In the previous papers, the authors reported on the experiments of the coiled springs of steel wire under pulsating compressive load, and obtained the fatigue strength and the deformation due to fatigue. Through the tests, the authors found that the deformation limit diagram of miniature coiled spring differed from that of the ordinary round steel bar.
In the same way as reported in the previous paper, the authors wanted to find the effect of three different conditions, that is, untreated, heat-treated and set conditions of the coiled compression springs of piano wire (PI). The authors also examined the deformation due to fatigue under various mean stresses of the coiled springs of piano wire, heat-treated and annealed at 800°C in vacuum, and compared this with the previous results.
The results of these tests are as follows:
1) The proportional limits, the fatigue limits and the creep limit of the three kinds of coild springs are as follows:
The untreated spring showed the lowest values in all these, the heat-treated one was the second, and the set spring was the higher than the others.
2) The residual shear strain of coiled springs under repeated compressive load was the largest in the pulsating test (τ_a=τ_m), but it decreased gradually as the mean stress increased up to τ_a/τ_m=0.6, and it appeared to increase more or less for the values of τ_a/τ_m smaller than 0.6.
3) A stress amplitude τ_a at endurance limit decreased gradually as mean stress increases.
4) τ_a in the fatigue creep limit diagram of the heat-treated spring kept approximately constant when τ_a/τ_m=1-0.6. While, τ_a began to decrease rapidly when mean stress increases beyond such a range, and the slope was about 60° to the abscissa τ_a=0.
5) In the coiled spring, annealed in vacuum at 800°C, the deformation limit diagram kept almost horizontal when τ_a/τ_m=1-0.6 and began to decrease remarkably beyond such a range and the inclination was approximately 45° to the abscissa τ_a=0.
6) It seems that the endurance limit diagram consists of an inclined line, and the fatigue creep limit diagram consists of a horizontal line (τ_a/τ_m=1-0.6) and an inclined line (τ_a/τ_m<0.6).

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The present test was carried out in order to investigate the following two points. (a) What changes take place in the macro-residual stress distribution of plastically twisted and stress aged carbon steel specimen due to strain-aging and cyclic torsional stress? (b) Which gives a larger effect on the static strength and the fatigue yield strength of carbon steel, the macro-residual stress or micro-structural change? The results obtained may be summarized as follows;
(1) If the degree of pre-torsion is kept equal, the skin residual stresses of the plastically twisted and stress-aged specimens are almost equal in value and residual stresses distribution in these specimens are working in the direction of increasing the pre-torsion in the outer layer and in the direction of recovering the pre-torsion in the inner layer.
(2) The residual stress was recognized to be decreased because of the strain-aging.
(3) The macro-residual stress of both plastically twisted and stress-aged specimens is affected very little by the pulsating torsional stress, but is influenced remarkably by the reversed torsional stress. These changes in residual stress may be attributed to the redistribution of piled-up dislocation by the minimum stress which acts towards the reverse direction of pre-torsion, the interaction of the redistributed dislocations or other lattice defects with solute atoms, and the plastic fatigue deformation which occur in the direction against pre-torsion.
(4) In spite of the results that work-hardening, metal structure and the macro-residual stress are nearly equal in plastically twisted and stress-aged specimens of the equal degree of pre-torsion, a large difference in the static strength and the fatigue yield strength of these materials was recognized in this test. In giving explanation on these difference, the effect of the atomic-microstructural change on these strength should be highly estimated.

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The problem of failure of structures subjected to cyclic thermal stresses is of interest both from fundamental and also practical points of view. The thermal stress arises because of thermal strain which is produced in consequence of constraint of free thermal expansion or contraction of structures. Thus, the thermal strain is an independent variable in the present problem.
The authors, as well, have been interested in the problems of lifetime of metals subjected to varying temperature or combined variation of temperature and stress, and have published some reports in this line. The present study is undertaken to obtain information regarding fundamental problems of thermal cycling of heat resistant alloys. For this purpose, a thermal cycling test apparatus was manufactured and the tests were performed, using 22Cr-1 Cu ferritic and 18-8 austenitic stainless steels. The followings are the results from our studies:
(1) The apparatus for thermal cycling give good patterns of both temperature cycling and thermal stress cycling for the thermal cycling test.
(2) Temperature dependency of failure life of thermal cycling on AISI 318 type stainless steel is not so remarkable as compared with the case of creep rupture life under temperature cycling for the same material.
(3) From a series of thermal cycling tests for mean temperature of 650°C on 318 type stainless steel, it is found that variation of wall thickness of hollow specimen from 0.5mm to 1.5mm has no significant effect on the failure life.
(4) If mean temperature is adopted as a basis of test temperature, the data of tests where minimum temperature is taken during temperature cycling as a basis of test condition can be estimated as well, provided that temperature dependency of failure life of thermal cycling is known.
(5) In thermal cycling tests, adoption of total strain amplitude which is to be composed of elastic and plastic strains is practically more useful than that of plastic component of strain.
(6) From the present experiments some pertinent problems which are to be developed are clarified.

