Stress Amplitude Ratio Research Articles

シリンダカバー用鋳鉄材の室温および高温における重畳波疲労強度

The fatigue strength of two kinds of gray cast iron under superimposed stress wave forms in tensile and compressive stress regions at room temperature, 300°C and 450°C were discussed.Obtained main results through the investigations are following.(1) In tensile stress region at room temperature the influence of the superimposed secondary stress over the fatigue strength for primary stress wave appears about at not less than 0.1 of stress ratio of secondary stress amplitude to primary stress amplitude.(2) In compressive stress region at room temperature the influence of superimposed secondary stress over the fatigue strength for the primary stress wave is smaller than that in tensile stress region. For evaluation of mean stress in compressive stress region. Gelber's equation is most suitable.(3) In tensile stress region at room temperature and elevated temperature the fatigue life under superimposed stress wave form is evaluated by using the modified Miner's rule and Yamada's evaluation equation of mean stress. In the case of elevated temperature the frequency modified S-N curves and the frequency modified evaluation equations of mean stress are used.(4) In compressive stress region at elevated temperature the superimposed stress wave fatigue life can be estimated by the use of the next equation.1/Nf=Aσα*1⋅exp(-m*σpe2)where Nf: number of cycles to failure for primary stress waveσ1: primary stress amplitudeσe2: equivalent secondary stress amplitudeA, ρ*, m*, p: material's constants

Cyclic Undrained Triaxial Strength of Sampled Sand Affected by Confining Pressure

The effects of confining pressure on the cyclic undrained triaxial strengths of intact sandy specimens obtained by sand sampling methods were investigated. It was found that the ratio of cyclic deviator stress amplitude to initial confining pressure required to cause a certain value of cyclic axial strain amplitude at a certain number of loading cycle in an intact triaxial specimen increased significantly with the decrease in confining stress when the laboratory confining pressure was less than the in situ confining pressure. However, it was also found that cyclic undrained triaxial strength expressed by stress ratio decreased only slightly with the increase in confining pressure when the laboratory confining pressure was larger than the in situ confining stress. (Author/TRRL)

A Study on the Transition of Fracture Mode during Fatigue Crack Propagation in Polycarbonate Plate

The transition from flat to shear mode fracture occurs during the crack propagation in polymeric materials under repeated tensile loading. The present study has been made under the condition fulfilling Irwin's criterion for fracture mode transition, to clarify how the transition behavior of an edge-slotted polycarbonate plate is affected by temperature, strain rate and stress amplitude. It has been found that the ratio of the size of plastic zone at the transition point to the plate thickness increases with increasing the temperature or the ratio of stress amplitude to yield stress, but decreases with increasing the strain rate. This size ratio is roughly equal to one half at the static fracture under a given strain rate, but it is less than one half at the fatigue fracture. However, its value calculated with consideration for the local yield stress at the tip of fatigue crack approaches to one half and becomes in good agreement with the ratio of the measured plastic zone size to the plate thickness.

パルセイティングストレスクリープ特性に関する研究(I)

Most of machines, vessels and others for high-temperature service are not static in their load or stress in actual service but vary in some regular or irregular manner depending on the operating conditions. In designing and manufacturing these products, therefore, the flow or creep properties and rupture characteristics of used materials and their welds under varying stresses at elevated temperatures are very important.Such varying stresses as mentioned above may be classified into two types; one is the alternating static stress. which has as long period as several minutes or several hours and another is the continuous and dynamic stress which has as short period as less than one second. The latter stress condition was adopted in this study.Both static and tensile pulsating stress rupture tests were carried out on mild steel at 450°C as a fundamental research before the investigations on heat resisting alloys and their welds were made.Pulsating stress rupture tests were conducted in which four values of stress amplitude ratio A (σa/σm) were selected for a given mean stress; stress amplitude ratio A=5, 10, 20 and 30%. The frequency of pulsating stress was 380 rpm.The mean stress versus time for rupture diagram in log-log plots showed that the rupture life decreases as the stress amplitude ratio A increases for the same mean stress. For a given mean stress, therefore, the static creep has longer rupture life than pulsating stress creep in which the rupture life decreases with increase of stress amplitude. But it is not appropriate to consider that decreases in rupture life mentioned above are only caused by the dynamic stress or fatigue behaviour due to alternating stress.In view of fatigue behavior, a stress versus time for rupture diagram was plotted for maximum stress (crest stress) in the stress cycle on log-log scale. It was shown that the rupture life in pulsating stress creep became longer as the stress ratio A increased and the static creep had the shortest life. This is incompatible with fatigue behavior.Rupture characteristics in pulsating stress creep were also investigated from creep behavior, assuming that ruptures under pulsating load were caused by accumlation of static creep damage produced by the instantaneous stress and the dynamic effect did not influence the rupture behavior. Consequently, the rupture strength of pulsating stress creep, regardless of stress ratio A, turned out approximately equal to that of static creep.Ductilities of test specimens, determined by true strain at the fractured section, were compared in both static and pulsating stress creep. But no remarkable difference was found.For the mild steel at 450°C, ruptures under the stress conditions of a relatively small pulsating stress being superimposed on the static stress which is sufficient to produce a creep rupture may be caused by static creep damage, and dynamic stress effects may scarcely affect the rupture behaviors.

