Type Fatigue Testing Research Articles

Effect of Notches on the Creep-Rupture Strength and Fatigue Strength at High Temperature

We have carried out creep rupture tests and high temperature fatigue tests for notched bars, to examine the general effect of notches on high temperature strength over prolonged periods of time.Test materials are ten kinds of heat resisting steels, ranging from low alloy steels to super alloys, and we have given them regular heat treatment in these tests. Notched specimens have U-type and V-type circumferential groove by machine cutting, which are respectively about 1.8 and 3 in stress concentration factor.The creep rupture tests of multiple tension type were carried out aiming at 1000 hours, and fatigue tests of rotary-bending type was practised at a revolving speed of 3000rpm up to N=107 cycles.The typical values of the abave tests are tabulated in this report, with notch rupture strength ratio and strength reduction factor (β). Here, we obtained several conclusions as follows;(1) The notch rupture strength ratio is generally larger than unity, as in the short time tensile test. But, it correlates closely with ductility variation of materials, depending on progressive precipitation and aging, with the increase of applied temperature and time.(2) Within the range of the tests, all the test points of ten materials have about the same correlation for the notch rupture strength ratio to the value of plain bar contraction, at corresponding rupture time, and it falls under unity when the plain bar is contracted below approximately 30% in V-notch, and 20% in U-notch. It is convenient to assume notch effect from plain bar creep rupture tests.(3) This ratio has a tendency to decrease after it has once increased, with the increase of notch sharpness. When it has large ductility the maximum value is high, but with small ductility, it gradually falls.(4) In general, the ratio for austenitic alloys is lower than for ferritic alloys. Particularly, 25-20 stainless steels showed the highest notch sensitivity of all the test materials.(5) Concerning the high temperature fatigue, there was a tendency in the strength reduction factor (β) for ferritic alloys to decrease, and for austenitic alloys to increase at high temperature.

Experiments on the Fatigue Strengths for Press-fitted Specimens

Canti-lever type fatigue tests were carried out for various cases of press-fitting of collar with 50 mm-dia. specimens. Thicknesses of collars were of two kinds-3 and 12.5 mm, the material of shaft being of practical axle (SF55, normalized). Results obtained were as follows : (1) Hardness of collar was taken as Hv=140 and 600, but the effect was very little.(2) The fatigue limit where the collar was press-fitted over the fillet portion of shouldered shaft was lower than in the case of only press-fitting. Lowest fatigue limit was obtained for the specimen of which the collar was press-fitted closely to the shoulder portion with the fillet radius of zero, and the magnitude was not affected by the press-fitting of collar. (3) The fatigue limit was lowered even by light press-fitting, and very little effect of radial pressure at the press-fitted surface was recognized.

X線による切欠材の疲労破壊に関する研究

In the present paper, the results of the X-ray study on fatigue fracture of notched specimen that has been performed as a preliminary examination on the mechanism of the fatigue fracture are described.The specimen used was 0.07%C carbon steel plate, having 60-deg. V notch, and was loaded under full reversed bending stress by the Nishihara's type fatigue testing machine. The change in micro-structure at the notch root of specimen was observed by optical micro-scope and the dimension of plastic region appeared in the vicinity of the notch root, that meant the region enveloping the slip band which came in sight until the macro-scopic crack appeared, was measured, by optical micro-scope as for cases with various stress concentration factors and various nominal stress magnitudes.On the other hand, the change of half-value breadth of X-ray diffraction line taken at the notch root in fatigue process was measured with notched specimen having a stress concentration factor of 2.0. The obtained results are summerized as follows:(1) At the very early stage of fatigue process, the slip lines appears in quite localized region close to the notch root, and as the number of stress repetitions increases this plastic region develops wider and longer progressively until macro-scopic crack is initiated.(2) When stress concentration factor is constant, the plastic region that comes in sight until the crack initiation occurs depends upon the magnitude of nominal stress applied. On the contrary, the plastic region decreases as stress concentration factor increases, when the maximum stress at the notch root is constant.(3) The plastic region at the tip of crack is much smaller than that of the notch root, and is nearly constant in the course of propagation of fatigue crack. The plastic region at the tip of crack depends on the magnitude of nominal stress, and the plastic region measured in the direction of crack propagation is likely to converge into about 0.02mm when nominal stress is small.(4) The half-value breadth of X-ray diffraction line at the notch root of specimen increases with the number of stress repetitions, corresponding to the structural change, that is, the halfvalue breadth increases rapidly at the very early stage of fatigue process and at the next stage it increases gradually, and nearing the point of crack initiation it increases rapidly again. After the crack initiation has occurred, the half-value breadth increases very slowly with the increase of the number of stress cycles.

