A high tension species of steel called“Hi-Strand”, for prestressed concrete was developed, and its mechanical properties and fatigue strength were studied.“Hi-Strand”, as its commercial name stands, has tensile strength about 20% higher than conventional PC steel called JIS PC strand. In spite of this high tensile strength, it is proved that Hi-Strand has as much strength as JIS PC strand in many points. In this report (the first of three reports), its mechanical properties are discussed, and in succeeding reports its fatigue strength will be reported.
It is generally considered that the materials for electrical insulation get their arc resistance differently effected according to the kinds of the insulating materials and to their respective nature. The differentiation in the effective resistance of the materials is due also to the geometrical forms of the samples. It is considered that the latter give rise to somewhat divergent processes and forms of deterioration, and so affect the arc resistance of the electrical insulation materials considerably. The experiments were carried out accordingly about the effect of the geometrical forms of the samples on their arc resistance. To the experiments was adapted the method of ASTM D 495-61. Since it was required that the samples of insulating materials were to be of various forms, such materials as epoxy resin and polyester resin were preferred for the experiments as fit for being cast readily into required shapes. The following results were obtained from these experiments. (1) Considerable effect was given by the geometrical forms of the samples on the arc resistance of the insulating materials. The arc resistance in the case of columnar samples of from 10 to 15mm in diameter showed its drop to from 84 to 90%, even to from 60 to 68% of the arc resistance in the case of plates. Then there is larger rate of drop of arc resistance in the case of epoxy resin than that in the case of polyester resin. (2) No obvious effect to save the drop of arc resistance due to variation in diameter in the case of columnar samples was seen by reinforcing the fillers with such materials as CaCO3, SiO2, TiO2, mica and talcum. (3) The same effect of the forms of the samples on the arc resistance of the electrical insulating materials is observed in conic samples as in those columnar. (4) When the columnar samples had pleats and trenches, no obvious effect to save the drop of arc resistance due to variation in diameter in the case of columnar samples was seen, so long as the variation in the diameter due to the presence of the pleat or the trench is about from 0.55 to 1.30 times as large as that of the columnar samples without pleat or trench.
Various investigations have hitherto been reported of stress aging treatment, but they have been confined to its effect on the mechanical properties of the material, and to its effect on the fatigue limit of the material but little attention has so far been paid. The measurement of fatigue limit was performed therefore of austenitic and precipitation hardening stainless steels at the intervals respectively of 15min, 30min and 60min of loading and stress aging at 375C. The tensile stress 26.9kg/mm2 was given to the stress aging of austenitic stainless steel, and 52.0kg/mm2 to that of precipitation hardening stainless steel, which corresponded respectively to 130% and 60% of 0.2% yield stress. The fatigue test was carried out at room temperature by means of Ono type rotary bending fatigue testing machine (3400rpm). The results obtained are outlined below. (1) In austenitic stainless steel the progression rate of its loading time 15min, 30mm and 60mm showed increase in percentage 17, 30 and 33 of the fatigue limit of the specimen only subjected to solution treatment. (2) The fatigue limits of precipitation hardening stainless steel showed an increase of 7, 6 and 12% the value of the specimen subjected to only solution treatment. (3) With increse in the loading time there is less difference between the time strength(2×105) and the fatigue limit (107). (4) The treatment of stress aging at stress level higher than 0.2% yield stress is considerably effective in a short time, but for the treatment of stress aging at stress level lower than 0.2% yield stress a longer time is necessary in order to be effective. (5) In spite of the fact that the time required for stress aging is subject to variation, there is no variation of the tensile strength of austenitic stainless steel subjected to stress aging, nor of its 0.2% yield stress. Though there is a little decrease in the tensile strength of precipitation hardening stainless steel, there is no variation in its 0.2% yield stress.
