This paper studies the effect of vacuum on the creep rupture time of SUS 316 unnotched and notched round bar specimens. Creep and creep rupture tests were carried out using three types of notched specimens in air and vacuum at 873 K. Creep rupture times of the unnotched and notched specimens in vacuum were shorter than those in air. The reduction of rupture time due to the vacuum was smaller In notched specimens compared with unnotched specimens. The rupture time ratio, tr (notched)/tr (unnotched), in vacuum was larger than that in air. The environmental rupture time ratio, tr (air)/tr (vacuum), decreased with increasing the elastic stress concentration factor. The effect of vacuum was attributed to the crack initiation and propagation periods.

抄録全体を表示

The effects of both vacuum environment and shape of stress wave of “cp” and “cc” as shown in Fig. 2 on push-pull cyclic creep fracture of SUS 316 stainless steel were examined experimentally in both vacuum of 0.1Torr and 1 atmosphere at 600°C.
(1) The cyclic creep failure time of the material tested was remarkably dependent on environment and shape of stress wave. The time to failure under cp stress wave with tensile creep hold in 0.1Torr was shorter than that in air. On the contrary, the failure time under cc stress wave with both tensile and compressive creep hold in 0.1Torr was larger than that in air. The latter fact was also revealed under pc stress wave with compressive creep hold.
(2) The above behavior under cp wave resulted from both early crack initiation and fast crack propagation in tension cycle in 0.1Torr, and the latter effect under cc wave resulted from late initiation and slow propagation of crack in 0.1Torr.
(3) The crack propagation rate of the material tested was well represented by a single curve on the basis of modified J integral, independent of stress levels, shapes of stress wave and environments.
(4) The variations of both crack initiation time and crack initiation time and crack propagation rate of the metal under vacuum environment of 0.1Torr seemed to result mainly from the variation of macroscopic creep rate of the specimen. It was considered that the increase of creep rate under cp wave in 0.1Torr resulted from the increase of dislocation density as in the case of static creep of the metal in 0.1Torr, and also that the decrease of creep rate under cc wave in 0.1Torr was analogous to the deformation and fracture behavior of high-temperature low-cycle fatigue of the metal.

抄録全体を表示

This paper studies the creep and creep rupture properties of 63 Sn-37 Pb solder. Creep and creep rupture tests were carried out using 63 Sn-37 Pb solder specimens at 313 K, 353 K and 398 K to develop a creep constitutive equation and to obtain rupture time. Short primary and long tertiary creep stages were found but there was no clear steady creep stage at the three temperatures. A creep constitutive equation was proposed as a function of stress and temperature. The proposed equation predicted the experimental creep strain within a factor of 1.25. A new equation for predicting creep rupture time was also proposed. The equation predicted the experimental rupture time within a factor of two.

抄録全体を表示

It is well known that the tool of extruding and the die of casting, of which the materials are Cr-Mo-V steel, are frequently broken by fire crack. It is believed that this is the result of thermal cycling during the hot working. In order to obtain basic information for a reasonable design and working of such material, a series of thermal fatigue tests have been made under large amplitude of temperature. In the present study, particular attention has been directed to the effect that the rapid change of deformability of the present material owing to the temperature level will have on the thermal fatigue characteristics. Attention has been paid also to microscopic observations of nucleation and propagation of thermal crack.
From the present experiments the following conclusions are marked:
(1) Thermal crack nucleates at the crevice of the scale on the surface of the specimen, in case of rich ductility and of fine structure (grain) of the material. The crack at early period grows by repetition of oxidation and at the latter period it propagates under thermal stress cycling. In case of coarse structure (grain) and of poor ductility, the crack propagates rapidly along the austenite grain boundary.
(2) Thermal fatigue life of the material, of which ductility is much dependent on the temperature level, is approximately estimated from the information of static tensile tests of the same material at various temperature levels. This is expressed as
[ΣNfN=1(Δεp)2]1/2=1/√2εf*=1/√2∫T2T1εf(T)dT/T2-T1,
or
Nf1/2(Δεp)=1/√2εf*=1/√2∫T2T1εf(T)dT/T2-T1,
where Δεp is plastic strain range, Nf the number of cycles to fracture, T2 and T1 the upper and lower temperature levels, εf(T) tensile elongation at temperature level T. The latter equation holds in case of constant range of plastic strain during the thermal fatigue test.

