Fatigue failure of materials in the plastic range is considered to have an increasingly important significance nowadays, since the results obtained from these investigations provide the fundamental data not only for the design of some pressure vessels, but also for the study of thermal fatigue failure of materials. Several experimental results have been reported concerning the fatigue strength of some materials in the plastic range at room and elevated temperatures.
We also planned these tests, and in the first place constructed a new device that made possible to carry out the completely reversed axial fatigue test with a conventional static testing machine. This device is necessary in most cases of this type of experiments, because the conventional static testing machine can not provide the tensile and compressive load to a specimen cyclically.
Secondly, we made two types of experiments with low carbon steel using this device at room temperature-constant strain range test and contstant stress range test.
The stress and the strain characteristics in the respective tests were compared in detail, and discussions were made concerning some hypotheses in order to interpret the relationship between these tests.
The main results are as follows:
(1) In the constant strain test the maximum tensile and compressive stress increase with the number of cycles due to the strain hardening effect. But in the constant stress test the mean tensile strain increases gradually with the number of cycles regardless of the initial loading directions'-tension or compression.
(2) The true stress based on the actual cross sectional area must be considered when a discussion is made about the shape of stress-strain curve especially in the case of high stress or high strain conditions.
(3) The total strain given to the specimen during the period up to the failure is found to be the most important factor affecting the fatigue failure in the plastic range. With this total strain we can interpret the respective test results of constant strain and constant stress together.

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High frequency cantilever beam rotating fatigue test was carried out with 0.44% carbon steel at a temperature of 500°C with a frequency of 10000rpm, and the effect of periodically applied intermittent loading upon the specimen was investigated at two stress levels that were chosen from the basic S-N curve obtained from the ordinary test without rest periods.
From the obtained results, it was concluded that the existence of rest periods which were given with definite periods in the loading cycles, rather reduced the fatigue life of steel at elevated temperature, compared with that of the continuous loading test, especially in the case when loading frequency is as high as 10000rpm.
Such tendency does not agree with that of the previous investigations by several authors. The cause and the meaning of these different tendencies seem to be caused by many factors, such as testing frequency, method of inserting rest period, kind of stress, but it is not easy to clarify these points at the present stage of study.

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We carried out rotatory bending fatigue tests on smoothed specimens of 0.26% carbon steel. Describing in details, at room temperature and 300°C many-fold multiple repeated load fatigue tests, and at 300°C double repeated load fatigue tests were made in two stress levels. The fatigue tests under varying temperature and stress amplitude in two conditions were also conducted at room temperature, 300°C and 500°C. In these tests, the cycle ratio at the primary temperature and stress was 40%, and the fatigue testing machine was operated at 3000rpm. The results of the experiment were as follows:
(1) In both cases of room temperature and 300°C, the cumulative cycle ratios in the many-fold multiple repeated load fatigue tests in two stress levels are 0.7-0.8 as for this material.
(2) When the fatigue life at the primary temperature and stress in larger than that at the secondary temperature and stress, the cumulative cycle ratio Σn/N is larger than 1. When the fatigue life at the primary stage is smaller than that at the secondary stage, the value of Σn/N is smaller than 1.

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In the present study, experiments were carried out to find the influence of stress on the micro-Vickers hardness, using carbon steel.
Mechanically applied stress, residual stresses due to heat treatment and cold-working were given to specimens. For measuring surface stress, the X-ray technique reported by Christenson and Rowland was employed. As an application of this experiment, the change in residual stress and hardness due to the aging at room temperature were measured. Experimental results were discussed from the standpoint of relief of stress due to indentation.
The results of experiments are summarized as follows: -
(1) Hardness increases proportionally to the magnitude of surface compressive stress and, on the contrary, inversely to tensile stress. A linear relationship holds between stress and hardness.
(2) The change in hardness due to cold-working is caused by residual stress and work hardening.
(3) The change in hardness of the quenched specimen due to aging, is influenced by the residual stress and the transformation of metal. Both hardness and residual compressive stress decrease until about ten days after quenching, and thereafter hardness increases gradually, while residual compressive stress maintains nearly a constant value except for a temporary increase at about 15th days after quenching.

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The sedimentation of clay materials has been considered, and the following results are obtained:
(1) Velocity of sedimentation is influenced by the plasticity of clay materials,
(2) Reynolds Number Relation is unable to be applied to the sedimentations of Chikuho clay and Zettlitz kaolin, but the sedimentation of Yakusa Kibushi clay and Sidare Kibushi clay can be analysed successfully by Re-Relation.

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