The authors have applied X-ray diffraction techniques to the study of low-cycle fatigue in order to discuss its macroscopic behavior from the microscopic point of view. In a previous paper, it was reported that a good correlation was obtained between the particle size measured by the profile analysis of the X-ray diffraction peaks and the damage fraction in 0.16% C steel under low-cycle fatigue of constant strain amplitude for some range of the strain range. It is found that the nondestructive measurement of the remaining life might be possible using this correlation. However, the above finding was not successfully used for the discussion of the mechanism of deformation and fracture in low-cycle fatigue, because the publications dealing with the analytical method and the application of this X-ray technique gave little available information for relating the data obtained by the authors to the microstructural changes reported by various investigators. In these circumstances, the authors applied the X-ray microbeam diffraction technique to a further study of the microstructural changes which occurred during the low-cycle fatigue. The behavior of the subgrain was observed by this technique, and was made to bear on the measurements of the X-ray profile analysis. The following was obtained as the results of this investigation: (1) Low-cycle fatigue before visible cracks occurred had two stages in its process, each of them having its own particular X-ray phenomenon. The first stage was the stage of subgrain formation indicated clearly by the changes in the subgrain size and the interaction function of dislocations. The subgrain size was reduced rapidly and was inclined to saturate. The interaction function, which was defined as the ratio of the dislocation density obtained from the value of microstrain to that from the particle size, was reduced considerably. On the other hand, the excess dislocation density at the subgrain walls increased remarkably in the second stage, which was termed the stage of the subgrain development. The number of cycles at which the first stage was completed was around 270 in the case of the strain amplitude of 2.0%. (2) The observation of surface patterns by electron microscopy showed a clear development of surface irregularities like extrusion and intrusion after the first stage was completed. This development may be related to that of the subgrain. (3) The particle size had a linear relation with the subgrain size, and the interaction function indicated well the state of the subgrain. However, for the practical use of these measurements of X-ray profile analysis for closer examination of the substructure which appears in plastically deformed metals, it is necessary that further investigation will be made along this line.
The mechanism of fatigue crack propagation has been investigated from the point of view of micro-structural change at the crack tip. The relation between the crack propagating mechanism and the developed sub-structure is discussed.
Recently, low-cycle fatigue of metallic materials has been widely studied, and several fracture criteria of fatigue have been presented as the basis for predicting low-cycle fatigue lives. The authors had previously proposed a fracture criterion of low-cycle fatigue based on strain, in stead of the Miner's linear damage rule based on S-N curve, and investigated the relation between the stress-strain behaviour and the fatigue lives by carrying out the low-cycle fatigue tests under rotary bending to measure the stress and strain during those tests, and the above mentioned criterion was verified experimentally. Although many ordinary fatigue tests have hitherto been carried out of aluminium alloys, still we have very few data of low-cycle fatigue tests of aluminium alloys performed under rotary bending. In the present report information is given of experimental studies that are usually performed on steels as applied to the studies on the aluminium alloy 17 S-T 4 under the condition of the constant- and varying-stress amplitudes. Moreover the effects of frequency distribution of loads and their sequence of application on fatigue lives were investigated. The applicability of the above criterion has been discussed by summarizing the data by means of“the reduced strain amplitude”and“the reduced stress amplitude modified by the stress correction factors”already reported.
Various changes in structure and in hardness of Cu-6.0wt% Al alloy at different stages of repeated bending fatigue test were studied by hardness test and by both micro-and electron microscopic observations. The experimental results are summarized as follows: (1) Irrespective of amplitude whether of high or low stress, the initial formation of slip bands are observed first during the stress cycling, and they are all of homogeneous slip. (2) After the appearance of slip bands, rippling phenomena of slip bands are observed and the phenomenon is remarkable in high stress amplitude. (3) After the formation of rippling bands, the intrusion and extrusion are observed principally along the grain boundaries in low stress amplitude, but in high stress amplitude, they come out irrespective of grain boundaries and slip bands. (4) The crack occurs initially along the grain boundaries in low stress amplitude, In high stress amplitude, they are observed both in the interior and along the grain boundaries. (5) The hardness of the specimens increase with increase in stress cycling numbers and the maximum hardness is obtained around the fractured area.
The tensile test of unnotched and round specimens in the liquid tin, lead, zinc and bismuth bath was carried out with an Instron type testing machine in order to investigate the susceptibility of a mild steel to liquid metal embrittlement for which some inconsistent results had been reported among investigators. In the present paper, in this context, various effects of test conditions on liquid metal embrittlement were studied. The results obtained are summarized as follows; (1) Propensity to liquid metal embrittlement is influenced by the test temperature, for which the optimum temperature exists. (2) The embrittlement is greatly affected by the strain rate, for which the optimum rate also can be found. (3) It is not considered that liquid metal embrittlement is essentially influenced by the holding time at the testing temperature before straining. (4) The embrittlement by liquid zinc is very sensitive to the surface roughness of the specimen, that may be ascribed to the notch effect. (5) The embrittlement by liquid tin has no relation with the diameter of specimen or the ratio of surface area to volume of test specimen. (6) Liquid bismuth does embrittle the mild steel. Consequently, it is not considered that the embrittlement in liquid metal of mild steel is merely controlled by the chemical diffusion that depends upon the acting time and temperature.