各種耐熱鋼の高温疲労およびクリープ・ラプチャー強度

We have practiced high temp. fatigue tests and creep rupture tests on seven kinds of heat resisting steels, and have examined the method of obtaining the allowable stress value and also various phenomenon incidental to these tests. Some of these specimens contains a chemical content non-standardized, but we have given them regular heat treatment in our tests. Fatigue tests of rotary-bending type was practiced at a revolving speed of 3000rpm, aiming at N=107-2×107 cycles, and creep rupture tests were carried out up to 1000-2000 hours.The results of the above tests are tabulated in this report, and the high temp. strength of respective test specimens are found to be the values generally acknowledged so far and they involve nothing particular to explain. Here, we only wish to add our opinion concerning the method of assuming the allowable stress against long term of life and also a few points we noticed about it, which are as follows:(1) In the high temp. fatigue, it seems that endurance limn of Ferritic Heat Resisting Steel is not observed above 400°C and that of Austenitic Heat Resisting Alloy above 600°C, and at higher temperatures than these degrees, it is considered preferable to obtain strength equivalent to 105 hours by straight line extrapolating of S-N curves on the double logarithmic graph and to regard this strength as the allowable stress against long term fatigue.(2) When we tried to extrapolate 105 hours creep rupture strength from 1000 hours data, it was noted that about 30% difference of strength was recognized among direct extrapolation on the double logarithmic graph, Larson-Miller Method and Manson-Haferd Method. Here it is to be noticed that Manson-Haferd Method, the method which is reputed nowadays as the most accurate one, brought about the lowest value, it is deemed the safest in the process of obtaining the allowable stress to use Manson-Haferd Method, although careful examination of this method is indispensable.(3) Besides, it is very convenient to draw the Fatigue-Creep Rupture Diagram in the form of a graph of allowable stress at high temperatures. If we plot on both axes of graph 80% of the 105 hours value extrapolated by the aforesaid method and connect them with an elliptical curve, an almost proper graph could be made.(4) Reduction of area at the creep rupture are apt to change regularly in accordance with the applied stress and the rupture time, To this also, we think the arrangement of master curve style may be applied. For trial, if we arrange the varying value in reduction of area according to the parameter of T(C+σ), we will be able to find a tendency that reduction of area at each temperature can mostly be gathered into a single curve. Further examinations would be necessary on this matter, but if they can be gathered into one single curve, it will be much convenient for estimating the variation of ductility for the long term.(5) Further, it was shown that there are differences between high temperature fatigue and creep rupture in their precipitation and rupture conditions, and that change of hardness is so complicated, having relations not only with time of heating but applied stress and value of deformation.The foregoing descriptions involve many problems and further examinations are required, and we expect the future study and discussion will be quite helpful to solve these ploblems.

試作したランダム疲労試験機と二, 三の試験結果

When the forms of the stress wave change or the order of the stress-amplitudes and stress-frequencies change, the fatigue strength becomes very complicated in comparison with that at the time of the normal sine wave.In order to clarify these problems, we devised for trial a random type fatigue testing machine. This machine is composed of two parts, that is, the ordinary Schenck-type machine (used the double excentric mechanism and drived by induction-motor) and a newly designed random loading apparatus.This random loading apparatus is composed of the following five parts:(i) Program oscillator is composed of ink-writing oscillogram to be entered in in special ink (it can lead the electric-current) and the tracing apparatus of oscillogram (on-off type).(ii) Thervo-amplifier(iii) Oil-pressure source(iv) Oil-pressure mechanism (Servo-valve and ram-cylinder)(v) Load-checker and load-indicator.We can carry on the fatigue test only by the Schenck-type machine, or only by the random-type loading apparatus, and also we can practise the same test superposing the two apparatus on one another.We tested some fatigue tests as follows:(1) Torsional fatigue test of C=0.34% steel (d=15mm plain specimen)(i) Ordinary fatigue test of 1450rpm by Schenk-type only.(ii) Ordinary fatigue test of 30cpm by the random-type only.(iii) Secondary sine wave by motor (1450rpm) superposed on the first sine wave of 12cpm by the random-type.(iv) Secondary sine wave by motor (1450rpm) superposed on the first rectangular wave by the random-type.(v) Secondary sine wave by motor (1450rpm) superposed on the first random wave by the random-type.(2) Plane-bending fatigue test of the induction-hardened V-notched specimen (low carbon steel)(i) Ordinary fatigue test of 1450rpm by Schenk-type only.(ii) Ordinary fatigue test of 60cpm by the random-type only.(iii) At first fatigue test of 1450rpm and secondly fatigue test of 60cpm.(iv) At first fatigue test of 60cpm and secondly fatigue test of 1450rpm.

大形動力用ローラチエンの疲労強度について

Fatigue test results on the large size roller chains for power transmission (Tsubakimoto RS 0206) was reported in this paper. The test results were discussed on the fatigue strength of large size roller chains and compared with those obtained on the small size roller chains reported in the provious issue.For fatigue testing two kinds of Losenhausen oil pressure type fatigue testing machines were used, that is, UHS type (capacity, static load 35ton, dynamic load 20ton) and UHP type (capacity, static load 20ton, dynamic load 10ton). The former was used for repeated load over 10ton, and the latter for under 9ton. Also fatigue tests on steel plates for chain links were carried out by Haighs fatigue testing machine (capacity 1.5ton) for comparison's sake. The driving speed of the testing machines was 800cycle/min at Losenhausen machine and 1800cycle/min at Haigh machine.The results obtained are as follows:1. The critical part of the chains applied to tests was the cross section of roller links perpendicular to longitudinal direction, where bushes were press-fitted.2. The endurance ratio of the large size roller chains is about 1/4. This value is larger than that for small size roller chains which was about 1/7, as previously reported. The reason is considered to be due to the fact that the endurance ratio of roller link plate materials for large size roller chains was larger than that for smaller ones.3. The effective stress concentration factor for the large size roller chain was 3.4 at 107, and 2.2 at 105.4. Eliminating the effect of fitting pressure, the value of the effective stress concentration factor was calculated to be 4.8 at 107, which is nearly equal to the value of the stress concentration factor for the eye end plate loaded by a pin with clearance.