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The failure life of metals subjected to thermal stress cycling is often discussed referring to the lifetime in strain-cycling. In this case, the comparison of both strength is made, taking the mean temperature of thermal cycling test is the same as the test temperature of strain-cycling. In this mode of comparison of strength for these two types of tests, a question arises as to whether the mean temperature may be used as the common basis of test condition. There opens another question to research; in this comparison, locarization of plastic strain due to uneven temperature distribution along the length of specimen is not taken into account. In this connection, special attention is paid to these particular points in the present study.
Thermal cycling tests on AISI 318 type stainless steel are performed by maintainig mean temperature of 650°C for various temperature amplitude. Strain-cycling tests are also made under steady temperature of 650°C which is equivalent to mean temperature of thermal cycling test, where temperature distribution along the length of specimen is uneven, being similar to the case of thermal cycling tests by using the same heating setup. From these two types of tests, it is found that, in the range of comparatively large value of total strain amplitude, both data are very close in log-log plots of total strain amplitude and number of cycles to fracture, but they are otherwise in the range of small total strain amplitude. A similar relation holds even when the total strain is replaced by plastic component of strain. From this, it is possible to mention that the difference of lives for both series of tests of thermal cycling and strain-cycling which has been emphasized by other investigators consists partly in the difference of temperature distribution along the length of specimens in both tests.

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Conducting the rotating bending fatigue test of mild steel specimens with a drilled hole of various size, we studied on bad influences exerted owing to the hole, and the others connected with it. The specimens are 12mm in diameter, 25mm in parallel length, and each hole drilled on them is 1, 2 and 3mm in diameter, and at intervals of 1mm from 1 to 12mm in depth.
The results obtained through our experiments are as follows:
(1) When the shafts with a hole of the same diameter and various depth are subjected to a rotating bending load, according as the hole depth increases, up to about 3mm in depth, the nominal fatigue limit σ_wn falls rapidly, and from about 4 to 8mm in depth, its lowering becomes very slow, and above about 9mm in depth, it becomes great again.
(2) In every size of hole, the fatigue limits σ_wk calculated on the cross section containing a hole axis are within the range of 15-18kg/mm², accordingly notch factor β is about 1.2-1.3. In Fig. 6 is shown an useful diagram for the design of shafts with hole to be subjected to the rotating bending load.
(3) When the shafts mentioned in (1) are subjected to the heavy load over fatigue limit, according as the hole depth increases, up to about 3mm hole in depth, the fatigue life is shortened at a great rate, and over about 3mm in depth, its lowering becomes very slow, especially for the shafts with a hole 1mm in diameter.
(4) In every size of hole, the starting point of the way to fatigue is always on either side of entrance of the hole, and so the fatigue limits are raised to some extent owing to hardening of this parts by shot-peening and others.

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Previously, the authors reported results obtained through the usual method of experimental strain analysis by photograting, applying the method to two-dimensional problems (using experimental values of interference fringe spacing and inclination), and also suggested a new method of experimental strain analysis by photograting. In the new method, the inclinations of two interference fringe patterns with the reference axis ∅₊ and ∅_-, where the master screen axis makes the angle +θ and -θ respectively with the reference axis, are used.
In this paper, two methods of experimental strain analysis by photograting are presented and the accuracy of strain measurement by the new method is described and discussed.
It has been clarified that the new method makes strain measurement easier and helps to improve the accuracy of measurement. It was also concluded that the angle θ should be 2° for strains less than 1% and 3° for about 1% strain and with increasing strain greater θ'_s must be adopted to get high accuracy.

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