A study has been made of creep damge accumulation in two casts of l%.Cr-1/2%i.Mo low alloy steel. Creep tests and creep crack growth tests have been carried out at 823K to determine the nature of the damage accumulation and to attempt to relate microscopic damage mechanisms to the macroscopic fracture parameters. Four types of specimen were tested and failure of all occurred by the continuous nucleation. growth and coalescence of grain boundary cavities. A mechanism for the growth of cavities is suggested. based on grain boundary diffusion coupled with geometric constraint. The influence of continuous cavity nucleation has also been considered and it is suggested that this phenomenon initially increases the rate of diffusive cavity growth. However continuous nucleation decreases the growth rate once the latter becomes constrained. The effect of stress-state is also considered and increasing triaxiality is shown to have little effect on the unconstrained diffusive growth but it decreases the constrained growth rate by increasing the overall constraint in the specimen. Predicted growth rates give good agreement to those observed experimentally for both notched and un-notched creep specimens. Reasonable agreement is also observed to the predicted rupture lives although the predictions suggest notch strengthening whilst experimentally notch weakening is observed. This is thought to be due to non-uniform damage formation on loading. Based on the above concepts of cavity growth, constitutive equations are presented to predict the time dependence of creep strain. These are found to give good agreement to the experimentally determined strain rates, lending further support for the development of continuum damage mechanics as a means of assessing creep crack growth behaviour.

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Welded steel pipes are an essential structural part of any power plant. Longer lifetimes and higher pressures in the pipes cause an increased probability of failure due to creep damage. To maintain safe operation, nondestructive evaluation techniques to detect creep damage are needed. Nonlinear acoustic techniques employing longitudinal waves have been proven to be sensitive to creep damage. The objective of this research is to develop a robust experimental procedure to reliably measure the acoustic nonlinearity parameter using longitudinal waves, and then to validate the procedure on samples of different materials and sizes. Finally the technique is applied to characterize creep damage levels around the weld of a welded steel pipe. While the experimental technique presented can only measure the relative nonlinearity, it is accurate enough to detect changes in nonlinearity due to creep damage. Measurements show an increase in nonlinearity in the heat affected zone (HAZ). Experiments after annealing the creep damaged specimen show a decrease in nonlinearity in accordance with a decrease in dislocation density. Measurements on an undamaged welded A36 steel component suggest that the heat itself is not responsible for the increase in nonlinearity.

Considering the unlikely core melt down scenario for a light water reactor (LWR) a possible failure mode of the reactor pressure vessel (RPV) and its failure time has to be investigated for a determination of the loadings on the containment. Worldwide several experiments have been performed accompanied with material properties evaluation, theoretical, and numerical work. At the Institute of Safety Research of the FZR a finite element model has been de-veloped simulating the thermal processes and the viscoplastic behaviour of the ves-sel wall. An advanced model for creep and material damage has been established and has been validated using experimental data. The thermal and the mechanical calculations are sequentially and recursively coupled. The model is capable of evalu-ating fracture time and fracture position of a vessel with an internally heated melt pool. The model was applied to pre- and post test calculations for the FOREVER test se-ries representing the lower head RPV of a PWR in the geometrical scale of 1:10. These experiments were performed at the Royal Institute of Technology in Stock-holm. The results of the calculations can be summarised as follows: # The creeping process is caused by the simultaneous presence of high tem-perature (>600 °C) and pressure (>1 MPa) # The hot focus region is the most endangered zone exhibiting the highest creep strain rates. # The exact level of temperature and pressure has an influence on the vessel failure time but not on the failure position # The failure time can be predicted with an uncertainty of 20 to 25%. This uncer-tainty is caused by the large scatter and the high temperature sensitivity of the viscoplastic properties of the RPV steel. # Contrary to the hot focus region, the lower centre of the vessel head exhibits a higher strength because of the lower temperatures in this zone. The lower part moves down without significant deformation. Therefore it can be assumed, that the vessel failure can be retarded or prevented by supporting this range. # The development of a gap between melt crust and vessel wall could not be proofed. First calculations for a PWR geometry were performed to work out differences and commonalities between prototypic scenarios and scaled experiments. The results of the FOREVER-experiments cannot be transferred directly to PWR geometry. The geometrical, mechanical and thermal relations cannot be scaled in the same way. Because of the significantly higher temperature level, a partial ablation of the vessel wall has to be to expected in the PWR scenario, which is not the case in the FOREVER tests. But nevertheless the FOREVER tests are the only integral in-vessel retention experiments up to now and they led to a number of important insights about the behaviour of a vessel under the loading of a melt pool and pressure.

Anisotropic tertiary creep damage formulations have become an increasingly important prediction technique for high temperature components due to drives in the gas turbine industry for increased combustion chamber exit pressures, temperature, and the use of anisotropic materials such as metal matrix composites and directionally-solidified (DS) Ni-base superalloys. Typically, isotropic creep damage formulations are implemented for simple cases involving a uniaxial state of stress; however, these formulations can be further developed for multiaxial states of stress where materials are found to exhibit induced anisotropy. In addition, anisotropic materials necessitate a fully-developed creep strain tensor. This thesis describes the development of a new anisotropic tertiary creep damage formulation implemented in a general-purpose finite element analysis (FEA) software. Creep deformation and rupture tests are conducted on L, T, and 45°-oriented specimen of subject alloy DS GTD-111. Using the Kachanov-Rabotnov isotropic creep damage formulation and the optimization software uSHARP, the damage constants associated with the creep tests are determined. The damage constants, secondary creep, and derived Hill Constants are applied directly into the improved formulation. Comparison between the isotropic and improved anisotropic creep damage formulations demonstrates modeling accuracy. An examination of the off-axis creep strain terms using the improved formulation is conducted. Integration of the isotropic creep damage formulation provides time to failure predictions which are compared with rupture tests. Integration of the improved anisotropic creep damage produces time to failure predictions at intermediate orientations and any state of stress. A parametric study examining various states of stress, and materials orientations is performed to verify the flexibility of the improved formulation. A parametric exercise of the time to failure predictions for various levels of uniaxial stress is conducted.

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