The position sensitive proportional counter (PSPC) is capable of providing accurate positional informations with high resolving power and high count rate capability. If the PSPC system is applied in the field of X-ray stress measurements, the following features can be anticipated; (1) No goniometer is necessary. (2) Stress analysis can be accomplished at a high speed unequaled by any conventional method in use today. In the present paper, the application of PSPC for stress measurement is reported. A new type PSPC having 85mm long of effective length was made, in which the position sensitivity is acheived by measuring the electric charge of output pulses from a proportional counter having a high resistance cathode. A spatial resolution of 230μm was obtained over the full length of the detecter. Stress measurement was performed on several bending type specimens of carbon steel. The difference in stress measured by this system between load on and load off was in close agreement with that measured by the wire strain gage method. The time required for the stress analysis was reduced to 1/5∼1/10 of that with the conventional counter method.
The X-ray stress measurement of austenitic phase was carried out on two-phase stainless steels, such as ferrite austenite stainless steel and strain-induced metastable austenitic stainless steel, by using the monochromator method. This method was performed by setting a graphite plain crystal between the receiving soller slits and the detector. The results obtained are as follows. (1) X-ray diffraction from Fe(211) by CrKα can be completely eliminated, when the graphite monochromater is set at CrKβ and γFe(311) is analyzed. (2) By using the monochromator method, the X-ray intensity decreases to 1/4∼1/7 compared with the case without it, but the S/N ratio improves from 0.1 to 4. (3) The monochromator method is particularly useful for the X-ray stress measurement of SUS 304 rolled materials and welded materials which shows wider half value breadth than 2° or texture.
The X-ray elastic constants of 18Cr-8Ni austenitic stainless steel were determined accurately from γ(311) diffraction of CrKβ radiation by the ψ0 oscillation method using a X-ray diffraction stress analyzer, and the amount of scatter in the measured values was examined. The results obtained are summarized as follows: (1) The value of constant K for 18-8 stainless steel is -34.10kg/mm2/deg, and the error in stress measurement, the error of K-value and the range of scatter of K-value are ±0.86kg/mm2, ±1.7kg/mm2/deg and 1.3kg/mm2/deg, respectively, if the limit of confidence is set at 95%. This K-value is considered appropriate in practical use for fine grain steels which have not been subjected to severe plastic deformation. (2) Even if the error in stress measurement Δσ is small, the amount of scatter in the measured values increases depending upon the conditions of diffraction peak intensity, grain size, texture, working history and the surface properties of test specimen. It is, therefore, necessary to evaluate the X-ray stress value by taking these conditions into consideration.
The effects of quenched and tempered microstructures on the diffusion coefficient and solubility of hydrogen in structural low-alloyed high strength steels have been investigated at room temperature by means of the electrochemical permeation technique. The results obtained are as follows: (1) The as-quenched martensitic structure gives a low diffusion coefficient, and the diffusion coefficient first increases with tempering temperature up to 200-300°C. Then the diffusion coefficient decreases until it reaches a minimum at about 450-550°C, after which it increases again at temperatures of 600°C and above. The mixed structure of bainite, martensite and ferrite obtained by furnace cooling of HT80 and HY130 steels above Acl gives a very low diffusion coefficient. (2) The solubility of hydrogen changes inversely with the diffusion coefficient. (3) The variation in diffusion coefficient or solubility can be interpreted in terms of the hydrogen trapping process at the lattice imperfections introduced by martensitic and/or bainitic transformation, such as dislocations, faults, subgrain boundaries and lattice vacancies, etc., or at the interface between the ferrite and carbide precipitates, depending upon the tempering temperature.
