Tensile and fatigue tests under the conditions of various testing speeds and temperatures have been performed on a thermoplastic resin (polyacetal) to investigate the effects of testing speed and temperature on its tensile and fatigue properties. The results obtained are: (1) The static tensile strength increases with strain rate but decreases with temperature due to softening. (2) The fatigue strength decreases with increase of frequency and temperature due to softening as a result of heat generation by fatigue stressing. (3) The variation (increase) of deflection during fatigue stressing is similar to that of surface temperature of the specimen. (4) The relation between the initial deflection and the number of cycles to fracture is represented by a curve regardless of testing temperature. (5) Softening as a result of heat generation due to repeated stressing contributes largely to the fatigue fracture of the material.
Studies on fatigue strength under varying stress conditions are very important in the fields of fatigue research and machine design. So, in the past many researchers have conducted a great deal of fatigue tests under varying stress, but there were few studies under varying stress on surface hardened steel. This paper describes an experimental study on the fatigue strength under varying stress using tufftrided specimens of S10C and S15CK. The main results are as follows: (1) S10C tufftrided specimen gives approximately the relation Σ(n/N)≥1.0, but S15CK tufftrided specimen gives Σ(n/N)=0.2∼4.0 under high-low stress sequence tests. (2) Both tufftrided specimens give Σ(n/N)≥1.0 under low-high and two level multiple repeated stress sequence tests.
In order to investigate the characteristic of the low cycle fatigue strength of gray cast iron, pulsating tension fatigue tests have been carried out by using notched and unnotched plate specimens of various sizes. The primary results obtained are summarized as follows: (1) The fatigue crack propagating rate dlc/dN is proportional to the n-th power of the stress intensity factor range ΔK regardless of the dimension of notch and the stress level. And the value of n is 7.5-9.1. (2) The apparent fracture toughness KIC is hardly affected by the dimension of notch, but depends on the size of specimen. That is, KIC of a large specimen is higher than those of smaller ones. (3) The notch factor β is about 1.1 in the case of the hole notch, and about 1.2-1.3 in the case of the key hole notch.
There exist two types of fatigue damage caused by the cyclic plastic strain. One is the damage which is accumulated in the form of a surface irregularity such as slip bands or random notches on the specimen surface and is considered to control the crack initiation life of an unnotched specimen. The other is the decrease of fracture ductility which is found to control the fatigue life at the very high cyclic strain region. So the latter type of damage may contribute to the propagation of the crack at the tip of which the high cyclic strain appears. In this paper, therefore, in order to obtain some microscopic information about fatigue damage, direct observation of fatigue damage on the specimen surface has been made. Furthermore, new techniques for measuring the amount of the surface irregularity and also the local strain distribution of the very small portion of the specimen has been developed using a scanning electron microscope (SEM). The new techniques have the following characteristics: (1) Electron signals due to the back scattered electrons which depend on the gradient of the reflection surface are analized and integrated along a single scanning line with aid of an A-D convertor and a mini-computer to give the profile of that section, and (2) a micro grid of about 1∼2μm spacing is printed on the specimen surface by so-called“the electron beam exposure of the photo-resist”technique by means of the SEM. Using this grid the microscopic strain distribution within a grain or at the tip of crack can be measured.
The effects of slip character in metal polycrystals on the cyclic hardening and softening behaviors were investigated under the multiple repeated tests at two strain levels, and the fatigue lives of copper and α-brass in the double repeated fatigue tests were considered. The main results obtained are as follows: (1) The stress range after changing the strain amplitude approaches gradually to the inherent value of the virgin material for S10C and copper which show a wavy slip mode. On the other hand, for α-brass having planar slip character, the stress range approaches to the same value as that of the virgin material as the strain level is increased. However, as the strain level is decreased, the stress range reaches the value larger than the inherent value of the virgin material after the transient increase in the early stage. These results can be explained by considering the slip character of materials and the aging effect in α-brass. (2) For S10C and copper, the stress range for a given plastic strain range just after the change in the strain amplitude corresponds to the state of hardening just before the change. On the other hand, for α-brass the former does not always correspond to the latter. This characteristic property for the cyclically deformed α-brass can be explained by considering both the slip character and the aging effect of this material. (3) The cyclic stress-strain relation for α-brass of planar slipping mode is sensitively affected by its strain history, while for copper of wavy slip character this relation is scarcely dependent on it. (4) The double repeated fatigue tests show that the cumulative cycle ratio does not depend on the stressing sequence for copper, while for α-brass it is larger than one in the High-Low test, but less than one in the Low-High test. These test results correspond well with the cyclic strain hardening and softening behaviors shown in (1) and (2).