抄録全体を表示

This paper describes the development of triaxial tension creep machine. The developed machine can make equitriaxial tension creep experiments, and has a loading capacity of 49KN and the maximum temperature of 923K. Three dimensional finite element (FE) elastic-creep analyses were made to determine the shape and dimension of the specimen having uniform equi-triaxial stress distribution at the specimen center. The specimen shape determined by the FE analyses has 2.0mm groove radius at the notch. Equi-triaxial tension creep test was performed using Type 304 specimen at 923K. The uniaxial, equi-biaxial and equi-triaxial tension creep lives of Type 304 stainless steel at 923K were compared. Creep rupture time increased with increasing J₁(=σ_x+σ_y+σ_z), so that the positive mean stress has an effect of increasing the rupture time. In the equi-triaxial tension stress condition, three principal strains of ε_x, ε_y and ε_z were smaller than in the uniaxial tension and equi-biaxial tension stressing conditions.

抄録全体を表示

As one of the series of study on the effect of hydrostatic pressure on plastic deformation of polycrystalline metals, in the present paper, the effect of hydrostatic pressure on tensile creep of polycrystalline metals at elevated temperatures was discussed based on the X-ray measurements. In the previous paper by the present authors, the remarkable decrease in creep strain rate of the materials subjected to combined hydrostatic pressure at room temperature was discussed from the viewpoint of continuum mechanics. It was also pointed out that the effect of hydrostatic pressure on tensile creep of metals at elevated temperatures was not fully explained from the extension of creep mechanics of the materials under confining pressure. Therefore, it became necessary to take account of the effect of hydrostatic pressure on the structural change of the metal in the creep process at elevated temperatures.
In order to elucidate the structural change in metals creep under hydrostatic pressure at elevated temperatures, in the present study, the experimental measurements of misorientation was performed on the coarse-grained, commercially pure aluminum sheet specimen by using the X-ray back reflection Laue method. It is specially noted that misorientation is a useful measure to study the effect of hydrostatic pressure on creep of metals at elevated temperatures, because it represents not only the density of both edge and screw dislocations but also the grobal imperfection density such as the density of vacancy in plastically deformed metals. It is suggested from the present experimental results that the decrease in the minimum creep rate of the metal under combined hydrostatic pressure at elevated temperatures results mainly from the decrease in jog density which is closely connected to the density of vacancy in the material.

抄録全体を表示

In the present paper is discussed the relative comparison of the strength of AISI 347 stainless steel in thermal fatigue combined with the mean stress and the reversed mechanical stress with its strength in simple thermal fatigue, low cycle fatigue and static creep.
The analysis is compared with experimental results.
Conclusions obtained are as follows:
(1) The fracture life of AISI 347 stainless steel in thermal fatigue under the reversed mechanical stress can be predicted from the test results in low cycle fatigue at equivalent steady temperature. In the case of tensile stressing at upper temperature side, however, the fatigue life coincides with test result of low cycle fatigue at that temperature.
(2) The strength of the material in thermal fatigue combined stress, the reversed mechanical stress and the mean stress has been discussed analytically and experimentally. The lifetime is predicted from the low cycle fatigue data at equivalent steady temperature and the creep rupture data.
(3) The deformation of the specimen under stress cycling combined with the thermal stress, the reversed mechanical stress and the mean stress is larger than what is shown in the result of analysis on the time consumption hypothesis. In this case, it is necessary to consider the effect of stress history during one cycle.

