Zone of Operation
Ultimate Strength or Mean Stress
Figure 11-.. Goodman .iagram
A Goodman diagram of the material is often used to determine the amount of alternating stress on the blades at different loadings. The Goodman Diagram is shown in Figure 11-5. The Goodman diagram is particularly helpful in determining the effectiveness of a material or component that will be subjected to a cyclic stress superimposed upon a non-zero mean stress. The horizontal axis is the Mean or Stress or Ultimate Strength of thematerial in psi or MPa， and the vertical axis is the Alternating Stress， which is half the ultimate strength or mean stress multiplied by any correction or safety factors.
Thermal Fatigue
Thermal fatigue of turbine blades is a secondary failure mechanism. Temperature differentials developed during starting and stopping of the turbine produce thermal stress. The cycling of these thermal stresses is thermal fatigue. Thermal fatigue is low-cycle and similar to a creep-rupture failure. The analysis of thermal fatigue is essentially a problem in heattransfer and properties such as modulus of elasticity， coefficient of thermalexpansion， and thermal conductivity.
The most important metallurgical factors are ductility and toughness. Highly ductile materials tend to be more resistant to thermal fatigue. They also seem more resistant to crack initiation and propagation.
Research programs are underway to demonstrate that brittle materialscan be successfully utilized in demanding， high-temperature structural appli-cations. From the work alreadydone， it has been established that siliconnitride and silicon carbide， in their variety of forms and fabrications， are the two most likely candidates for the future ceramic engine. Both exhibit asuitable workability， the desired strength at high temperatures， and havespecific resistance， availability， and manufacturing ease to make them likely prospects for gas turbine components.
The operating schedule of a gas turbine produces a low-frequency thermal fatigue. The number of starts per hours of operating time directly affects the blade life. Table 11-1 shows fewer starts per operating time increases turbine life.
Corrosion
The use of Ni-base superalloys as turbine blades in an actual end-use atmosphere produces deterioration of material properties. This deterioration can result from erosion or corrosion. Erosion results from hard particles impinging on the turbine blade and removing material from the blade surface. The particles may enter through the turbine inlet or can be loosened scale deposits from within the combustor.
Corrosion is described as hot corrosion and sulfidation processes. Hot corrosion is an accelerated oxidation of alloys caused by the deposition of
Na2SO4. Oxidation results from the ingestion of salts in the engine and sulfur from the combustion of fuel. Sulfidation corrosion is considered a form of hot corrosion in which the residue that contains alkaline sulfates. Corrosion causes deterioration of blade materials and reduces component life.
Hot corrosion is a rapid form of attack that is generally associated withalkali metal contaminants， such as sodium and potassium， reacting with sulfur in the fuel to form molten sulfates. The presence of only a few parts per million(ppm) of such contaminants in the fuel， or equivalent in the air， is sufficient tocause this corrosion. Sodium can be introduced in a number of ways， such assalt water in liquid fuel， through the turbine air inlet at sites near salt water or other contaminatedareas， or as contaminants in water/steam injections. Besides the alkali metals such as sodiumand potassium， other chemical elements can influence or cause corrosion on bucketing. Notable in thisconnection are vanadium， primarily found in crude and residual oils.
There are now two distinct forms of hot corrosion recognized by theindustry， although the end result is the same. These two types are high-temperature (Type 1) and low-temperature (Type 2) hot corrosion.
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