Fully three-dimensional, SCF ground-state computations for the Hartree equation are carried out by a finite element approach that completely avoids forming or storing the Fock matrix. A combination of strategies is used to reduce storage and computational requirements by orders of magnitude over the traditional finite element approach, which makes three-dimensional molecular orbital computations feasible. Results using the three-dimensional formulation and computer program are shown for one-electron systems: He' and H : , and for two-electron systems: He and Hz. The best results are within about 30-100 micro-Hartrees of the exact values of the total energies for the ground states of these systems, indicating that our three-dimensional approach has been correctly implemented in the computer code.
Gas Turbines -Materials, Modeling and Performance 56 the hot corrosion resistance. This is further complicated for marine applications by the aggressivity of the environment, which includes sulphur and sodium from the fuel and various halides contained in seawater. These features are known to drastically reduce the superalloy component life and reliability by consuming the material at an unpredictably rapid rate, thereby reducing the load-carrying capacity and potentially leading to catastrophic failure of components [3][4][5]. Thus, the hot corrosion resistance of superalloys is as important as its high temperature strength in gas turbine engine applications. Recent studies have shown that the high temperature strength materials are most susceptible to hot corrosion and the surface engineering plays a key role in effectively combating the hot corrosion problem [6][7]. Therefore, the two materials (base and coating) must be considered as an integral system and the interface between them is also equally important as it can often be the limiting factor. This chapter explains the critical issue of hot corrosion of superalloy components in gas turbine engines and methodologies to select appropriate materials and coatings for its prevention. It is followed by an assessment of current status of coatings, coating techniques and finally, some of the critical areas that need to be addressed for development of more efficient and smart coatings.
Quantitative prediction of environmentally assisted cracking such as stress corrosion cracking is one of the greatest concerns in lifetime management and consequent lifetime extension of light water reactors. Continuum mechanics has been applied to quantify the effect of crack tip mechanics on crack tip film degradation and its physical interaction with the oxidation kinetics. Besides such an effect, it has been realized that crack tip stress/strain can significantly affect the oxidation kinetics by a physical-chemical mode. The present paper focuses on optimizing crack tip asymptotic fields, oxidation kinetics laws, and their interaction modes for modeling stress corrosion cracking growth rates. Meanwhile, the physical-chemical effect of stress/strain on solid state oxidation kinetics at the stress corrosion crack tip is emphasized. The criteria for formulating stress corrosion cracking and optimizing input parameters are also discussed.
Locally delaminating cracks, i.e. cracks that grow along the plane that is parallel to the principal applied loading direction, are observed on the fracture surfaces of heavily strain-hardened austenitic alloys after stress corrosion cracking tests in hydrogenated high temperature water environments. These delaminating cracks often occur locally on mixed intergranular-transgranular stress corrosion cracking fracture surfaces. The crack tip stress field is analyzed by finite element analysis. The physical-chemical degradation effect of stress on oxidation kinetics is formulated. The combinations of local oxidation penetration, relatively slow crack growth on the principal plane, and locally high crack tip stress are favorable for the growth of delaminating cracks. These conditions can be satisfied in stress corrosion cracking for austenitic alloys with heavily deformed microstructures and high strengths in hydrogenated high temperature water environments.
Stress-corrosion tests of Cu-30 %Zn alloy under a constant strain rate were conducted in Mattsson's type solution of both non-tarnishing(pH 4.3) and tarnishing(pH 7.0) conditions, in order to examine the relation between the mode of cracking in tarnishing and non-tarnishing solutions. Fractographic examination showed that Intergranular cracking predominated in a non-tarnishing solution of pH 4.3, where it has been reported that U-bend specimen failured indicates the nature of transgranular cracks. It is thought that a single distinction of the cracking form by tarnishing or non-tarnishing conditions is not justified, due to the observed evidence of intergranular stress-corrosion cracking occured in both conditions. 1.
Uneven crack fronts have been observed in laboratory stress corrosion cracking tests. For example, cracking fronts of nickel-base alloys tested in simulated boiling water reactor (BWR) and pressurized water reactor (PWR) environments could exhibit uneven crack front. Analyzing the effect of an uneven crack front on further crack growth is important for quantification of crack growth. Finite-Element analysis shows that the local KI distribution can be significantly affected by the shape and size of the uneven crack front. Stress intensity factor at the locally extended crack front can be significantly reduced. Since generally there is a nonlinear CGR versus KI relationship, it is expected that crack growth rate at the locally extended crack front can be significantly different from those in the neighboring areas. There could be several patterns for the growth of an uneven crack front. For example, once initiated, the crack growth rate in areas other than the locally protruded front would become higher and then the whole crack front would tend to become uniform. On the other hand, if the crack growth in other areas is still low, there is a possibility that the crack growth rate at the front tip would slow down.
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