Materials, when subjected to reversed stress, reveals some magnitude of plastic strain even at the stress of the endurance limit. Then, if plotted in the stress versus strain diagram, deviation from the linear elastic line takes place, and the so-called elastic hysteresis curve is obtained. The area of this closed curve has a dimension of energy, and its value represents the work given from the external system to the material. In the course of fatigue of metals, most of this energy may be turned to heat, and lost away from the material. But the rest of it is considered to be accumulated in the material and to result in some structural changes, starting of cracks, and finally the failure of the material.
In this way, many investigators have supposed that there exists a close relationship between the fatigue strength and the above mentioned elastic hysteresis energy. In fact, a number of theoretical and experimental literatures have been issued for the study of this relationship.
However, the detailed or quantitative relationship between these values is not known practically at present. The cause of it seems to be based mainly on the minuteness of magnitude of the plastic strain at the stress near the endurance limit, and many difficulties occur in connection with the detection of it.
Recently, remarkable development of strain measurement by the electric wire strain gauges has made such an investigation possible, and the studies in this field are being conducted by many investigators.
Authors had also made the examination about the measuring method of this plastic strain, and has succeeded recently in developing the equipment to magnify the longitudinal deformation of the specimen more than 2000 times, using the principle of Martens mirror extensometer. Previous tests have proved the good accuracy and reliability for the long time measurement.
Fatigue tests were performed on 0.21% carbon steel and heat resistant Cr-Mo-V steel at room temperature under completely reversed direct stress, and the plastic strain of each test specimen was measured during the test.
The results show that there exists a linear relation between the plastic strain and the number of stress cycles to failure for respective steel, if plotted each in logarithmic scale.
Moreover, an interesting fact is that the results plloted in the figure of the elastic hysteresis energy versus stress cycles to failure indicate a good agreement for both steels, that is, the test results of each steel fall on the same straight line in the figure of logarithmic scale, in which the elastic hysteresis energy is calculated as the product of plastic strain and stress.

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This paper describes about a new testing machine devised for fatigue test under repeated impact at high frequency and the test results obtained by it.
The mechanism of machine is as follows:
When a crank mechanism makes the upper end of the spring reciprocate, an impact hammer, which is hung from the lower end of a spring, receives a forced oscillation in vertical direction, so that the hammer may impact a test roller-chain through a cross-piece.
The result on the theoretical consideration of the oscillation of the hammer and that obtained by experiments are as follows:
(1) When the crank radius and the clearance between the impact hammer and the cross-piece are suitably set, the machine operates steadily in a satisfactory condition.
(2) By raising the resonance point of the machine, we can make tests under higher frequency.
(3) Using the resonance by the spring, the machine does not bring unnecessary impact and consumes just a little power.
(4) The fatigue test result about test roller-chain was satisfactory with little dispersion.

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The study was carried out on the variation of the fatigue strength and the mechanical properties due to cold drawing and annealing for the specimens of nickel silver wire.
The specimens were cold drawn and straightened again into 2m/m∅, and Haigh-Robertson. testing machines were used for the fatigue test.
The followings are the results obtained:
(1) Due to cold working the nickel silver wire was increased in tensile strength, yield strength, spring limlt, hardness, and fatigue strength, but, on the contrary, decreased in elongation. The number of torsion attained to the maximum when the specimens were reduced to 20%.
(2) Low annealing effects do not make the fatigue strength increase for the cold drawn and straightened specimens. Especially, in the case of low drawn wire materials, the mechanical properties and fatigue strength will decrease with annealing temperature. As for high drawn wire, the mechanical properties and fatigue strength whose values are lower than those which are not low annealed were increased at 250-350°C.
(3) The value of the fatigue ratio of the nickel silver wire was 0.35 when cold drawn and approximately 0.25 when low annealed.
(4) The value of spring limit corresponds to the value of yield strength which may be calculated from the results of tension tests.

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The authors have investigated upon the variation of hardness value and microstructure due to fatigue fracture in nickel silver wire. Both cold-drawn and low annealed specimens with 2m/m in diameter were used. Also, Haigh-Robertson testing machine was used.
The followings are the results obtained:
(1) The cold-worked nickel silver wire may be decreased in hardness value due to fatigue fracture, but it is not affected by the amount of repeated stress. It would be because of the softening phenomena where the nonuniform worked structure may be turned into grain growth states due to the repeated stress.
(2) The 600°C×1hr annealed nickel silver wire may be increased in hardness value due to fatigue fracture but it is not affected by the amount of repeated stress.
(3) The low annealed nickel silver wire may be increased or/and decreased in hardness value, which is governed by the amount of repeated stress and the annealing temperature.

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Recently several studies of the photoplasticity are proceeded. In this paper, the fundamental law about the birefringent phenomena in plastic deformation of high polymer solids is investigated.
In the first place, as the definition of strain the Lagrangian representation of finite deformation is adopted according to V.V. Novozhilov. And it is induced theoretically from the four appropriate demands that the fring order per unit thickness N representing the birefringent effect is expressed, in the plane strain state, by the sum of a term in proportion to the principal stress difference (σ₁-σ₂) and a term in proportion to the principal strain difference (ε₁-ε₂):
N=C₁·(σ₁-σ₂)+C₂·(ε₁-ε₂).
Within the limits of the elastic deformation region, the above formula is reduced to the well known formula in photoelasticity, i.e. the fring order is proportional to the principal stress difference.
In our experiment, the celluloids of the five kinds different in the quantities of the camphor are adopted. In order to maintain the test piece in the plane strain state, it is held between two flat glass plates; and in order to maintain the constant loading direction, the ordinary loading apparatus of photoelasticity is improved to be able to shift the fulcrum of its lever with the increasing load.
The coefficients C₁ and C₂ are determined by the least square method. And the appropriateness of the above theoretical formula is proved in the elastic and the plastic deformation regions of the celluloid material.

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