Experiments were made with flat plate 13 Cr stainless steel specimens for their fatigue test in the reverse plane-bending, to study the effect of their edge shapes on their fatigue strength. The specimens had been heat treated to Rockwell hardness of C 45, and their edges were formed either by machining the specimens or by punching them. Some of the machined specimens were further machined to be rounded to various sizes of radius. The experimental result shows: (1) that the fatigue limit of machined specimens has risen about 10 per cent as the result of rounding their edges even slightly; (2) that the fatigue limit of punched specimens has remained about 26 per cent lower than that of machined specimens of right angular edges.
It is the object of the present work to develop a new device for evaluating the durability of polymeric materials of wide variety the materials of rigid quality under bending and those of flexible quality under compressive deformation. The advantageous features of this device are summarized as follows: (1) It permits parallel tests of five specimens simultaneously. (2) It is equipped with ultraviolet lamps for irradiation, and with apparatus for controlling temperature constant. (3) It allows continuous recording of dynamic load of the specimens at constant amplitude of deflection and of rise of temperature in them. (4) Frequency is variable within a range from 300 to 1500 c.p.m. By preliminary experiments with several samples, the tester has been found to be efficient for the present purpose.
Hereunder is described a new approach to evaluating the influence of strain-rate and temperature on the flow stress of mild steel. The flow stress consists of two components, thermal and athermal. Application of thermal activated process permits separation of flow stress into the two components developed during the deformation. Strain-rate cycling tests were performed during the tensile deformation in the temperature range 25°C∼450°C. The effect of strain-rate and temperature on the thermal component and of history on the athermal component were discussed.
The effect of precompression on the fracture ductility and the low-cycle fatigue properties was investigated with low carbon steel. The fracture ductility in the transverse direction of the longitudinally precompressed material decreased very slowly with precompression in a range of intermediate and high magnitude of precompression, while this decrease was rather rapid at small prestrains. As a result, when subjected to a longitudinal precompression as high as 120%, the fracture ductility of the low carbon steel investigated still retained in the transverse direction a magnitude of about 30%, which was about 1/2.5 of the ductility in the transverse direction of the initially unstrained condition. The low-cycle fatigue strength of the longitudinal specimen made of the carbon steel highly compressed in the longitudinal direction, the fracture ductility of which was about 55%, was found to be approximately represented by the Manson-Coffin type relation, as was also found in the case of initially unstrained materials, including the point plotted as twice the fracture ductility value at a quarter cycle. On the other hand, the transverse specimen, the fracture ductility of which was about 11%, showed a strange behavior. The plastic strain range sharply dropped at cycles ductility between 102 to 2×102, and little increase in the number of cycles to fracture was observed with further decrease in the plastic strain range. At cycles less than 102, however, a straight line relation in the log-log co-ordinates was observed, although the inclination of the straight line was much lower as compared with that of the foregoing longitudinal specimen and also of the initially unstrained specimens. This strange behavior was interpreted as an instability phenomenon resulting from the cyclic strain softening characteristics of the material subjected to extremely high prestrain.
It is suggested that fatigue failure mechanisms in the face-centered cubic metals are greatly affected by stacking fault energy. It was from this viewpoint that the fatigue crack initiation and stage I type crack propagation in 18-8 austenitic steel which had the lowest stacking fault energy among the face-centered cubic metals on the market, were discussed by the X-ray microbeam diffraction technique and the replica electron microscopic method. The fatigue slip line at room temperature tests has fine and straight configurations which are characteristic of low stacking fault energy materials, while the wavy and coarse slip line is typical of fatigue tests at 200°C where screw dislocation is more easily prone to cross slip. In both of these cases very complicated deformation configurations are caused by double slip and cross slip near the grain boundaries, and frequent extrusion and intrusion are observed in these areas. Fatigue crack nucleation might be caused by stress concentration at the notch and peak topography. The stage I type fatigue crack propagates immediately after the initiation, mainly along and frequently across the intense slip lines. Its growth in the area neighbouring the grain boundary runs in the direction that depends on both the crystal orientations of the grain containing the crack and the grains adjacent to it. Total misorientation and micro lattice strain measured by X-ray micro-beam diffraction technique increase abruptly at early fatigue stressing, then equilibrium values are attained and finally increase rapidly. These three periods during the fatigue deformation might be corresponding to those of the fine slip line initiation, the intensification of each slip line without new slip line formation and of final initiation of micro crack, respectively. Furthermore it will be noticed that the equilibrium value of total misorientation of 18-8 austenitic steel during the fatigue process is about one-tenth of that of carbon steel. The area of substructural zone near the fatigue crack of 18-8 austenitic steel is smaller than that of aluminum of high stacking fault energy and body-centered cubic iron. Therefore, the formations of subgrain and martensite could not be detected by X-ray micro-beam diffraction technique when irradiated by Cr-Kα of 200μφ beam size. In order to clarify these points the detailed observation at the fatigue crack tip must be made by using transmission electron microscopy. These results will be reported elsewhere.
