In order to obtain fatigue strength data in long life range over 10⁸ stress cycles under multiple two-step varying load conditions with high frequency of Hi/Lo block cycles, an automatic load changing device for uniform rotating bending fatigue testing machine was developed. With this device can be conducted the repeated Hi/Lo varying load fatigue tests with short interval of the higher stress level of about 100 cycles. It was confirmed, through experiments done by using JIS. S45C round bar specimens, that the level of performance of the newly developed load changing device was high. Furthermore, the discussion on the results from the viewpoint of the linear cummulative damage laws showed that, in some cases, the effect of understressing on fatigue lives appeared in two opposite manners, i.e., fatigue lives became considerably longer than estimated values or vice versa, especially when the number of understressing cycles was sufficiently large. To reveal such a inconsistent effect of understressing, and further, to clarify the fatigue behaviors in long life range up to 2×10⁹ stress cycles, the authors intend to conduct a series of experiments systematically.

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As a basic study on fatigue damage accumulation of structural members subjected to service load, fatigue tests under superposed stress and varying stress amplitude have been carried out in the past, and various fatigue life prediction methods have been proposed.
To investigate the applicability of six different cycle counting methods to more complicated stress pattern than the superposed stress, the in-plane bending tests were carried out on the specimens made of low carbon steel S10C under the combined superposed stress having such a pattern that the maximum stress amplitudes of two superposed stresses were varied stepwise or that the superposed stress and sine wave were mutually repeated. The cycle counting methods investigated were zero cross range pair mean method (ZRPM), range pair mean method (RNPM), peak method (PEAK), range pair method (RNPR) among others. The modified Goodman's method was used to evaluate the effect of mean stress on stress amplitude. The lives estimated by using the linear cumulative fatigue damage law were compared with the experimental lives, and the applicability of the cycle counting methods was examined.
The results obtained were as follows;
(1) For the superposed stress wave, the ratio of the experimental life (N) to the estimated life (N_es) was within the range of 1/3≤N/N_es≤3 except the PEAK method.
(2) For the combined superposed stress wave and the superposed-sine stress wave, the ratio of the experimental life (N) to the estimated life (N_es) was within the range of 1/3≤N/N_es≤3 except the PEAK and RNPR method. The PEAK method showed a conservative life against the experimental results, but RNPR method was not.
(3) The scatter band of the estimated-experimental life ratio by the ZRPM and RNPM method was smaller than that of the other 4 kinds of cycle counting methods.

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In order to evaluate the fatigue strength of blades of an axial flow compressor, we have done the strain measurements in service condition, the fatigue tests on the materials of blade under several conditions (under varying stress amplitude and superimposed stress, in corrosion, etc.) and the fatigue life prediction.
In this paper, the results of strain measurement of 1st and 14th stages of rotor blades and prediction of life are explained. Also the problems in evaluating the fatigue strength of the blade are treated. The main results are as follows;
(1) Stress-frequency curves depend very much on the cycle counting method and the rate of resonance of blade.
(2) Fatigue life is determined by low level stress having many number of cycles rather than the maximum stress level.
(3) The S-N curve estimated can be used to evaluate the service life of blades.

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With the increase in the speed of railway trains higher equipment is demanded for the safety of the axles of the rolling stock against fatigue strength. The crack initiation of fracture on the axles usually occur in press fit parts, and the ratio of the crack initiation in these parts have formed over 90% of the total. It is necessary then to investigate the fatigue strength on the press fit part, i.e. the wheel-seat, the dust guard seat etc. in order to study fatigue life of the axles. But in estimating the fatigue life of the axles the usual fatigue tests on specimens are inadequate for data since the press fit parts are subject to the effect of stress concentration and fretting, and there are not sufficient data concerning fatigue strength on the press fit parts.
In this monograph, the current researches in practice at the present time on the strength of the axles are reported, and the problems concerning the estimation of fatigue life of the axles, the initiation and propagation of crack and the application of cumulative damage law are discussed. To solve the problems concerning the fatigue strength of the axles, researches are required on the following items.
(1) The estimation of fatigue damage due to service load.
(2) The effect of the load variation in the initiation and propagation of crack in the press fit parts.
(3) The fatigue strength under superposed mini sinusodial stress wave and shock stress wave.
(4) The fatigue strength under the fretting phenomena.
(5) The fatigue tests repeated 10⁸∼10⁹ times.

