found for coarse grained materials. As the strain rate is decreased, the initial transient (primary creep) disappears and the creep curve is linear until necking occurs and tertiary creep ends in failure. In the principal superplastic range, at medium strain rates, creep curves are linear to at least 50 % strain. The recovery rate is immediately equal to the strain hardening rate and there is no primary creep. At low strain rates the creep curve is slightly convex as the creep rate decreases with time. This may be due to the self extinguishing nature of diffusional creep or possibly strain induced grain growth. These results are consistent with the grain boundary sliding theories of superplasticity although details of the accommodation processes are not known. At the lowest strain rates, diffusional creep may operate. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Two sets of data are analyzed in the thesis. The first set was derived from the long term deformations of 2 in x 6 in x 12 ft (40 mm x 140 mm x 3600 mm) joists of Douglas-Fir loaded under constant bending stress to levels lower than or equal to 3110 psi (21.44 MPa). The second set was derived from the long term deformations of 2 in x 6 in x 12 ft joists of Hemlock loaded under constant bending stress to levels of 3000 psi (20.68 MPa) and 4500 psi (31.0 2 MPa) . The analysis shows that the creep behaviour of structural size beams depends upon the material characteristics; specifically, material with a strength lower than 5000 psi (34.33 MPa) appeared to creep 1.5 times more than, material with a strength higher than that level, over a three month period. In addition, the test results support the assumption of a linear relationship between the creep deformation of a structural-size timber beam and applied stress. A method is presented to predict the creep behaviour of a structural-size specimen at discrete times over a three month period. The method consists of expressing the creep deformation, Δ[sub c], in terms of the elastic deformation, δ[sub e], or equivalently, the fractional creep (f = δ[sub c] / δ[sub e]) in terms of δ[sub e] or in terms of the modulus of elasticity. This work is limited to the stress levels investigated and to specific temperature (10°C< θ <30°C) and moisture content (8%<MC<12%) conditions. While this method could be employed in preliminary design procedures, it has been especially designed for more complex studies of the creep behaviour of structures including floor systems, trusses, etc... The advantage of the method is that in this kind, of analysis the modulus of elasticity of the individual components can be used. This thesis also presents a set of creep curves that cover a three year span. These creep curves show that the average total deformation of beams loaded to a stress level of 3110 psi (21.44 MPa), at this time, is approximately 1.6 times the elastic deformation. / Applied Science, Faculty of / Civil Engineering, Department of / Unknown