抄録全体を表示

The following conclusions have been derived from the present analytical and experimental studies on the creep of a cylindrical tube of low carbon steel subjected to internal pressure at 450°C. In the cylindrical tube with an arbitrary radius ratio K, subjected to combined axial load and internal pressure, it was found that the following assumption was efficiently suitable for the creep analysis of the tube in the transient or steady state creep stage. The present assumption is the constancy of both the equivalent stress σ_av and the stress ratio A at the mean radial distance of the cylindrical tube during the creep, where the stress ratio A was defined as the ratio of average tangential stress σ_θav to the axial stress σ_zav.
The first of the present verification was made through the aspect of the influences of K and A on the analytical results of the creep of the cylindrical tube. It was concluded that the analytical results were in good agreement with the experimental results. The second of the present verification was performed from both the types of the creep tests of the tubular specimens under the periodic stress variation along the single Mises-stress ellipse and under that along the two Mises-stress ellipses. It was also concluded that the analytical results were closely like the results of the experiments made under the general loading condition of both the variation of axial load and of internal pressure during the creep tests. Therefore, it was found that both the influences of the change in the equivalent stress at the mean radial distance of the tube during the creep and of the multiaxiality of the stress or the stress ratio at the mean radial distance were not predominant in the creep analysis of the high pressure tube. By such simple assumption, the creep strain distributions along the radial distance of the tube were easily calculated from the information of simple tensile creep. It should be noted, however, that the present analytical result gives a slightly higher estimation of creep strain than the measured values in the case of variation of the equivalent stress.

抄録全体を表示

The effects of hydrostatic pressure on deformation and fracture energy at the notch bottom of single V-notch test specimens were investigated experimentally on commercial 7-3 and 4-6 brasses by means of the tension tear and three point bending tests. The specimens were soaked under hydrostatic pressures up to 200MPa for one hour before measuring at atmospheric pressure the transient behaviors of such parameters as the fracture toughness, the unit crack initiation energy (UIE) and the unit crack propagate energy (UPE).
The following conclusions were obtained:
(1) The initiation of unstable deformation was delayed by applying hydrostatic pressure.
(2) The pressure soaking treatment dispersed the dislocations piled up at the grain boundaries. This caused a temporary work-softening, and then the increase of fracture ductility and the decrease of flow stress.
(3) A remarkable influence of hydrostatic pressure was observed on the fracture toughness K_c, UIE and UPE in the tear tests. These parameters increased with increasing hydrostatic pressure. This means that both the ductility of fracture and the flow stress increase with increasing hydrostatic pressure. The same results were obtained in the three point bending tests, and they were explained well by the linear fracture mechanics.

抄録全体を表示

In the present paper, the analytical and experimental studies on tensile creep of polycrystalline metal under hydrostatic pressure at elevated temperatures were presented, in a series of studies on the influence of hydrostatic stress on plasticity and fracture laws of the metals. From the tests performed of commercial pure aluminum at 200°C, the following conclusion has been derived.
(1) The effect of hydrostatic pressure on minimum creep rate at elevated temperature is more intensive than that on tensile flow stress at the same temperature. This is considered to be mainly due to the pressure on the diffusion of vacancies in the creep process.
(2) In order to predict the effect of hydrostatic stress on the creep rupture of the metal at elevated temperature from that on the steady state creep at the same temperature, it is required that the pressure effect on the structural change during the tertiary creep, such as inhibition of growth of void as the result of reduction in the diffusion of vacancies will be taken into consideration.

抄録全体を表示

This paper points out the problem of the plural external evaluation system for Japanese universities. Japanese evaluation system is internationally the peculiar case that both of chartering in government and accreditation in government and non-government agencies exist in parallel. It is the biggest problem that the accreditation agencies secure both the autonomy and quality control and the appropriate cooperation with chartering goes well. This paper also reviews, as the experience in the world and Japan, the activity in the INQAAHE and the JUAA 2003 Version, which aims at the quality assurance of internationally viable higher education and foresees the new accreditation system recognized by government after 2004. From these experiences, it is prospected to develop the plural external evaluation system in future.

抄録全体を表示

Fatigue fracture processes are divided into 7 stages. Fatigue promotive and defensive factors of each stage were considered systematically. Based on this consideration, new methodology to increase fatigue limit of components which subjected to cyclic load of stress ratio R≥0 was proposed. That is: (a) To increase the Vickers Hardness of the component as high as possible. (b) To introduce high compressive residual stress as high as possible. (c) To fine a grain size as fine as possible. To achieve this condition, new surface treatment methods were developed by authors and the methods were applied to coil spring and gear. By using these methods, fatigue life of coil spring was increased by about 10 times and fatigue limit of gear was also increased up to 250%, successfully and economically.

