Hereunder is reported the investigation made of the effect of grinding conditions and aftertreatments, such as barrel-finishing and grit-blasting, on the reversed plane-bending fatigue strength of hardened 13Cr stainless steel. Two grinding conditions were considered, the gentle and the severe. The grinding direction was always in parallel to the length of the specimen. The results obtained are summarized as follow: (1) The fatigue limit of severely ground specimens has remained about 42% lower than that of gently ground ones. (2) The surface residual stresses in parallel to the length of the specimens, both those gently-ground and those severely ground, were of tensile sort. Severe grinding, in particular, induced exceedingly high tensile stresses on the surface layer. (3) The barrel-finishing and grit-blasting after grinding have greatly raised the fatigue limit. (4) The barrel-finishing induced compressive residual stress on the surface, while, the distribution of residual stress on the surface layers of grit-blasting specimens was almost even for the two grinding conditions, and the stress was of compressive sort of considerably high degree.
In a series of studies on the influence of strain history on the plastic deformation of polycrystalline metals, the following experiments were performed in order to make clear the mechanism of plastic deformation of the material subjected to low-cycle tensile pulsating stresses. The measurement of misorientation was made by the use of back-reflection Laue X-ray method, employing the coarse-grained sheet specimens which were constructed with polycrystalline structure and monocrystalline structure respectively. From the viewpoint of crystal plasticity, it will be considered that the aggregate plastic deformation of the polycrystalline metals subjected to low-cycle tensile stressing is mainly interpreted upon the changes in the structure of the material and statistical inhomogeneity of microscopic stress and strain distributions in the material space. From the present experimental examinations, the following conclusions have been derived. (1) The measured values of misorientation in the polycrystalline structural grain under repeated tensile stressing have been found as larger than those in those in the monocrystalline structural grain. Moreover the farmer are dependent on the direction angle of principal strain in the grain. (2) Inhomogeneity in distribution of misorientation is more remarkable in the grains of polycrystalline structure than in those of monocrystalline structure. (3) The difference between the plastic deformation of polycrystalline structural sheet specimen under low-cycle tensile pulsating stresses and that of monocrystalline structural sheet specimen will be interpreted upon the model of mobility of lattice defects which is considered as the changes in structure of the material subjected to the repeated tensile stresses.
The effect of shot-peening, surface rolling and induction hardening on the rotary bending fatigue strength of sintered ferrous alloy was investigated. The main results obtained are as follows: (1) The fatigue limit of plain specimens is raised by shot-peening, but the increment of the fatigue limit is small due to the notch effect of the pores flattened by the shot-peening. The notch effect of flattened pores increases as the original density decreases. The fatigue limit of specimens resintered after shot-peened is higher than that of specimens processed only by shot-peening, because the flattened pores disappeared during the resintering after shot-peening. (2) The fatigue limit of notched specimens is considerably raised by surface rolling. (3) The fatigue limit is raised by induction hardening. The rate of increase of fatigue limit is larger in the case of notched specimens than plain specimens. The analysis of induction hardened plain specimens shows that the change of microstructure of the specimen caused by the induction hardening contributes little to the increase of the fatigue limit due to the reasoning that, as is easily predicted, the notch effect of the pores is more serious in the case of martensitic structure.
The corrosion fatigue tests of mild steel have been carried out under various mean stresses, and electrochemical discussions have been made on the progress of corrosion fatigue damage with corrosion potential, corrosion current and polarization characteristics. The corrosion potential under static stresses remarkably shifts to the anodic, irrespective of tension or compression, when the applied stress exceeds the elastic limit, and the deformation becomes plastic. In corrosion fatigue, the mean tensile stresses accelerate the increase of fatigue damage, which is found to be caused by the greater increase of the slope of anodic Tafel lines. On the contrary, the mean compressive stresses suppress the increase of its fatigue damage when the work hardening by the repetitive stresses has been completed, which is found to be caused by the smaller increase of the slope of anodic Tafel lines and by the delay of fatigue crack propagation. Under the catholic protection of corrosion fatigue, the incubation period before the occurrence of iron dissolution is found longer under the mean compressive stresses than under the mean tensile stresses, which suggests the easier applicability of the cathodic protection under the mean compressive stresses.
The writer reported some time ago on the natural cleavage of KCl single crystal that had been derived by rapidly cooling the melt in the crucible. The edge length of the crystal pieces d depended on that occasion on the cooling velocity v as d∝1/v2. On the other hand the commercial steel had grain structure. In this paper it is demonstrated that, in spite of the fact that both the materials differ remarkably from each other in several physical features, e. g. brittle or ductile, their apporoximate grain size can be formulated in a single expression representing the temperature gradient and the cooling velocity of the specimens in operation. In consequence of their numerical estimation their practically fit value have been obtained. Furthermore the standard cooling velocity has been theoretically studied and established by reference to which the cooling operation may be defined either as rapid or as slow.