It is well known that when a structure attains such a state of residual stress that no further plastic flow occurs, the structure is said to have shaken down. But on a real material, it is often observed that an effective state of residual stress is ruined by a small load cycled many times. To apply the shake down theorems on elastic-plastic design of structures, the stability of a state of residual stress must be examined. For this purpose, fatigue tests have been carried out on a frame structure which consists of three members of 7075-T6 aluminium alloy bending specimens. The results obtained are as follows: (1) The state of residual stress of the structure can be computed from a static moment-curvature relation of a specimen. But a small difference was observed between the computed and experimental results. And this difference is proved to result from a cyclic creep which is relatively small on this 7075-T6 aluminium alloy. (2) The fatigue life of the weakest member of this structure is estimated from that of single specimens. The experimental results are always a little greater than the estimated values.
The effect of stress change on the fatigue crack propagation of an aluminum alloy (2017-T4) was studied in detail by means of striation analysis of its fracture surface and the result was interpreted on the basis of the crack closure concept. The effect of stress change was large when the maximum stress was changed. The decrease or increase of stress caused the retardation or acceleration of fatigue crack propagation, respectively. The retardation of crack propagation after the maximum stress decrease was in good agreement with the analysis based on the crack closure. The acceleration of crack propagation accompanied with the maximum stress increase, on the other hand, could not be accounted for by the crack closure concept alone, but the effect of monotonic loading also had to be considered. The combination of these two effects gave reasonable agreement with the experiment. The effect of stress change under constant maximum stress was not as large as the effect of maximum stress change. A reasonable explanation was given also for this case. An analysis of striation morphology indicated that the crack closure appeared at least in the vicinity of the crack tip.
The through fatigue-crack propagation rate for a centrally slitted low carbon steel plate has been determined. The rate, d(2a)/dN, is correlated with the stress intensity factor range, ΔK, as follows: d(2a)/dN=C(ΔK)m Strictly speaking, it appears that the larger the plate width (from 50mm to 200mm), the smaller the value of m, and that the higher the stress ratio (from 0.06 to 0.36), the larger the value of C. But, the plate thickness, slit size, stress wave, tensile/shear fracture mode transition behaviour and reduction of plate thickness resulting from general yielding do not affect the correlation. The fatigue life, Nf, is correlated with the initial apparent stress intensity factor range, ΔK0, as follows: Nf=4.46×10-12(ΔK0)3.5 The predicted life from the rate is consistent with the correlation.
Through the study of the push-pull type programmed fatigue test having the stress amplitude below the fatigue limit with the intermittent high level stress on a medium carbon steel S35C, the authors previously obtained the following primary conclusions: In the constant stress amplitude test, there is the fatigue limit to the plastic strain as there is to the stress. In the programmed test the plastic strain range-pair corresponding to the stress below the fatigue limit becomes larger than that in the constant stress amplitude test, because of the influence of the intermittent high level stress. Moreover, the fatigue limit in terms of the plastic strain disappears and the fatigue damage caused by the plastic strain range-pair below the fatigue limit is linearly accumulated. In this study, the programmed fatigue test has been conducted more systematically on the same material in order to investigate the effects of some secondary factors on the fatigue damage in the programmed fatigue test. As a result, it is suggested that there exists the critical modified S-N curve which may always give a conservative estimation of fatigue lives under all kinds of actual loadings, and also this can be obtained by carrying out the test under a special condition on small number of specimens. The proposal has been examined on the results of programmed fatigue tests of a low carbon steel S25C and is found to be applicable for estimation of fatigue lives under actual loadings. In addition, the programmed fatigue test has been carried out on a high strength alloy steel SNCM 8.
