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.

Creep degradation of steam carrying vessels in the power generation industry is a concern that needs to be constantly monitored. The Weldcore® process has been earmarked as a potential method of creep sampling which will allow for thick-walled sections to be analysed. A component of the process involves plugging the resultant hole after removing a creep sample using a novel welding technique called friction hydro pillar processing. At the commencement of this study, insufficient data was available to warrant safe industrial application of the process. This research was conducted to evaluate the performance of 10CrMo910 friction hydro pillar process welds. The effects of downward force, stud taper angle, hole taper angle and hole base diameter on process response, defect population, static properties and dynamic performance were evaluated. The variation of downward force showed that higher forces produce significantly smaller defects and higher fatigue life. The occurrence of defects was linked to process parameters and geometry thereby identifying the correct parameters for safe use in the power generation industry. Flash formation was identified as an early indicator of weld defects and can assist with quality control in industrial applications. Methods of standardising the plunge depth and forge force were developed to identify the correct magnitudes for different geometries, without the need for testing. Defects were shown to populate specific regions of the weld and produce variations in fatigue life. Crack initiation sites were detected which will aid in identifying areas of focus in further research and development. Temperature measurements were linked to the occurrence of defects and crack initiation sites and have been identified as a method of identifying defective welds. The effects of process parameters and stud and hole taper angles on energy inputs and near interface temperatures were statistically evaluated. Downward force was shown to have the largest effect on energy input rates, total energy input and temperatures at the 11.5mm and 20.5mm positions. Smaller hole and stud taper angles produced lower energy inputs and were identified as more energy efficient than the larger taper angles. A regression model was also developed to predict the fatigue life of welds and can assist with critical process related decision making. A range of hole base diameters were identified which produced welds with low defect populations and fatigue performance similar to that of the parent plate. Larger hole base diameters were shown to produce significant defects along the hole bottom fillet, in the weld nugget and along the bond line. Temperature measurements of the larger diameter welds showed a delay in response and are attributed to a delayed contact of plasticised stud material with the sidewall. Welds with hole base diameters larger than 11mm produced unrepeatable and defective welds, and also required higher energy inputs making smaller diameters more desirable. Analysis of all welds in this study revealed that clearance and interfacial pressures characterise the quality of friction hydro pillar process welds, therefore models were developed to aid in critical decision making with respect to downward force and geometry. This study has successfully evaluated the effects of process parameters and geometry on the properties of friction hydro pillar process welds and thereby has increased understanding of the process.

The stress/strain relationship for most engineering materials is known to be time dependent. This is most evident during a creep test in which continual deformations are observed under constant stress conditions. In the laboratory, a specimen of cohesive soil subjected to a constant shear stress may fail after having deformed alt relatively slow rates for a considerable time. This type of failure, termed creep rupture, is also known to occur in the field. Results of drained and undrained triaxial creep rupture tests are presented in this thesis. These tests were performed on a sensitive marine clay from western Canada which was consolidated to various stress histories. Pore pressure measurements were taken during undrained tests using an electrical transducer. In addition to the creep rupture tests, incremental load and constant strain rate triaxial tests were performed for comparative purposes. The strain rate during a creep rupture test was observed to initially decrease as the specimen strained, reach a transient minimum strain rate, and then increase until rupture. Failure was found to be inevitable whenever the strain rate started to increase after having reached a minimum value. Pore pressures measured during the undrained tests did not reflect the onset of creep rupture at the transient minimum strain rate, and therefore, the onset of creep rupture cannot be explained in terms of effective stresses. A relationship was found to exist between the deviator stress, strain and current strain rate during undrained triaxial tests having the same consolidation history. This relationship permitted the prediction of the results of constant strain rate tests based on the results of creep rupture tests. This resulted in an understanding of the interrelation between the transient minimum strain rate of a creep rupture test and the maximum deviator stress of a constant strain rate test. Once the transient minimum strain rate had been reached, the results of creep rupture tests showed that the strain rate was inversely proportional to the time remaining before rupture. This relationship is independent of stress level, consolidation history and drainage conditions. As a result, it is suggested that measurement of deformations in the field can be used to predict the time until a sudden failure would be anticipated. The upper yield strength, defined as the maximum compressive stress which will not cause a creep rupture failure, was evaluated from both creep rupture and constant strain rate tests. It was found that the compressive strength increased as a linear function of the cube root of the strain rate. / Applied Science, Faculty of / Civil Engineering, Department of / Graduate

A compressive creep study of cold compacted colloidal boehmite has been carried out during the dehydroxylation reaction. The creep tests were made as a function of temperature, applied stress and the relative density of the cold compact. The activation energy for creep has been found to be 9.1 ± 1.5 Kcal/mole. The total creep rate was due to the stress associated with the neck formation at the points of contact and the applied stress. The creep rate is proportional to the applied stress. The final form of the total creep rate equation is ė =[0.144 exp ( formula omitted )+2.2x10ˉ⁷σ] sec ˉ¹. Electron photomicrographs of fractured surfaces of deformed specimens revealed the presence of contact points in the aligned fibers, confirming the existence of the driving force for shrinkage. Equations relating the change in length and the strength of a compact with time have been tested with the experimental data, which indicated that the rate controlling mechanism may be volume diffusion for the creep process. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

A theoretical and experimental investigation into the creep of a sensitive clay under different deviator stresses is presented. The effect of deviator stress on pore-pressures within the soil is also studied. An explanation of the phenomenon of creep is offered which is based on a consideration of the micro-structure of the clay. Experimental evidence to support this explanation is presented. The experimental work consisted of standard triaxial "consolidated undrained tests" performed on cylindrical specimens. Pore-pressures were measured by means of a pore-pressure panel based on the M.I.T. design. The observed data show that, with other variables remaining constant, the strain rate increases with increase in applied stress. Experimental results do not conclusively prove the existence of an upper yield value for the clay tested, but they do suggest it as a possibility. With the temperature remaining constant, the test results show that the pore-pressure increases with increase in applied stress. Under a given applied deviator stress the pore-pressure increases at a decreasing rate and does not attain a maximum or constant value at any time during the test. / Applied Science, Faculty of / Civil Engineering, Department of / Graduate

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Several theories of creep and creep rupture are reviewed. Specific attention is devoted to the brittle damage theory proposed by Kachanov. Creep, damage and life predictions for rectangular or circular cross section beams under bending and tensile loads are presented. Comparison with data for a Ni Superalloy showed life predictions could be 30X in excess of experimental values. This beam model also revealed that it is imperative that no bending moments be inadvertently applied during tensile creep testing. The creep-damage material model is extended to multidimensional situations. A refinement, whereby no damage accumulates in compression, is incorporated. A User-Material subroutine for this constitutive model has been formulated, and incorporated into the ABAQUS FEM package. Several verification examples are presented; one example is the creep-damage behaviour of a notched bar in tension. The value of reference stress techniques is discussed. Reference stress estimates for a centrifugally loaded bar, as well as for a cantilever under distributed loads, are presented. These could be useful in turbine blade design. Bibliography: pages 91-92.

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