Static tensile tests and cantilever bending delayed fracture tests were performed on Cr-Mo structural steel V notched specimens which had been dipped into 5% H2SO4 solution to introduce hydrogen beforehand and tempered at the range from 250°C to 600°C. The results obtained are as follows. (1) The materials tempered at lower temperatures or having higher strength levels show a noticeable decrease in notch strength by hydrogen charge. (2) There is a good correlation between the delayed fracture limit and the notch strength after hydrogen charge. (3) The delayed fracture sensitivity varies remarkably with tempering temperature from 350°C to 500°C, corresponding to 120∼160kg/mm2 in tensile strength, 320∼480 in Vicker's hardness, and 2.6∼4.2 degree in half value breadth. (4) Hydrogen sensitivity is affected by the residual stress near the surface caused by heat treatment and it increases with tensile residual stress. (5) The microfracture mode changes depending upon the magnitude of delayed fracture sensitivity.
Crystallographic deformation near the fatigue crack tip in annealed and cold-rolled aluminium was observed by the X-ray microbeam technique. The crack propagation rate is expressed by a power function of the stress intensity factor. However, the exponent and the coefficient vary with the materials and thus the propagation rate of fatigue crack cannot be uniquely defined by the stress intensity factor. On the other hand, according to the observation of microstructure, the recovery of substructure introduced by cold-rolling is not homogeneous in a grain at the crack tip in the heavily cold-rolled material. In this case, if attention is focused only on the microstructure just ahead of the crack tip, the same relation holds for all materials between the crack propagation rate and the subgrain size or the excess dislocation density in a grain at the crack tip. Therefore the propagation of fatigue crack can be said to be ruled by the substructure ahead of the crack tip.
The residual stress measurements by X-ray technique were performed on the fatigue fracture surfaces of heat treated high strength steels. It is known that a plastic deformation area is formed at the tip of fatigue crack. The residual stress at the fatigue fracture surface is due to plastic deformation accompanying with crack propagation. Therefore, it is expected that the residual stress at the fracture surface is associated with the mechanical properties of a material and the applied stress condition. The following results were obtained. (1) It is possible to measure the residual stress at the fracture surface with surface roughness under 20μm. (2) The residual stress at the fatigue fracture surface is related to K-value, but in the final fracture or static fracture surface the residual stress is nearly constant independently on K-value. (3) The residual stress can be expressed as a function of the maximum elastic stress intensity factor Kmax. (4) In the fracture analysis, the residual stress measurement is an effective method to know the fracture mode and the applied stress condition especially for high strength steels.
A new method for measuring stress at local area of about 100μ in an diameter by the X-ray microbeam diffraction technique was proposed. The method can be regarded as an improvement of the single incident angle method and uses information on lattice strain along the whole Debye ring in order to determine the stress value. The accuracy of the stress value measured by this method was found to be nearly the same as that of the sin2ψ method. As an example of the application of the method to fatigue crack growth problems, the residual stress field near the fatigue crack tip was measured after the application of several cycles of overloads. The compressive residual stress became higher as the number of overload cycle increased. The higher compressive residual stress may be responsible for the greater amount of growth retardation of a fatigue crack due to increasing number of overload cycles.
This paper describes the results of observation of dislocation structures near the cracks having the known stress intensity factor ranges (ΔK) in fatigued bulk specimens of iron. The ΔK-values were between 80 and 140kg/mm3/2, which correspond to the crack growth rate from 0.10 to 0.08μ/cycle. A sort of cell structure was observed near the crack tips. Although the average size of the cells decreased with increasing the value of ΔK, the general feature of the cell structure did not seem to change much in spite of the difference in ΔK-value; it was characterized by the formation of highly dense dislocations in the cells just ahead of the tips and a narrow band composed of cells elongated along the direction of crack growth in front of the crack. In the vicinity of the crack sides, a clear cell structure was observed and the interior of the cells has a relatively low dislocation density. These results suggest that some of dislocations ahead of the crack tip disappear when the crack tip has passed, and, therefore, some extent of strain relaxation takes place at that moment. Some cracks were found which propagated along a cell-or grain-boundary, or penetrated into a cell. It appeared that the route of fatigue cracks in iron was not always along cell-boundaries. This work may be considered as a follow-up of the previous investigation on fatigue structures around the cracks, in which the ΔK-values were difficult to obtain.