A Study on the Effects of Induction Hardening on Torsional Fatigue Strength

Previously the authors pointed out that the endurance limit under rotating bending of a solid circular specimen of plain carbon steel, especially the one with circular groove was enormously improved by Induction Hardening. Since then Induction Hardening has been applied to miscellaneous machine parts, particularly force-fitted or notched parts under such condition. This method was also expected to be useful for the helical springs made of medium-carbon steel. This time they investigated torsional fatigue limit of induction-hardened medium-carbon steel (C=0.41% steel) by a 200kg-m Schenck type fatigue testing machine, with the intention of applying it to helical springs. From the results, it could be confirmed that there was a possibility of manufacturing induction-hardened helical springs.

線材の疲労強度に及ぼす引抜条件の影響

This paper deals with the experimental research on the effects of wire drawing conditions, mainly the shape of die wall and the amount of reduction of diameter in drawing on the fatigue strength of drawn wire.We have investigated the endurance limit of 0.41% carbon steel wires drawn under several kinds of drawing conditions, using a Haigh-Robertson type fatigue testing machine.Results obtained may be summerized as follows;(1) The endurance limit of the wire drawn with a die of larger radius of curvature of drawing surface revealed higher than that of smaller radius.(2) The endurance limit increased sharply with the increase of reduction of diameter in the range of low reduction, tending to a constant value when the reduction of diameter was more than 10%.

引抜線材の疲労試験

This paper deals with the experimental research on the effects of wire drawing conditions, mainly the shape of drawing surface of die and reduction of diameter, on the fatigue strength of drawn wire.Using a Haigh-Robertson type fatigue testing machine we tried to investigate the endurance limit of 0.10% carbon steel wires drawn under several kinds of drawing conditions.As the results, we have made it clear that:(1) the endurance limit of the wire drawn with a die of larger radius of curvature of drawing surface was higher than that of smaller radius.(2) the endurance limit increased sharply with the increase of the reduction of diameter in the range of low reduction, tending to a constant value when the reduction of diameter was more than 10%.

The Effects of the repeating Speeds on the Fatigue of metallic Materials

1. Preface When the metallic materials were broken down due to fatigue under repeated stresses, it was previously published partially that the repeating speeds had influences upon the fatigue-curves, especially the endurance limits. But they were not investigated systematically. The causes lay, it was though, in the fact that no reliable high speed fatigue testing machine was available. In this research, some carbon steels from 0.07% to 0.6% carbon has been studied under the new Nishihara type high speed fatigue testing machine, which was designed as satisfying the purposes, and also under the old Woler type fatigue testing machine, and some other materials were also studied. 2. Effects of the repeating speeds We have studied on the range from 5, 000 rpm to 15, 000 rpm by the new Nishihara type high speed fatigue testing machine, and studied on the range from 50 rpm to 2, 000 rpm by the old Woler type fatigue testing machine. It was imperfectly known that the endurance limits were rising as the repeating speeds were rising. By this research, the facts that the endurance limits were rising as the repeating spceds were rising were confirmed, and these rates of the rising are different according to the contents of carbons or the internal crystal systems or the results of the heat treatment. When we consider all states of fatigue curve at the high stresses, the materials are fatigued sooner in the cases of repitition of high speed than of low speed, i. e. at the high stresses, the fatigue strengths are lower than at the low stresses, at the high speed repitition. But at the endurance limits, or at the lower stresses near such limits, the materials are fatigued sooner in the case of repitition of low speed than of high speed. As the cause that the fatigue strength is varied by such variations of the repeating speeds, we consider 2 causes. At the high stresses the facts that the fatigue strengths at the high speed repetitions are lower than at the low speed repetition, are considered as the next. The heat quantity which is accumlated in the material in unit time is much larger at high speed repetition than at lower speed repetition, and then the temperature of the material rises and the material is softened. So the strength becomes smaller. At the low stress near the endurance limits, the rising of temperature of the material is very much smaller, but on the contrary, the variations of the internal crystal system are larger at low speed repetition than at high speed repetition, so that the material at low speed repetition is fatigued sooner than at high speed repetition. 3. Conclusion To save the testing time of fatigue test, we perform the fatigue test under high speed repeating stresses. But by such a method, the phenomena, that the endurance limits were varied by the repeating speed, were discovered, and the facts that according to the internal crystal system of material, the rate of influences was different, was also discovered. we now believe that we must study another materials, ana discuss the effects of the repeating speed that influences the fatigue strength.