In many cases of actual service loads, machine parts are being used for a longer time than 108 cycles. Most of the stress levels in these cases are lower than the endurance limit under a constant amplitude. However, in the case when the minor large-stress and the major small-stress are repeatedly loaded in a random manner, the small stress gives comparatively a large damage on the parts. The present tests were conducted in the condition that the stress level distribution was similar to the actual one, that is, the minor over-stress and the major under-stress were repeated alternately more than 2×106 cycles. These tests were planned by the subcommittee of Fatigue Strength of JSME, and have been carried out by its members. The results of these tests have been analysed with the fatigue function using a criterion that the fatigue process consists of both the damaging and strengthening processes. Furthermore, the new endurance limit which has been brought about by the damage under the over-stress repetitions is assumed to be smaller than that under the constant stress amplitude, and the strengthening factor of the damaged materials is assumed to vary largely with the stress amplitude. On the basis of the above processes and assumptions, a new fatigue curve is derived from the original fatigue curve. The fatigue lives of the respective tests are estimated from the new fatigue function which satisfies the new fatigue curve. These estimated values have a good agreement with the test results of the region of not only the finite lives but also the fatigue limits.
Fatigue tests under superposed stress have been considerably conducted in the past. The present authors also carried out the fatigue tests under the superposed stress on many engineering materials and proposed a fatigue life estimation method. But most of these investigations under the superposed stress have been concerned with the fatigue life reduction, and the studies on the mechanism of fatigue failure under the superposed stress are few. It is the purpose of this paper to clarify the behavior of stress-strain response and plastic strain amplitude under the superposed stress. Fatigue tests were conducted on two kinds of specimens made of cast steel under sinsoidal and superposed stress, and the stress-strain behavior was measured during the fatigue tests. The experimental results are summarized as follows: (1) When the plastic strain amplitude caused by the maximum stress amplitude (primary stress+secondary stress) and that by the secondary stress amplitude are taken into consideration, the experimental results under superposed stress agree with those under sinsoidal stress. But further investigations under the condition that the secondary stress amplitude is less than that of primary stress will be necessary to draw final conclusion. (2) When the material elongates to the tension side under fatigue test (such as creep-wise), it is necessary to take account of such behavior into fatigue life estimation.
Studies on fatigue behavior of carbon steel under varying stress conditions have been done up to now by various workers. The majority of those studies, however, were conducted under rotating bending or tension-compression, and there were only few studies under torsion. The purpose of the present study is to investigate the torsional fatigue behavior under multiple repeated load in two stress levels. Carbon steel S30C was used as the specimen and fatigue tests were conducted under rotating bending, torsion and combination of them. The conclusions obtained are summarized as follows: (1) Independently of the stress sequence, the cumulative cycle ratio Σn/N is larger than unity under torsion. (2) Variation of surface hardness during secondary stress repetition respectively corresponds to the cumulative cycle ratio Σn/N under either rotating bending, torsion or combination of them. Namely, when the cumulative cycle ratio Σn/N is larger than unity, the surface hardness increases, and when smaller than unity, decreases. (3) Variation of plastic strain amplitude during secondary stress repetition roughly corresponds to the cumulative cycle ratio Σn/N under torsion. When the cumulative cycle ratio Σn/N is considerably larger than unity, the plastic strain amplitude is smaller than that in constant stress amplitude test.
The rotating bending and push-pull fatigue tests were carried out on 0.29% carbon steel specimens with four kinds of surface roughness at room temperature, 250°C and 350°C, where the strain aging is most remarkable. The results obtained are as follows: (1) At room temperature, the fatigue strength decreases as the degree of the surface roughness increases. (2) At 250°C and 350°C, it is not possible to find any general tendency for the relation between the surface roughness and the fatigue strength, because of the scatter of fatigue strength data, the difference in stress level between the rotating bending and push-pull, the change in surface roughness caused by the formation of oxide scale, the change of hardness by repeated stress and so on. (3) At 250°C and 350°C, in spite of the difference in surface roughness, the S-N curves can be divided remarkably into two groups, one for the mechanically finished specimens and the other for the specimens annealed in vacuum after mechanical finishing.
