Investigation of Thermal Fatigue : Behavior of Cracks during Thermal Fatigue

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In this paper, as an attempt at the quantitative representation of the behavior of fatigue cracks during thermal fatigue, the concept of "crack density"is proposed. This can be defined by n^^-=1/(2πr)∫^<2πr>_0 nds where n denotes the number of cracks intersecting a meridian line drawn on the surface of specimen, and r indicates the radius of cylindrical specimen. Experiments were carried out in two different temperature ranges ; 200〜600℃ and 300〜700℃. The values of crack density on the surface of a specimen, after a certain number of repetitions of thermal load before failure, were measured by optical microscope. Then relations between the values of crack density and crack depth were investigated. The behavior of cracks at a certain stage is discussed also. It was found that the crack density n^^- is one of the appropriate parameters to represent the damaged state of specimen.
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