High-Mn non-magnetic steel is anticipated as a new structural material which sustains the development of future industry, e.g., an atomic power generator, linear motor car, et al., but the fatigue strength characteristics of this sort of the material has not been clarified yet sufficiently. So, the fatigue strength of a newly developed 18Mn-5Cr non-magnetic steel, one of the high Mn non-magnetic steel, was investigated under the push-pull full reversed fatigue load condition. The material used in this experiment was especially developed as the material for the nonmagnetic retaining ring of a power generator, and was machine-finished to the ring-like shape after the cold working-stress relief annealing treatment done to obtain the high static strength. Therefore, to clarify the whole fatigue strength feature, a series of the experiments were conducted by using the material before and after the cold working. At first, the fatigue strength of the material before cold working was studied by using the two sorts of the plain specimens: one was the specimen of 4mm in diameter and another was of 5mm. Here, it was observed that the S-N relation of this material well resembled to that of Ni-Cr austenitic steel and that the size effect on the fatigue strength appeared despite that there existed only a slight difference in diameter of 1mm. Secondly, from the comparison of the fatigue strength between the materials before and after the cold working, it was clarified that the fatigue strength of thus treated material was much higher than that of the material before cold working, but that the fatigue limit of the cold worked material disappeared in the range of the experimental number of stress cycles in this study. Additionally, the impact fatigue strength of the cold worked material was also researched. It seems to be possible to give a reasonable explanation for the fatigue strength behavior of 18 Mn-5Cr non-magnetic steel revealed in this study, when the knowledge on the fatigue behavior of ordinary Ni-Cr austenitic steel and that of the prestrained steel are refered to.
Axial fatigue tests were carried out on tufftrided and non-tufftrided S15CK steel specimens. The main results obtained are summarized as follows. (1) Tufftriding of steel specimens had obviously an effect of improving their fatigue strength under axial load, but the effect was not so pronounced as in the case of rotating bending fatigue tests which had been reported by many researchers. (2) Large notch effect was observed on the tufftrided small size specimens having a diameter of 5mm, but for the large size specimens with a diameter of 25mm, notch effect was rather moderate. (3) Among the tufftrided specimens, the plain specimens showed larger size effect than the notched specimens. And, among the notched specimens, the non-tufftrided specimens showed larger size effect than the tufftrided ones.
In this paper, the derivation of the equations of temperature distribution and stress intensity factors in a material was shown based on the correlative equations of maximum thermal stresses and Biot's numbers and the equations of thermal conditions at abrupt cooling or heating. Then, the case that the material was subjected to cyclic thermal stresses was examined and the correlative equation between the time exposed to thermal stress σ(=0.9σmax, σmax=maximum thermal stress) and Boit's numbers was obtained. It was shown that the cyclic lives of the materials could be easily evaluated by use of these equations. It is considered that the present method is widely applicable to estimate the possibility of thermal fracture of a material at abrupt cooling or heating.
The effects of quenched and tempered microstructures on the diffusion coefficient and solubility of hydrogen in high tensile strength bolt steel have been investigated by means of the electrochemical permeation technique. The results obtained are as follows: (1) The diffusion coefficient was minimum when the steel had the as-quenched martensitic structure or the low temperature tempered martensite, and it increased with increasing tempering temperature. The mixed structure of ferrite and lamellar pearlite obtained by furnace cooling from a temperature above Ac3 gave a higher diffusion coefficient than the tempered spheroidal pearlite. On the other hand, the solubility of hydrogen showed a behavior opposite to the diffusion coefficient. (2) The low diffusion coefficient and high solubility are attributed predominantly to hydrogen trapping at the lattice imperfections, such as dislocations and faults etc., which were introduced by martensitic transformation during quenching. (3) With regard to the effect of catholic current density, i.e., the hydrogen concentration directly below the surface on hydrogen diffusion in tempered steel, the diffusion coefficient hardly depended upon the hydrogen concentration but the solubility of hydrogen increased up to a certain value with increasing catholic current density. On the other hand, the hydrogen permeation current efficiency decreased with increasing cathodic current density. (4) The activation energies for hydrogen diffusion in the tempered martensite, tempered fine and/or spheroidal pearlites in the temperature range of 6 to 61°C, were almost constant (about 8.1 kcal/mol) but the pre-exponential factor increased with tempering temperature. The heats of solution of hydrogen in the tempered martensite and tempered pearlites were almost the same (about-10.3kcal/mol), indicating the exothermic reaction. However, the pre-exponential factor decreased with increaing tempering temperature.