In our previous paper were shown the electron micro-fractographies of aluminum alloys which were fatigued under actual service loadings, and statement was made that the striations of the fracture surface corresponded to the loading cycles as one to one, and their spacings to the stress peak values. In this paper are shown the fatigue fractographies of other metals, low carbon steel and manganese bronze. The applied loadings were of bending on their circular beam test pieces. The loading patterns were two level amplitude variations, square wave loadings and two superimposed sinusoidal ones. Though the striations of the carbon steel test pieces were irregular, only partial correspondence of the striations and loadings could be discerned. It was confirmed that the fatigue crack propagation length in one loading cycle of the square wave increased and its fatigue lives became short as the time was prolonged during which the constant stress was maintained. The striations of manganese bronze were of two types, one was regular which was discerned in α phase grain, f.c.c., and the other was irregular in β phase, b.c.c.. There was good correspondence of the striations and the loadings on this material. On these fracture surfaces rows of pressed marks were discerned, which were produced by pressure against the fracture surface on the counter side and sliding over each other if slightly after the crack grew. These rows of pressed marks also corresponded well with the loading cycles.
The temperature dependences of Vickers hardness of tungsten, molybdenum and the reactor pressure vessel steel (Coltuf-28) were examined in comparison with the geometrical characteristics of the deformation around the impression by using a salerometer. As a result, it is made clear that the deformation range (lm) around the impression increased generally with increasing temperature for a constant size of impression (dm) and became maximum between the two junction points in the logHV-1/T relation, and that the rise height (hm) around the impression indicated the maximum between the above mentioned junction points and decreased abruptly above that temperature. It is made clear that the average surface strain (ε) obtained from the deformation around the impression is generally 2∼5% for most materials and becomes maximum between the above mentioned junction points for a constant deformation range (lm) and decreases at other temperatures. It is considered that the plastic strain increases abruptly with increasing temperature over the temperature range between the two junction points in the logHV-1/T relation. This corresponds to the abrupt increase in the strain and the reduction of area in the tensile test.
The authors have recently reported a new method for the separation of principal stresses, which combines the conventional method with the scattered-light method applied in only one direction. The separation of principal stresses can be carried out by combining the isochromatic fringe order and the isoclinic parameter measured by the conventional method with the fringe order distribution obtained by the scattered-light method. By using the circularly polarized light in the scattered-light method, fractional fringe orders may be determined from the rotated angle of the polarizer, similar to Tardy's procedure for the conventional method. In this paper, as an extension of the above-mentioned new method of principal stress separation to the plastic range, a photoelasto plastic experimental result is reported. The model material used was nitro-cellulose with good transparency, whose optical and mechanical behaviors, such as the variation of stress sensitivity in the plastic range and the yield criterion, had been examined previously. Clear fringe patterns could be obtained in the scattered-light method by using the ordinary mercury lamp as a light source. As the shape of the specimen, a flat plate with a longitudinal slot was adopted, the ratio of length to width of the slot being varied in four ways. Tensile loads were applied to the specimens in seven steps from the incipient yielding to the penetration of plastic regions. From the experimental results, the principal strains were calculated by using the Prandtl-Rueβ strain incremental equation. The experimental results are summarized as follows. In the stress distribution along the cross section through the upper edge of the slot, the principal stress σ1 showed the peak value at some distance from the boundary of the flat plate, due to the strengthening interference between the hole and the free edge. In all the specimens the penetration of plastic regions occurred in the same direction. The load required for the penetration of plastic domains became smaller with the increase in slot length, which was contrary to the tendency at the primary yielding.
When brittle lacquer coating is to be employed to strain analysis of machines and structures, it is required that the coating will, without resorting to heat treatment, present preeminent strain sensitivity, so far as the object bodies to be analysed have large heat capacity, particularly when they are in running state. But since the lacquer coating hitherto processed has been left to air-drying, it is deficient in brittleness, and cracking does not easily develop on the coated surface, because the lacquer used for the coating has, as its basic ingredient, limed wood rosin which contains neutral materials. In the previous study a report was made of an experiment that was performed with micro structure of the coating with a view to inducing the principle to improve its brittleness. The principle was then applied to preparation of improved lacquer coating, and was verified by presenting the excellent sensitivity simply by curing it at room temperature. The authors have been engaged in the systematical study of the influence that the basic ingredient of the lacquer coating and the plasticizer will make on the sensitivity by the relative proportion of its content at various levels of temperature and humidity. As the result of the study improvement of lacquer coating has been made available and its preparation duly made that will always present sensitivity to circ. 800 micro strain under whatever condition of temperature and humidity without subjecting it to heat treatment. Moreover the improved lacquer, with advantage of leaving the cracks made on the coating applied with it staying open after the removal of the load, has made it more easily practicable to work dynamic analysis of the machines and structures. Coating with the improved lacquer was once applied to stress analysis of an aeroplane with wingspan 33000mm in its test flight with successful result.