It is intended in designing the present model to make clear the macroscopic features of nearly homogeneous materials when they yield to simple tension and break down leaving many small cracks. (1) Let us assume that a certain material contains only one part where the defect structure is mechanically of nonlinear character, and that in that defective part Young's module E is dependent on the relative elongation ε as is to be represented in the formula E=E0(1-βε) where E0 and β are constant. Then it is found that the material breaks down at stress E0/4β and that β is nearly in the range 10∼350 with glass, several metals and wood. The physical import of the present model is shown in the interatomic potential curve for the one dimensional single crystal. (2) Or let us suppose, on the other hand, a number of areas of defect structure in the material for so many small cracks, which are considered as nuclei respectively of the defect, their distribution in the material can be expressed in a nonlinear factor as function of the coordinates to be defined by three parameters. (3) The defect structure seems to consist of various common kinds in the case of single crystals, but in the case of polycrystals, of cavities of size described as of optical order. It is found that the present model will efficiently explain macroscopic structure in the material from the distribution of the defect structure.
The radiation damage to materials insulating electric current presents serious problems affecting the delicate functions of such electric apparatuses and measuring instruments as are constantly exposed to electronic radiation. One of the problems concerns epoxy resin which is appropriate material of wide application for manufacture of electric apparatuses by reason of its insulating capacity. Studies have been made, accordingly, of the effect of γ-ray radiation on epoxy resin with this excellent property. Our experimental studies will best be stated in three respects, first regarding the insulation, secondly regarding the test material, and thirdly regarding the radiation. Regarding electric insulation, the resistance against electric insulation, the strength against dielectric breakdown and the resistance against arc were respectively examined. Regarding the test material three kinds of epoxy resins were used, that of the bisphenol type, that of the novolac type and that of the cycloaliphatic type. Regarding radiation the dose of γ was prescribed between 106∼109. The results obtained from the experiment are briefly as follows. (1) With increase of γ-ray radiation in dose there is general decrease in insulation resistance. It is observed, however, at the same time that the maximum value of γ-ray radiation so far produced has been about 4×106∼5×107, and it is found also that there can sometimes be restoration of insulation resistance by virtue of its temperature dependence. (2) The strength in voltage against dielectric breakdotwn decrease slowly with increase of γ-ray radiation in dose. (3) Regarding the amout of γ-ray radiation that can have an effect on arc resistance of epoxy resin, as far as such an effect is observable, its arc resistance begins to drop at exposure to radiation of ca. 108-γ and over, though there can sometimes be epoxy resin that is intact in arc resistance despite exposure to radiation even of ca. 109γ. (4) When aromatic amine and acid anhydride are used as curing agents for epoxy resin there is but little effect γ-ray radiation to be observed on its arc resistance. (5) The amout of curing agents consumed by epoxy resin when it shows its maximum arc resistance in value, and their amount when epoxy resin shows its minimum in value, both under the influence of γ-ray radiation to which the material is respectively exposed, vary from the amount respectively when the material shows its maximum arc resistance in value, and that when it shows its minimum in value accordingly, both free from such influence. This is attributed to the relative variation in the corresponding quantity of epoxy resin itself owing to the molecular split and cross-linking.
To examine how far the effect of mix proportion in fresh concrete and in hardened concrete, centrifugally placed, upon its properties is dependent on its conditions, six sorts of sand graded according to grain size were selected and several rates of mix proportion s/a were determined for the experiments, which have been performed with these selected materials thus variously mixed. The tests of fresh concrete have been performed to investigate the mutual relation between the different grade of sand and different rate of mix proportion s/a on one hand and on the other the water content required for maintaining constant consistency. Investigation has also been made of the efficacy of water-reducing admixture. The tests of hardened concrete have been made to find the effects of the above mentioned conditions in mixing proportion on the segregation and separation of centrifugally placed concrete, and to ascertain its manifest gain in strength. The main results obtained by them are as follows. (1) Of the fine aggregates of different grades in grain size of sand, the finer part has effects on the water content, while the coarser part on the mix proportion s/a respectively, and when the mix proportion is properly determined, the centrifugally placed concrete with gap grades of sand will occasionally make better concrete of higher strength with smaller segregation. (2) The use of water-reducing admixture of good quality will remarkably raise the workability of fresh concrete, and considerably reduce the segregation and separation of centrifugally placed concrete.