Push-pull and plane bending fatigue tests in the high cycle fatigue region were carried out on annealed polycrystalline pure aluminum. The relationship between fatigue-induced dislocation structures and macroscopic hardening (unidirectional and cyclic) was investigated in the whole process of fatigue before the initiation of macro-crack. The stress amplitude σa and the plastic strain range Δεp tended to saturate at the early stage of fatigue in the both strain control- and stress control-tests. On the other hand, the Vickers hardness HV, the dislocation bundle width BW and the dislocation density ρ increased gradually until the initiation of macro-crack. Such behaviors of σa and Δεp can be explained from the dislocation shuttling model for the mobile dislocations, and the behaviors of HV, BW and ρ from the increase of the immobile dislocations. Furthermore, it is shown that the relationship between HV and ρ is expressed as HV-HVO=C3(ρ-ρ0)h where HVO and ρ0 are the values of HV and ρ at the annealed state, respectively, and C3 and h are the experimental constants.
The influence of the sub-structure developed during the recrystallization heat treatment on tensile properties was investigated on high-purity aluminum (99.99%) having the sub-structure of various sizes. These sub-structures were introduced by adopting different annealing temperatures and times suitable to the given prestrain. The results obtained are summarized as follows; (1) The flow stress σf is related to the sub-grain size t by the Petch's equation σf=σ0+KSt-1/2 where the value frictional stress σ0 becomes negative when the amount of re-strain is small, such as 0.2%. (2) The flow stress σf is related to the product of the square root of misorientation α and the inverse square root of sub-grain size t by a linear equation σf=σ0+KMα1/2t-1/2 which can be derived from the dislocation forest theory. (3) The flow stress σf increases with the excess dislocation density Db. This relation is expressed by a linear equation σf=σ0+α'GbDb1/2 which is of the same form as that introduced from the work-hardening theory. (4) The tensile strength is expressed by a linear function of the inverse square root of the sub-grain size t.
Most industrial materials are usually polycrystalline and thus inherit the preferential crystallographic alignment of individual crystals, or preferred orientation and texture, from workings and heat treatments. If materials have such a texture, their physical and mechanical properties are affected not only by the anisotropy of single-crystal properties, but also by the interactions of each grain with the neighboring grains and grain boundaries. So, these anisotropic materials provide effective informations concerning the mechanism of deformation. In the present paper, the mechanism of elastic deformation of anisotropic materials has been studied by using hot rolled steel plates having such anisotropic texture that would show more notable behaviour of deformation than in isotropic ones. The specimens were stressed stepwise in parallel as well as perpendicular to the rolling direction by using a tensile testing machine. At each stage of the stress application, X-ray beams were radiated to the center of specimen surface, and the X-ray elastic constants εψ/σ on the (211) and (310) diffraction planes were measured. The behaviour of elastic deformation was discussed by comparing the experimental results with the analytical ones, and the effect of texture on εψ/σ was explained by this theoretical analysis of the results of (211) diffraction plane. On the other hand, from the results of (310) diffraction plane, it was concluded that the actual behaviour of elastic deformation of the anisotropic material used in this study fits comparatively closely to the uniform local stress model rather than to the uniform local strain model. And the disagreement between the uniform local stress model and the actual one remains less than the experimental errors.
Push-pull fatigue tests were carried out on two kinds of specimens of 0.33%C carbon steel subjected to a static high tensile load prior to stress cycling. The main objectives were to clarify the behaviour of cyclic plastic strain and to compare the results of the push-pull fatigue tests with those of the rotating bending fatigue tests reported previously. The test results were discussed on the basis of plastic strain change, hardness change, residual stress change and surface observations during fatigue tests. The conclusions obtained are summarized as follows: (1) The lower yield point pre-load reduces the push-pull fatigue strength. However, when the pre-load is above the yield point, the fatigue strength becomes higher. This increase of fatigue strength is most pronounced in the case of pre-load close to the tensile strength. (2) The plastic strain range of the specimen preloaded above the yield point increases gradually up to its failure during stress cycling. On the other hand the specimen preloaded up to the lower yield point shows a similar cyclic plastic strain behaviour to that of the unprestrained specimen. (3) The cyclic plastic strain becomes smaller with increasing pre-load. This shows that the work hardening caused by pre-load restrains the plastic deformation during stress cycling. (4) Independently of the magnitudes of pre-loads, there is a good relation between the mean value of plastic strain range 2Δεp and the number of cycles to failure Nf, as given by the following equation: 2Δεp·Nf0.264=0.013 (5) Though the static slip lines generated by pre-loads are subjected to stress concentration during stress cycling, they do not grow to a main crack for failure.