It is the purpose of this experiment to study how the growth of evaporated BaTiO3 deposit in vacuum into crystal films varies dependening on the difference between the degree of vacuum and the deposition rate. The degrees of vacuum applied were from 2×10-3 to 2×10-6Torr. A tungsten boat filament was used for evaporation heater. The deposition rates were varied from 1 to 30Å/s by changing the current intensity applied to the filament. The thickness of the films was 1μm to 2.5μm. The substrate used was a platinum plate and its temperature was kept at 500°C constant. As the result the condition of the films is found as follows as they have been examined by X-ray diffractions: In the cases of 10-3 and 10-4Torr only the diffraction lines of the platinum plates are observed, but almost no trace is observed of crystal of BaTiO3, regardless of deposition rates, while in the cases of 10-5 and 10-6Torr, when the deposition rates are relatively smaller strong diffraction lines of BaTiO3 are seen. However the electron micrographs show that as the deposition rate increases the corresponding sizes of the crystals become larger until the rate reaches 10Å/s. At this peak, the size begins gradually to be smaller as the rate increases. These crystals are all of cubic system. Impurities of BaWO4 are present in all the samples irrespective of the degree of vacuum and the diposition rate.
The disturbance of electric field in the vicinity of the test electrode by the Luggin-Haber capillary was studied mathematically and empirically with the current interrupter. The equipotential line near the capillary tip was predicted, and considerable deformation was observed over about one third of the diameter of the capillary. The capillary located close to the surface of the test electrode disturbed the uniform current distribution, and the build-up curve of the oscilloscope trace was deformed. Its critical distance was about twice the diameter of the capillary.
Attempts have been made to use a rod specimen in place of any conventional type of specimens for estimating changes in mechanical properties of plastic materials consequent on various sorts of their degradation. There are several advantages with rod specimens as follows: (1) The specimens are available by utilizing common commercial rods simply by cutting off the required pieces. (2) Their mechanical properties can be measured with a torsional apparatus made for trial by us. (3) The rod specimens are better suitable to surface resistance measurement. The rod specimens have proved to be efficient means for estimating durability in the experiments with polypropylene and polycarbonate for photo- and thermal degradation, for immersion in solvent and alkali, and for torsional fatigue.
Dental plastic materials (most of them are polymethyl-methacrylate polymer) are commonly used for partial and full denture as prosthetic appliance. For the basic design of raising efficiency of these materials it is important to promote their experimental knowledge as to their fatigue strength which has not hitherto been sufficiently developed. So in this paper a report is made on the fatigue strength of these materials, the relation between the surface roughness and strength and the influence of the polymerization method employed on these materials. As the result of the experimental studies the following conclusions are made. There has been remarkable influence of the polymerization method on these materials which were finished with the emery paper, number 04. The wet heat polymerization is superior to the dry heating on the static and fatigue strength. The degeneration of fatigue strength of the dry heat polymerization is greater than the static properties and the surface roughness influences for fatigue strength. The influence of the finishing methods on the surface roughness is not seen in the results of the surface inspection but in their fatigue strength. In the comparative study of the dental materials with the engineering materials, the S-N curves of the dental materials show their continuous behavior, and their difference from polymethyl-methacrylate (PMMA) of the engineering materlais. And the degeneration of fatigue strength of dental PMMA materials is less than that of their static strength.
It is the main purpose of this study to ascertain whether or not the maximum size of coarse aggregate will affect the stability of asphaltic concrete and the optimum percentage of its coarse aggregate. In this study, crushed stone, natural river sand and limestone powder were used for aggregate. The maximum sizes of crushed stone were 25, 20, 13 and 10mm. The percentages of coarse aggregate (>2.5mm) were 65, 60, 55, 50, and 45. The asphalt content varied from 5.5% to 9.5%. The properties of asphalt mixtures were evaluated by the Marshall test. The main conclusions to be drawn from the date obtained are as follows: (1) The maximum size between 13 and 25mm had negligible effect on the Marshall stability, whereas considerable reduction in stability was obtained when 10mm size was used. Therefore, 13mm size can be used for dense-graded asphaltic concrete. (2) The maximum size between 10 and 25mm had little effect on the optimum percentage of coarse aggregate. Consequently, it is not necessary to change the ratio of fine aggregate to coarse aggregate, when 13mm size is used instead of 20mm size. (3) The authors regard the specific limit of Japan Road Association for gradation of overall aggregate of dense-graded asphaltic concrete as appropriate.