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The predicted life of a rolling stock truck frame under service load history is greatly influenced by the cycle counting method selected and the S-N curve used. However, little words have been done to evaluate and select the most suitable counting method and S-N curve for the prediction.
In this paper, six kinds of cycle counting methods and three kinds of S-N curves were investigated by using the measured service stress data, and the predicted lives (a modified Goodman's method for evaluation of mean stress effect and a modified Miner's rule for fatigue damage calculation were applied) were compared to the service life.
The following results were obtained;
(1) Two cycle counting methods (the zero cross range pair and the zero cross range pair mean methods) showed the lives close to the service life, and the S-N curve B (which was used for the fatigue life prediction of fillet welded joint of truck frame by S. Tanaka) was good for the life prediction of the truck frame.
(2) When the effect of mean stress was taken into account, the lives under full load and vacant load conditions became a half of those without considering the effect.
(3) By using the zero cross range pair and the zero cross range pair mean methods for cycle counting and the S-N diagram B, the life under the full load condition was about 15% less than that of the vacant load condition.

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In order to study the development of the fatigue strength of steel due to the induction hardening, it is necessary to investigate the effect of tempering upon the fatigue limits σ_w1, σ_w2 and the condition of crack propagation.
This paper contains the experimental results and the considerations concerning the induction hardened steel specimens (0.16%C) with V-notch, tempered at several kinds of temperatures.
The data and considerations may be summarized as follows;
(1) The fatigue limit σ_w1 has little change, while σ_w2 decreases rapidly with increase of tempering temperature. This fact shows the fact that the initial crack of specimens tempered at high temperature develops easily to final fracture.
This is closely related with the experimental results concerning the residual compressive stress-release due to the tempering at the V-notch root.
(2) At σ=30kg/mm² stress level the curves of crack depths to stress cycles of specimens with two different tempering conditions (no-tempered and tempered at 300°C) completely coincide.
At higher stress level σ=50kg/mm², however, the rate of the crack propagations of the tempered specimens is larger than that of the non-tempered ones, while the stress cycles before crack formation remain almost the same under both conditions.
(3) Based upon the above facts, the effects of the residual compressive stress at the crack tip upon the crack propagation have been discussed.

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As a basic study on fatigue damage accumulation of structural members (for example; truck frame and plate spring of rolling stocks) subjected to service load, the authors have carried out fatigue tests under superposed stress wave on fifteen metalic materials (about 400 experimental points.)
In this paper, a few factors having influence on the predicted fatigue life have been discussed by comparing the experimental life N with the predicted life N_es with regard to these experimental points.
For the fatigue life prediction method, three kinds of stress cycle counting methods were used; the zero cross range pair mean method (ZRPM method), the range pair mean method (RNPM method) and the range pair method (RNPR method).
The modified Goodman's method was used to evaluate the effect of mean stress on stress amplitude. The lives were predicted by using the linear cumulative fatigue damage law.
The results obtained were as follows.
The most suitable cycle counting method was RNPM method, because of the scatter band of N/N_es being narrowest in three cycle counting methods. The mean stress effect on fatigue life amplitude was very complex, and the modified Goodman's method was not always adaptable for the prediction of the above experimental points.
In order to make narrow the scatter band of N/N_es, some discussions were made on the fatigue damage accumulation caused by the secondary stress amplitude cycling.

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On the blades of gas turbines and blowers, the vibrating stress superposes on the centrifugal stress which is repeated with the start-stop of machine. This combined stress wave is called the intermittent stress wave, and number of studies have been conducted under this stress wave. However, some unclarified problems are left regarding the intermittent stress fatigue.
The present report aims at the fatigue life prediction of a notched specimen made of carbon steel (S35C) under the condition of superposed stress cycles 10²-10⁴.
The following conclusions were obtained from this experimental study.
(1) Miner's rule does not predict the fatigue life under the intermittent stress.
(2) The experimental life is a little shorter than the life predicted by the modified Miner's rule.
(3) The proposed life prediction method may well predict the crack initiation life by using the interaction coefficient C of 0.2-0.4.

