In the present paper is presneted a report of an electronic type high temperature tensile testing machine which we have been enabled to design, and which is capable of automatically controlling strain rate of the test specimen between its gage marks. The capacity of the machine is 10000kg, with the following capabilities. (1) Driving System of Servo-Mechanism utilizing Silicon Controlled Rectifier with high response. (2) Automatic Strain Rate Control ranging from 0.05 to 20%/min. (3) Strain Rate Control Accuracy within ±0.5%/min. of set speed. (4) Duplex Type Heating System for increasing efficiency. In view of the fact that ISO, BS and JIS require that the strain rate between the gage marks will be maintained constant during the high temperature test, the conventional testing machines are hardly adequate for the purpose of performing the test so as to meet the requirement. With this machine the test can be made as designated by these standards with high accuracy automatically, and moreover, it is better fit for studying the effect of the strain rate on the high-temperature properties of metals.
Since conventional indenters used for hot hardness testers by which the measurement of indentations can be performed in the furnace heating the specimen are of diamond indenter points fixed into metallic mounts, the temperature on the region of the indentation inevitably falls mainly as the result of heat conduction during the indenting process even if the indenter point has been adequately preheated, so that correct hot hardness value of the specimen cannot be obtained at the predetermined testing temperature. On the basis of theoretical considerations on the behavior of heat transfer, highly effective heat insulating diamond indenters to be used for the measurement of the hot hardness of the materials harder than sapphire have been produced on trial in order to obtain a closely correct hot hardness value at every testing temperature up to about 1000°C. The results of hot hardness measurement showed that the heat insulating effect of the indenters produced on trial was closely similar to that of the solid sapphire indenter. Moreover the measurement of hot hardness of various ceramic tool materials up to 1000°C could be perfectly carried out by means of these heat insulating diamond indenters.
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.
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 σav 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 σθav to the axial stress σzav. 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.
Some time ago we previously proposed what may be termed time-temperature parametric method, by which stress relaxation data with an equal level of initial stress or strain, but with different temperatures, can be combined into a single correlation curve with the coordinates of X(=logσ/σ0) and parameter P(=log Et-Q/4.6I), where σ0 is intial stress, σ is residual stress, E is Young's modulus, t is time, T is absolute temperature, and Q is parameter constant. In this paper we report a study that was made of the methods for calculating parametric constant Q by an electronic computer, assuming that X (or P) could be mathematically represented by a polynominal of P(or X). Consequently parametric constant Q was readily computed. We further disscussed on the method for analysing the isothermal relaxation data with different initial stress and strain, and elucidated the fact that these data were correlated as well into a single curve by using a modified parameter π=m logσ0+logt, instead of parameter P.
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.
There is extensive use of liquid metal at elevated temperatures as the coolant of heat exchanger, as the information and pressure medium, as the electro-magnetic fluid, as the lubricant and so on, for their excellent thermal and electrical characteristics. In using the liquid metal, however, there are many problems. One of them is that, in the case of structural materials, the metal weakened not only by the corrosion effect but also by combined effect of the alternating succesion of stress and the embrittlement by liquid metal. In the last mentioned respect, the present paper gives the fatigue tests of low carbon steel, 13Cr steel and 18Cr-8Ni steel in air and in contact with liquid tin and lead, in order to examine the fatigue resistance of various steels that were brought into contact with molten metals at elevated temperatures. The 18Cr-8Ni steel showed the smallest percentage decrease in fatigue limit of the three kinds of steels tested as the results. The discussion based on this experimental result has led to conclusion that the rich ductility of austenitic steel due to its having been employed in solution heat-treatment and the notch insensibility of 18Cr-8Ni steel for liquid metal embrittlement resist the fatigue damage.