When 18-8 stainless steel is soaked in sea water, a local attack is formed by adhesion of microbes comparatively in a short time. In order to investigate the effects of corrosion defects on the fatigue strength of this material, we carried out the rotating beam fatigue tests of plain specimens and notched specimens with stress concentration factor, α=2.0, 3.3, 4.9, which were soaked in sea water for 3 months, 6 months and 12 months. The results obtained are as follows. (1) The fatigue limit of plain specimens and that of notched specimens with α=2.0 and 3.3 are on the decrease with the soaked periods shortened. The lowering ratio during the 12 months stood at 33%, 27% and 19%, respectively. (2) The corrosion defects in the order of crack, pin hole, line and pit have a deteriorative influences upon the fatigue strength. (3) The lowering of fatigue limit of notched specimens with α=4.9 is not recognized, for this is the strength with nonpropagating crack. (4) In the case of notched specimens with α=2.0 and 3.3 notch strengthening is found out, because of formation of martensite by plastic deformation at the notch root.
The method of straining grooved tensile specimens suggested by Hill has been applied to the finding of the criterion for necking of polycarbonate. The specimen is in the form of a strip grooved obliquely on its surface. If such a strip is pulled in tension, deformation will be confined to the grooved regions. The state of stress produced in the zone of deformation is biaxial, owing to the constraint of the surrounding material. It is by the measurement of the maximum load, for a range of groove angles, that the deduction of the criterion for necking of polycarbonate under any state of combined stress is possible. The results are as follows: The due stress at the maximum load increases appreciably, as the state of stress in the grooved region changes from that of thin-walled tube subjected to internal pressure to that of uniaxial tension of solid bar. In order to examine the anisotropy of polycarbonate plate, the small specimens cut at five different angles with respect to the longitudinal direction were each tested in simple tension. The strip specimens were found approximately isotropic. In consequence of these investigations, it is concluded that necking of polycarbonate is due to mechanical instability, in so far as the stress of the specimen is in a state of tension between the biaxial and the uniaxial.
The break of tungsten filament and deformation of the electron tube heater in their on-off cycling test are the subjects of our present study with respect to their tensile strength at room temperature after their heat treatment, and also with respect to their metallographic structure. The heat treatment of the tungsten filament was made by the method presented in our previous report5). The on-off cycling test of the heater was carried on by applying heater voltage higher than normal rating. The results are summarized as follows. (1) The tungsten filament with tensile strength that remains high at room temperature after the heat treatment does not break even at the on-off cycling test where heater voltage higher by 60% than normal rating is applied. In order to prevent the break of the filament or the deformation of the heater, it is necessary that the tungsten filament will have tensile strength higher than 50g/MG at room temperature, and maintain it after being subjected to heat treatment at 1600°C for 20 seconds. (2) The tungsten filament with low tensile strength at room temperature easily breaks and the heater gets deformed after heat treatment or at on-off cycling test, and their metallographic structure shows grain growth considerably. (3) As evidenced by repeated experiments the tensile strength of the tungsten filament has a great deal to do with the condition of the filament itself and of the electron tube heater after their heat treatment and on-off cycling test, whether both the filament and the heater are safe and sound or whether there will be the break of the filament and deformation of the heater. This evaluation method of tungsten filament will thus guarantee prevention of break of the filament and deformation of the heater.
To examine the effect of reduced cement content upon the properties of concrete containing admixtures, as in the air-entraining agent, five water-reducing admixtures, four water reducing-set controlling admixtures, and five brands of normal portland cement have been selected and used with some combinations. Comparative tests have been performed of fresh concrete with respect to its water-reduction, bleeding, time of initial and final setting, and of hardened concrete with respect to its strength gain concretes with the above-mentioned admixtures with cement contents of 260, 280 and 300kg/m3, were prepared and compared with the control concrete having the cement content of 300kg/m3 and same consistency of 7.5cm without admixture. The results of the test show that the good quality of water-reducing admixtures can reduce water requirement by 10 to 15 percent, irrespective of the cement contents, and that the strength gain, the time required for setting and bleeding are affected by reduction in cement contents, and by the quality of water-reducing admixtures.