Minoru Kawamoto

On a New Rotating Bending Fatigue Testing Machine

In fatigue test strain usually arises a little later than stress applied to the specimen. Owing to this strain lag, in rotating bending fatigue test, the specimen is bent in the direction otherwise than what it was at the moment stress was applied.However, since the conventional rotating bending fatigue testing machine of uniform moment type loads up the specimen through only the knife edges, the specimen can be bent only in the direction at the moment stress was applied and not at right-angle to it; therefore the specimen is given an unknown restrictive load and the test will be futile.On the other hand, the new rotating bending fatigue testing machine of uniform moment type described in this paper loads up the specimen through the knife edges and a pair of pivots, and the sustaining frames of the rotating axis which keeps the specimen supported with a pair of angular contact radial ball bearings. So the specimen can be bent in any direction and therefore no unknown restrictive load is applied to it, and consequently our test will not be futile.Fatigue strengh of S55C steel specimens has been tested by the new testing machine and compared with the results obtained by the conventional testing machine. It has been found that the fatigue limit obtained by the former machine, equal to the one by the latter, but the fatigue strength at N cycles obtained by the former machine is smaller than the one by the latter.The difference between these fatigue strength values at N cycles is considered to be due to the unknown restrictive load mentioned above and this difference is to increase with the applied stress.

Fatigue Strength of Steel Wire under Increased and Decreased Loads

The cumulative cycle ratio is usually larger than unity, when steels are tested under gradually increased stress amplitude, and is less than unity when tested under decreased stress amplitude. This fact seems to indicate that the strengthening effect, due to the work-hardening, during the test, prevails in the case of increased stress amplitude. What, then, is expected on the steel which was heavily work-hardened beforehand, when it is tested under the same stress conditions?With respect to this problem, the fatigue tests were carried out on the steel wires which have received different treatments, under the stepwise increasing and the stepwise decreasing stress amplitudes. The results on the heavily cold-drawn steel wire showed that the average of the cumulative cycle ratio is very close to unity under the increased stress amplitude, and is approximately 0.7 under the decreased stress amplitude. Therefore, it is clear that the heavily work-hardened material has no capacity for further work-hardening when it is subjected to the increased stress amplitude and the material is rather weakened or softened by the decreased stress amplitude. This inference was also confirmed by the examination of the hardness of specimens fractured under the respective stress conditions.On the other hand, the results on the steel wire, which was annealed at 400°C in vacuum after the same cold drawing, showed the increase of the cumulative cycle ratio more than two, in both cases of the increased and the decreased stress amplitudes. Therefore, it is inferred that the work-hardening can still take place for this annealed one. The increase of the cumulative cycle ratio in the case of the decreased stress amplitude is probably the result of the work-hardening, that particularly was occasioned by the large stresses applied at the initial stage. However, no more confirmation of the results was available from the examination of the hardness of the fractured specimen on this annealed steel wire.

A Method of Calculating Fatigue Lives under Multiple Rotating Bending based on Variation of Strain Amplitude

In many cases, the life of machine members under fatigue tests with random loadings is usually estimated on the basis of the so-called“linear damage criterion”. In most cases of the experimental results already reported, however, the linear damage criterion does not necessarily agree with the experimental results. It seems that this fact does not depend upon the scatter of the strength of its material, but upon the mutual interaction between the stress levels.As a method of discussing this interaction of stresses, the authors noted the change of stress and strain-amplitudes during the fatigue stressing under the multiple repeated load of multi-step, and tried to lead the method of calculating the fatigue life.

Thermal Fatigue Behaviors under Cyclic Variation of Stress or Mechanical Strain

Thermal Fatigue behaviors under cyclic mechanical strain were reported previously. In this report thermal fatigue behaviors under cyclic stress are compared with that under cyclic mechanical strain. The following results are obtained. The life on thermal fatigue under cyclic stress is shorter than that under cyclic mechanical stram. It has been found that the variation of elongation of the specimen under 700-300°C temperature cycling is different from that under 500-200°C temperature cycling. In 500-200°C temperature cycling the increase of elongation at the beginning of cycling is quick, then it turns slow and just before fracture it becomes quick again. In 700-300°C temperature cycling, however, the quick increase of elongation at the beginning of cycling has not been found.

