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The mechanism of formation of striations on the fatigue fracture surface is considered from a crystallographical point of view for striations observed by scanning electron microscopy on a Type 316 stainless steel fractured by low-cycle fatigue at high temperatures. It is observed that striations change their directions abruptly and regularly at parallel lines which are considered to be twin boundaries. The directions of striations in relation to twin boundaries and the stereographic relation of the neighbouring fracture surfaces are obtained. Straight interstriation markings parallel to the main striations are sometimes observed. In some regions striations with sharp saw-toothed edges and saw-toothed interstriation markings are also observed. One part of these striations is considered to be formed by Pelloux's mechanism. The other part is considered to be formed by a mechanism that premised deformation zones consisting of cross slip planes in front of the crack.

In order to collect the results of micro-fractographic studies of austenitic stainless steel SUS 32 with fractured surfaces fatigued at high temperatures, and to find how far the number of cycles to fatigue failure is dependent on temperature and strain rate, the material was submitted to push-pull low cycle fatigue tests at high temperatures, and observation of the fractured surfaces was carried out by a scanning electron microscope.For fatigue test conditions the strain rates were set at 40, 4 and 0.4%/min, and the temperatures were set at room temperature, 450°, 600° and 700°C.The conclusions obtained are as follows;(1) Striations are observed in each fatigue condition. The striations observed clearly at high temperature are also corresponding to the respective strain cycles.(2) The striations are bent at twin boundaries.(3) Under the conditions of 600°C, 0.4%/min and large strain ranges, grain boundary fracture is observed clearly, which is of the same sort as that which is observed on the fracture surface at the creep rupture test made at 600°C.(4) Uuder the condition of 700°C, grain boundary fracture is also observed, but its surface is not always smooth.(5) Under the strain rate of 40%/min at 600° and 700°C, striations are observed more easily than grain boundary fracture.

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 order to investigate how mechanical and temperature conditions influence low cycle fatigue fracture, it is necessary to give attention to the process of fatigue fracture. The behaviour of fatigue cracks was observed in order to clarify the process of fatigue fracture. Fatigue tests were carried out using a AISI 321 stainless steel by a push-pull testing machine. According to the present experimental results, it is clarified that when fatigue tests are carried out under the same plastic strain range but different temperature conditions, the similar state of cracks are observed, provided the same ratio of number of repeated cycles to the number of cycles to fracture is given. Thus, it is concluded that the process of the low cycle fatigue fracture are influenced mainly by the plastic strain range, and that the rate of fatigue fracture depends on temperature.

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.