Inhomogeneity and initial residual stress are considered to be the important factors controlling the fatigue fracture of laminated inhomogeneous metals such as induction hardened steels and carburized hardened steels. It is important to discuss quantitatively the relationship between these two macroscopic factors and cyclic plastic strain connected with fatigue life, on the basis of the valid relationship between stress and strain of laminated inhomogeneous metals. The effects of inhomogeneity and initial residual stress on the nominal stress-strain curve were experimentally discussed from the tensile tests of the clad plates composed of low carbon steel and medium carbon steel. Some specimens were quenched in oil and others were successively tempered for the purpose of separating the above fundamental two factors. The analytical results of elasticity at the hardened part and plasticity at the softened part were compared with the experimental results. The problems concerning the biaxial residual stress and the hysteresis loop of laminated inhomogeneous metals were discussed analytically.
The stress of each grain in anisotropic materials having a texture under uniaxial tension was analyzed by using G.I. Taylor's model. The average lattice strain in grains subjected to the X-ray diffraction was also calculated by taking account of the stress condition and the volume fraction of grains. The results were examined experimentally by means of the X-ray diffraction method for hot rolled steel sheets. The results obtained are summarized as follows. (1) The lattice strain of anisotropic materials depended upon the tensile direction of the specimen and differed from that of the materials without a texture. (2) The calculated values of the lattice strain agreed well with the experimental results, indicating that the stress in each grain depends upon its orientation of crystal and the critical resolved shear stress in each slip system can be regarded as equal in the initial stage of the plastic deformation. (3) The values of the residual lattice strain calculated on the assumption that the stress in each grain decreased uniformly in the unloading process after the plastic deformation coincided qualitatively with the experiment.
Load relaxation experiments were performed on single crystals of NaCl (at -30°C and 100°C), KCl (at room temperature and 60°C) and MgO (at room temperature). The resulting logσ-logε curves were convex upward for NaCl (at 100°C) and KCl (at room temperature and 60°C). It was slightly convex downward for MgO (at room temperature). The curvature for NaCl (at -30°C) could not be determined owing to the experimental difficulty in keeping temperature constant for a long duration. All the logσ-logε curves for a given sample (examined for NaCl at 100°C and KCl at room temperature and 60°C) were superimposed by a translation along a straight line of slope μ. These behaviors indicate that Hart model is satisfactorily applicable to these materials. The reason why the Gupta-Li analysis was not successful for KCl at room temperature in our previous experiment was also discussed.
The through crack propagation rate in creep is known to be nearly proportional to the modified J-integral J'. By paying attention to this fact, a propagation law of small creep crack was proposed based on a dislocation multiplication model. Creep crack propagation lives were calculated by the use of this propagation law. The theoretical results obtained were in well agreement with the experimental data for type 304 stainless steel.
Studies of fatigue fracture for rigid plastics have indicated that fracture takes place by two mode. The first fails by softening accompanied with generation of heat in high stress level, while the second does by crack propagation in low stress level. The purpose of this paper was to investigate the fatigue crack propagation properties and discuss a fatigue crack growth equation in low stress level for rigid plastics. Flexural fatigue tests of the specimens with a small blind hole of 0.3mm in diameter were carried out under a constant stress amplitude at room temperature. The materials used in the test were polycarbonate (PC), polyvinyl chloride (PVC) and polymethyl methacrylate (PMMA). The main results obtained are as follows: (1) In the range that the crack length l on the surface of specimen was small, the rate of crack propagation dl/dN was nearly proportional to σnl. The value of n was 3 for PC, PVC and PMMA. (2) In the case that the crack lengths l were equal, the forms of crack surface were similar each other even though the specimens were subjected to different stress levels. (3) The relation between the crack length l and the relative number of cycles (N-Nc)/(Nf-Nc) was independent of stress levels, where Nf is the number of cycles to failure, and Nc is that at crack length l of 0.4mm.