The application of spheroidal graphite cast iron to heavy duty parts has increased recently, and many researches have been done on the fatigue strength of that material. The reason why cast iron has weaker fatigue strength than steel has been considered that graphite in cast iron acts as internal notches and causes stress concentration. For this reason, cast iron is considered to be hardly influenced by external notches. However, beside this action, the reduction of effective sectional area can be also considered as another great action of graphite in cast iron. The authors made various kinds of cast iron containing different graphite shapes and investigated the influence of graphite shape on the fatigue strength of cast iron, especially from the stand-points of the external notch effect, the notch effect of graphite, and the effect of the reduction of effective sectional area caused by graphite. The results obtained are as follows: (1) The fatigue strength of cast iron increases with the increase of the degree of graphite spheroidization. This is related closely to the effective sectional area of cast iron. (2) An external notch makes the static strength of cast iron increase, but it makes the fatigue strength decrease largely in case of a cast iron with a degree of graphite spheroidization of over 40%. (3) The internal defect factor in fatigue due to graphite (βgd) can be divided into the notch factor of graphite (βgg) and the reduction of effective sectional area (Aef), and they can be expressed by following equation. βgd=σs/σc=βgg·1/Aef where σc is the plain fatigue strength of a cast iron, and σs is the plain fatigue strength of the matrix of the cast iron. (4) In case of a cast iron with a low degree of graphite spheroidization, the internal defect factor is large and the external notch factor is small, but in case of a cast iron with a degree of graphite spheroidization of over 40%, the external notch factor becomes larger than the internal defect factor. (5) The internal defect factor of a cast iron is dependent upon the reduction of effective sectional area mostly. The influence of the notch effect of graphite is far smaller than what has been considered before. This influence can be recognized slightly in case of a flake graphite cast iron, but hardly in case of a spheroidal graphite cast iron.
In order to clarify the mechanism of the molecular chain scission of polycarbonate (PC), the pelletted and the flaky PC were decomposed by sulfuric acid, ball mill, and vibration ball mill. The degree of scission was detemined by measuring the average molecular weight of PC at various stages of chain scission. In the case of the scission of PC by sulfuric acid, the reaction rate depends on the rate of diffusion of sulfuric acid aqueous solution into PC. The degradation of the pelletted PC is appreciably slow compared with that of the flaky PC. In this case, the degradation occurs at the surface of PC with the release of bisphenol A. On the other hand, the degradation of the flaky PC takes place rapidly with an increase of concentration of sulfuric acid and with an increase of reaction temperature. It was found that the degradation of PC proceeds with the random scission of weak bonds and normal ester bonds. In addition, the grinding of pelletted and flaky PC was also carried out by a ball mill or a vibration ball mill. It was found that the random scission of PC occurs at the early stage of grinding.
Frictional behaviors of Nylons having different chemical structures were followed by observing the change of frictional coefficient in the glassy, transition and melt-flow regions of the system. In the glassy region, a constant value of frictional coefficient was observed in spite of the variation in contact pressure. Only shearing fracture was observed in the region. A good correlation was observed between the frictional coefficient and shearing modulus of elasticity or hardness of Nylons. In the transition region, the maximum value of frictional coefficient was observed at Tg and Tm. Stick and slip behavior was also observed in the region. A good correlation was observed between the maximum values of frictional coefficient and damping. Orientation of Nylon layer was also observed in the region.