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Although corrosion fatigue has been studied by many investigators, no report so far describes the corrosion fatigue under different corrosion atmospheres and stress conditions. In this study, a few corrosion fatigue tests have been made to clarify the effects of environmental conditions and of the superimposed wave or varing stress.
The results obtained are as follows;
(1) The atmospheric corrosion fatigue strength of S35C specimen (ρ=10^R) under rotating bending (n=3600rpm) decreased by about 20∼30% as compared with the value obtained by the non corrosion fatigue test. And the fatigue strength decreased markedly after one month's exposure to atmosphere.
(2) The fatigue life of SS 41 specimen, subjected to repeated wetting with 3% NaCl and drying under superimposed wave, became very short when the stress of the second wave was very small. This arises from the double effects of corrosion and superimposed wave.
(3) The corrosion fatigue test on 13% Cr-Mo steel under varing stress in 3% NaCl showed that the fatigue life decreased extremely as compared with the corrosion life under constant stress.

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Axial fatigue tests were carried out on tufftrided and non-tufftrided S15CK steel specimens. The main results obtained are summarized as follows.
(1) Tufftriding of steel specimens had obviously an effect of improving their fatigue strength under axial load, but the effect was not so pronounced as in the case of rotating bending fatigue tests which had been reported by many researchers.
(2) Large notch effect was observed on the tufftrided small size specimens having a diameter of 5mm, but for the large size specimens with a diameter of 25mm, notch effect was rather moderate.
(3) Among the tufftrided specimens, the plain specimens showed larger size effect than the notched specimens. And, among the notched specimens, the non-tufftrided specimens showed larger size effect than the tufftrided ones.

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Compressor discs of jet engines and turbine discs of gas turbine engines are usually subjected to a combined stress σ_comb of centrifugal stress σ_t and thermal stress σ_th in the tangential direction during flight or start-stop time. However, there has been few studies on the life prediction for such components made of Al-alloy (2014-T6) at the condition of about 120°C and σ_comb of 21 to 22 kg/mm² during rotation. In this report, the life was estimated from the creep rupture data at 212, 400 and 600°F using the prediction method proposed by the authors.

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The surface treatment by tufftride effectively improves fatigue strength, resistance to corrosion and abrasion. This treatment is being used for many machine parts, but few studies on fatigue strength of tufftrided specimen have been made.
We have been conducting the experimental studies on fatigue behavior of tufftrided specimen since few years ago, and found many, experimental facts which have many problems to be studied. This paper describes some of the problems about the fatigue behavior of tufftrided specimen.

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As a preliminary examination for evaluating the effect of combinations of several varying environmental factors on fafigue cumulative cycle ratio Σ(n/N), the plain bending fatigue tests were carried out with various combinations of stress amplitude and temperature. The combinations used were: (1) a constant stress amplitude and step-up-2 stage or step-down-2 stage temperatures, (2) a constant temperature and step-up-2 stage or step-down-2 stage stress amplitudes, (3) a constant stress amplitude and multiple repetition of 2 stage temperatures, (4) a constant temperature and multiple repetition of 2 stage stress amplitudes, and (5) multiple repetition of 2 stage temperatures and 2 stage stress amplitudes.
The following conclusions have been obtained.
(1) Σ(n/N) became larger than unity under the tests for combinations of the step-up temperature environment and constant stress amplitude, and of the constant temperature and step-up stress amplitude.
(2) In the completely opposite cases to the conditions discribed-above, the values of Σ(n/N) seem to be smaller than unity.
(3) In the cases of the multiple step-down-2 stage for the both factors (500°C_??_300°C, 32kg/mm²_??_28kg/mm²), if the period of stage for the stress amplitude 28kg/mm² at 300°C is short, the values of Σ(n/N) are smaller than unity but, if this period is long, the values of Σ(n/N) are extremely larger than unity.
(4) The scatter band of the cumulative cycle ratio for all the test results was in the range of about 0.2 to 100.
Moreover a new idea for predicting fatigue life of machine parts employed under various combinations of temperature, corrosion environments and stress amplitude is proposed on the basis of the knowledge obtained previously from the tests under various combinations of corrosion environment and stress amplitude and the present informations.

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