Fatigue Strength under Multiple Repeated Stress : In the Case of Double Change of Amplitude in Two Stress Levels

The problems of the fatigue of materials have been studied by many investigators, but there still remain many unknown factors, especially on the damaging effects due to the repeated stress with varying amplitude. In this study, we carried out the fatigue tests on the medium carbon steel by superposing the higher stress reversals upon the fundamental repeated stress levels in order to discuss the process of fatigue stressing, and investigated the effect of the cycle ratio of the primary stress, the effect of the secondary higher stress level, and its reversals on the cumulative cycle ratio to failure. As a result, it has been found that the more the cycle ratio of the primary stress amplitude is and the lower the secondary stress level is, the greater the cumulative cycle ratio to failure amounts. Besides, we have interpreted these test results by the use of the cycle ratio-damage curves (D-R curves).

残留応力の変化に及ぼす繰返速度および硬度の影響

In the study of fatigue, the effect of cyclic speed on the fatigue strength has been investigated at room temperature as well as at elevated temperatures. In the fatigue at elevated temperatures and in corrosion fatigue, cyclic speed seems to have a considerable effect on the fatigue strength. In this study, the effect of cyclic speed on the change in residual stresses was, at first, investigated with plate specimens. Three cyclic speeds of 2000, 1000 and 780cpm were given to specimens. Specimens used were of quenched S45C steel and low-temperature quenched S35C steel. Alternating stressing was given at two stress levels, below and above the fatigue limit, for both steels. The results obtained revealed that cyclic speed had a considerable effect on the change in residual stresses, and that the fading of residual stresses was larger under a lower cyclic speed. It was found that the fading of residual stresses under a low cyclic speed was nearly in agreement with that under a high cyclic speed, when the equivalent number of stress cycles under the high cyclic speed obtained from the condition of an equal duration time was used. Since the inverse is also true, it may be concluded that the fading of residual stresses under any cyclic speed can be predicted from the known fading under a definite cyclic speed by using the equivalent number of stress cycles.The fading of residual stresses is assumed to occur in a larger degree for soft material than for hard one. To confirm this experimentally, second investigation was made on the fading of residual stresses for two materials with different hardness. In the previous experiments on the residual stresses produced by shot peening treatment and also on those produced by plastic torsion, experimental formulas on the fading of surface residual stresses in the second stage were established. Materials used were S25C steel (shot peening) and S35C steel (plastic torsion), each having a smaller hardness than that of the material used in the previous experiment. In choosing the stress levels of alternating stessing, alternating stress ratio (ratio of stress amplitude to the fatigue limit) was made the same for the two materials. The results obtained verified the above mentioned assumption, that is, the fading of residual stresses occurred in a degree inversely proportional to the hardness number of the material. Basing on these fading lines, experimental formulas for the fading of surface residual stresses involving the three variables of alternating stress ratio, cycle ratio and hardness number were established. Moreover, to obtain experimental formulas which are applicable to a wider range of hardness, the results of the previous experiment on the residual stresses produced by carburizing and quenching, and a new experiment on the residual stresses produced by plastic tension were employed. The calculated values from these formulas were compared with several experimental results obtained by other investigators, and the approximate applicability of these formulas to all cases was proved.