To investigate the effect of cyclic-strain aging on the fatigue strength of carbon steel, two kinds of cyclic-strain-aged carbon steel were fatigued in rotating bending at room temperature as well as at elevated temperatures. The annealed and surface-rolled specimens were aged at 375°C and 250°C during cyclic stressing above or below the fatigue limits. The strengthening effect due to the aging treatment depends on the aging conditions; the temperature, the cyclic stress amplitude and the number of cycles. The strain-aging treatment at 375°C raises the fatigue strength at room temperature. The strengthening effect at room temperature through the aging treatment in the range of cycles 5.9 ×103∼3.2×106 under the stress just below the fatigue limit at 375°C is independent of the number of cycles at 375°C. In the range of stress 1.15Sw∼0.58Sw for 5.9×103 cycles at 375°C, the higher the stress level at 375°C the more effective the strengthening effects. The strengthening effect through the treatment at 250°C is less than at 375°C and depends on the number of cycles at 250°C. It can be considered that there is not a substantial difference in the strengthening effect through strain aging between annealed and surface-rolled conditions although the surface-rolled specimens have the strongly hardened layers and the residual stress, but that there is a large difference between the treatment at 375°C and 250°C. The strengthening effect seems to be based on dislocation-solute interaction which operates afresh at each cycle at 375°C, and on any cumulative effect of aging at 250°C.
The push-pull fatigue behavior of a carbon steel S45C was studied by using the annealed, prestrained and strain-aged specimens. The prestrain was 5% or 10%. The aging treatment after straining was done by heating the specimens at 100°C, 250°C or 300°C. Under these conditions, S-N curves, the temperature rise of specimens during cyclic stressing and the repeated plastic strain range were discussed. As a result, it was found that the fatigue strength of the strain-aged specimen increased by about 34% at maximum in comparison with the annealed, and that the temperature rise corresponded with the increase of the repeated plastic strain range. In addition, the sum of the repeated plastic strain range of the strain-aged specimen was smaller than that of the prestrained or the annealed.
Recently, very high fluid pressure is being used in some chemical and metal working processes. The fatigue design of a very high pressure container is one of the most important works for applying these processes to industrial fields. The object of the present work is to study experimentally the effects of cylinder material, diameter ratio, inner bore protection and mean hoop stress on the fatigue behaviour of elastic open-ended thick wall cylinders subjected to very high internal pressures. Cylinders with the diameter ratio from 1.4 to 1.8 made from 0.4%C-Ni-Cr-Mo steel (SNCM 8), 18 Ni maraging steel (KMS18-17) and 1.5% C-12% Cr tool steel (SKD11) were tested. The inner bores of some monobloc cylinders were protected by an elastmer film. The compressive mean hoop stress was applied by autofrettaging and static fluid supporting. From the viewpoint of hoop stress at inner bore, the fatigue strength of the monobloc cylinders decreases with the diameter ratio. The protection of the bore surface with the elastmer film is effective to improve the fatigue life of the cylinders because the elastmer prevents the penetration of high pressure liquid into the fatigue crack. It seems, however, that long fatigue lives cannot be achieved with simple monobloc cylinders made from any kind of high strength material under very high cyclic internal pressure. On this point of view, prestressing is a powerful means for obtaining long life. The test results show that the fatigue life of the cylinder is significantly improved with the compressive mean hoop stress. The effect of the mean hoop stress on fatigue life becomes larger as the static strength of the cylinder materials increases. The autofrettaged cylinder has long fatigue life in comparison with nonautofrettaged one, but the effect of autofrettage is far less than that of fluid supporting. This trend may be caused by the redistribution of residual stress of the autofrettaged cylinder during pressure cycling. When the cylinders made from the hard tool steel are sufficiently strengthened in hoop direction, axial tensile stress at sealing position of inner bore causes a circumferential cracking of the cylinders.
In order to evaluate the influence of materials and methods of case hardening on the bending fatigue strength of full size gears, induction hardened, carburized and quench-tempered gears made of various plain carbon and low alloy steels are subjected to pulsating bending stress. The gears used for the test are of 20° pressure angle with 34 teeth and 8 module. The strengths at the root of gears are raised more than twice by case hardening due to the induced compressive residual stress and the increased hardness. The effects of these factors are evaluated by a regression analysis. The anlyasis indicates that, regardless of the materials used, the contribution of the residual stress to the increase in fatigue strength is about 68% in the case of induction hardening and about 55% in the case of carburizing, and the remainder contribution is due to the increased hardness.