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Thermal fatigue failure is the result of repeated thermal cycling which generates repeated stress gradients within a free component or stress cycling within a component constrained to fixed dimensions. Thermal fatigue fractures are characterized by surface cracking formed by multiple initiation sites that join randomly by edge sliding to form the main crack. Other features to identify thermal fatigue are as follows:

Figure 5.10 Stress concentration at corners.

 Fractures are planner and transverse with no visible plastic deformation.

 Fracture is mostly transgranular.

 Oxidized fracture surfaces and oxide wedge filled cracks further characterize thermal fatigue failures

Fatigue resistance is affected by a number of controllable factors:

 The chemistry of the material and its resultant microstructure have a profound effect on fatigue strength. In fact, they can equally influence on mechanical strength (tensile and yield). Alloying elements, such as chromium, nickel, and moly, have the greatest effect on the iron base system. Solid solution alloys show the maximum increase in fatigue strength.

 Grain size appears to be a strong determining factor in inhibiting the plastic deformation process that occurs with crack propagation.

 Environmental factors such as cyclic temperature, temperature gradient, and corrosion pitting that result in stress concentrations. The thermal fatigue failure shown in was due to the temperature gradient across the thick wall section.

 Reduction of localized surface stress concentrations by such techniques as case hardening, shot peening, auto frottage, and thread rolling.

 Proper heat treatment can markedly improve fatigue resistance. As an example, for steels, a tempered martensitic.