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In order to clarify how the number of strain cycles to failure is influenced by temperature and frequency of straining in low cycle fatigue, it is necessary to observe the process of the fatigue failure. To obtain the knowledge of the process, crack appearances existing on the failed specimens were compared with each other. Fatigue tests were carried out in a push-pull testing machine on the AISI 321 stainless steel under various temperatures and frequencies of straining conditions. According to the results, the crack appearance does not depend on the temperature and frequency of straining conditions but on the plastic strain range. If fatigue tests are carried out in the same plastic strain range but at different temperatures and frequencies of straining conditions, similar distributions of crack density from the outer surface to the interior of specimens are always observed in the failed specimens. Thus, it is concluded that in low cycle fatigue the behaviour of fatigue cracks is governed primarily by the plastic strain range and it is deduced that the rate of fatigue process depends on temperature, strain rate, and plastic strain energy per cycle.

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)∫2πr0 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°C and 300∼700°C. 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.