抄録全体を表示

The present study is the sequel of the authors' previous papers on the influence of strain history on metallic creep at elevated temperatures, with further examination of the materials and detailed discussion on them in a series of studies on the laws of plasticity in creep of polycrystalline metals. From the present analytical and experimental studies on low carbon steel at 450°C, the following conclusions have been made.
(1) In regard to the experimental examination of initial isotropy of the material tested, the determination of anisotropic parameter A in the yield function of Eqs. (1) and (2) is simpler and more reasonable than that of usual Hill's parameter.
(2) No remarkable development of anisotropy of the material crept under fixed principal stress axes has been observed.
(3) In general loadings, in which the principal stress axes are rotated and the level of the equivalent stress is varied during the creep, the creep curve under the condition fairly deviates from the analytical curves based on the strain-hardening hypothesis. It is considered that in such loadings the development of anisotropy of the material crept must be more remarkable than that under the fixed principal stress axes, and that it is therefore dependent on strain history.
(4) In the creep tests unber combined constant axial tension and repeated torsion reversals, it does not necessarily follow that the transient increase in shearing strain rate drops immediately after the stress reversal with repetition of the reversal. It is considered that the influence of Bauschinger effect on metallic creep is remarkable when the stress reversal is imposed on the material of which anisotropy is heavily developed.

抄録全体を表示

This paper describes a method of predicting low-cycle fatigue life in long-term hold-time tests at elevated temperatures. In order to develop a proper life prediction method under creep-fatigue conditions, especially for the case of long hold-time, the correlation between the static creep rupture data and the low-cycle fatigue data in hold-time tests was examined. The specimens used were an unnotched and three round notched ones of austenitic stainless steels of SUS 304 and SUS 316 having elastic stress concentration factors of 2.6, 4.2 and 6.0. Zero-to-tension low-cycle fatigue tests were carried out by four modified creep machines at 873K using trapezoidal stress waves with 10, 30, 60, and 1440min hold-time. Static creep tests were also carried out on those specimens.
Two fracture regions were found to exist in the failure cycles-test frequency plot: one is the cycle-time dependent fracture region where the elastic stress cocnentration reduces the low-cycle fatigue life and the other is the pure time dependent fracture region where it does not. A new prediction method of fatigue life of the unnotched and the notched specimens in low-cycle tests with various hold times from the static creep rupture time and low-cycle fatigue life data obtained in no hold-time and comparatively short hold-time tests was proposed.

抄録全体を表示

As one of the series of studies on the influence of hydrostatic pressure on the mechanical behavior of polycrystalline metals, the transient behavior of plastic flow stress of prestrained polycrystalline aluminum at atmospheric pressure immediately after hydrostatic pressure soaking of 2000kg/cm² for 1 hour was discussed experimentally in the present paper. From the present study, the following conclusions have been obtained:
(1) At reloading after the pressure soaking, it was observed that the yield stress of the metal at reloading decreases transiently in comparison with the flow stress of the metal in the tension test at atmospheric pressure but it returns asymptotically to the original flow curve as the deformation advances. It was also observed that the pressure soaking has no influence on ductility of fracture of polycrystalline aluminum.
(2) It was observed that the yield stress at reloading after pressure soaking decreases remarkably with increasing the prestrain formed by pulling at atmospheric pressure. This becomes more and more evident for the test specimen with finer grains.
(3) As a structural change caused by pressure soaking of the material, the transient increase in number of moving dislocations in the metal resulting from decrease of frictional stress and locking force in the material can be considered. It is suggested that the yield stress at reloading after pressure soaking decreases because of the accelerative effect of moving dislocations.
(4) As for the strain-rate dependence of the influence of pressure soaking, it was obtained that the decrease of the yield stress under high strain-rate at reloading after pressure soaking is smaller than that under low strain-rate. This result was explained in terms of the increase of frictional stress caused by the strain-rate dependency of the material.

抄録全体を表示