A New Direct Stress Fatigue Testing Machine of Resonant Type

A new direct stress fatigue testing machine which operates in a perfectly resonant condition under a new principle is devised and constructed. In this machine repeated load is applied to a specimen by the inertia force of the mass forcibly vibrated by a crank and link mechanism just in a resonant condition. The amplitude of the vibration can be adjusted mechanically from zero to an arbitary magnitude in operating condition without stopping the machine. So the starting is easy and the amplitude is kept strictly constant without any automatic control apparutus. The machine is a horizontal type, one end of the specimen is fixed on the dynamometer side and the other on the spring side. The end of the dynamic load spring is forcibly vibrated by the vertical link oscillated around a movable support. When the support is situated at the top of the vertical slit, the amplitude becomes zero. In this state the machine is started. After the driving speed arrives at resonance, the support is gradually lowered, then the amplitude increases centinuously. Fixing the support at an appropriate position, the expected load amplitude can be obtained. The resonant frequency can be regulated by adjusting the oscillating mass. A preload spring is located outside of the dynamic load spring, so fatigue tests under any mean stress can be carried out. The machine is proved to be satisfactory in operation. Some experiments are made on smooth specimens, notched specimens, and others.

Plane Bending Plastic Fatigue of Ni-Cr-Mo Steel

The plane bending plastic fatigue testing machine was developed for a further systematic study of the fatigue problem in the plastic range. A constrained strain test and a constrained stress test were carried out on the Ni-Cr-Mo steel. From the discussion of the results, it was concluded that the test under constant strain amplitude is the most basic one in the sense that it gives a fairly stable hysteresis loop throughout the test, and the data of the other tests under various conditions such as varying strain amplitude and constrained stress amplitude should be discussed in comparison with those of the test under constant strain amplitude. The treatment using the parameter etotal, which is the total strain summed up from the beginning to the occurrence of a fracture regardless of the sign, seems to offer a method for further analysis.

New Resonance Type Fatigue Testing Machine

A new direct stress fatigue testing machine which operates in a perfectly resonant condition under a new principle is devised and constructed. In this machine a repeated load is applied to a specimen by the inertia force of a mass forcibly vibrated by a crank and link mechanism just in a resonant condition. The amplitude of the vibration can be adjusted mechanically from zero to an arbitrary magnitude in operating condition without stopping the machine. So the starting is easy and the amplitude is kept strictly constant without any automatic control apparatus. The machine is proved to be satisfactory in operation. Some experiments are made on smooth specimens, notched specimens, and others.

Studies on the Stress of T-Section Beam under the Bending Loads

We have investigated experimentally by electric wire strain gauges the stress distribution in the T-section beams in the case of bending loads, when they were supported at both ends and carrying a centrally concentrated load as the upper chord members of E.O.T. Crane girders. The principal results of the investigation may be summarized as follows : (1) The near part of the loaded point of the upper horizontal plate (flange plate) has a tendency to bend separately from the total bending state. (2) The reduction ratio of strength of the T-section beam was about 10∼20%, in which the width of flange plate is smaller than three times the depth of web plate.

Effect of Heating and Cooling Speed and Hold Time on Thermal Fatigue

The effect of a heating and cooling speed and that of a hold time at the highest temperature on the thermal fatigue life of an 18-8 stainless steel were investigated. The strain cycling was so applied to the specimen mechanically as to produce a tension at a higher temperature. Four different heating and cooling times ranging from 5 to 18 seconds and three different hold times ranging from 1 to 30 seconds were selected in order to see their effects, while the temperature was cycled between 300 and 700°C and between 200 and 500°C. Another emphasis was put on the study of the microstructures of the specimens which experienced various test conditions. With respect to the effect of the heating and cooling speed, little difference in the number of cycles to fracture was observed by changing the cooling speed from free cooling in the atmosphere to the forced cooling by the air jet. However, there were recognizable changes in the microstructures of the specimens which were tested under different cooling conditions. The hold time at the highest temperature had a different influence on the fatigue life owing to the magnitude of the applied mechanical strain ; for instance, when the highest temperature was 700°C, the longer the hold time, the larger the number of cycles to fracture.

Effect of Rest Periods on the High Frequency Fatigue at Elevated Temperature

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.