The rotating bending high cycle and the push-pull low cycle fatigue tests were performed on the cold-worked Al-2.4% Mg alloy, and its fatigue properties were examined and compared with those of the solution-treated one reported previously in another paper. The cold-worked Al-2.4% Mg alloy did not show the cyclic softening during cyclic straining. This fact might be closely associated with the strong dynamic strain ageing property of the present material. The cyclic stress-strain relation was not influenced by the amount of the reduction given to the material. The cold-work reduced the low cycle fatigue life of the solution-treated alloy, but it did not depend on the reduction ratio, as in the cyclic stress-strain relation. On the other hand, the exponent, α, in Manson-Coffin's law of Δεp·Nfα=C, which is also applicable to the present material, was about 0.84 and 0.64 for the solution-treated and the cold-Worked Al-Mg alloy, respectively. The difference in the value of α can be interpreted by considering the deformation mode of the two materials: the slip is concentrated at and near the grain boundary in the former, while it is uniformly dispersed, independent of grain boundary, in the latter. The effect of the cold-work on the high cycle fatigue strength of the Al-2.4% Mg alloy was very small, compared with those of other metal materials. This might be due to the small inclusions contained in the material, from which fatigue crack originates.
Pump shafts are used being fitted with impellers and sleeves. Since they are often made from different metals and form crevices, galvanic corrosion or crevice corrosion occurs in a corrosive solution. Therefore, corrosion fatigue strength (CF) of these components may differ from that of bare metal specimen. In this paper, effects of galvanic couples and crevices on CF were investigated. Round bar materials tested are a 0.35%C steel (S35C) and a 13Cr stainless steel (SUS 403). Sleeves used are Cu, Al and S35C for S35C specimen and brass and SUS 403 for SUS 403 specimen. All tests were carried out at 1450 or 2770rpm in 3% NaCl solution. Test results are summarized as follows. (1) The corrosion fatigue strength σwc of a S35C at N=107 was 10kgf/mm2 (33% of that in air) and considerably decreased at higher cycles. For SUS 403 σwc was 19.6kgf/mm2 (59% of that in air) and decreased little at higher cycles. (2) The effect of sleeve materials on CF of S35C bar specimen was explained by considering the difference in corrosion potentials between the specimen and the sleeve. Namely, the highest σwc was obtained for Al sleeve (12.5kgf/mm2 at N=2×107), followed by S35C sleeve (10kgf/mm2: the same as that in bare specimen) and Cu sleeve (6.7kgf/mm2). (3) In the case of SUS 403 bar specimen, the same σwc was obtained for two different sleeve materials, while both SUS 403 and brass sleeve considerably decreased σwc because of the formation of corrosion pits at the ends of the sleeves. (10kgf/mm2 at N=5×107, 30% of that in air) (4) To clarify the effect of pit on CF, corrosion fatigue test of etch pitted round bar specimens was carried out, in which etch pits were induced by the potential method and the size of the pits was controlled to be as large as those found on fracture surface of the sleeve fitted specimens. It was shown that σwc of etch pitted specimens was roughly equal to that of the sleeve fitted specimen in the high cycle regime.
Recently the application of the friction welded butt joint to mechanical structures has come into wider use. A further increase in demand for this sort of joint is anticipated because of its utility for some engineering purposes and the technical easiness of this welding method to make the butt joint of dissimilar metallic materials. However, there exist only a few papers concerned with the fatigue strength characteristics of this type of joint, for it has been recognized empirically that the fatigue strength of the friction interface exceeds that of the base material and thus no noticable problem on this type of joint exists from the view point of fatigue strength. In the preliminary study conducted by the authors on the friction welded butt joint specimens composed of 0.23%C carbon steel, however, it was revealed that the fatigue lives of these specimens showed a considerably wide scattering in comparison with those of the base material specimens. To clarify such statistical fatigue strength characteristics of the friction welded butt joint specimens, a series of experiments were planned and conducted on the base material specimens of 0.46%C carbon steel and the friction welded butt joint specimens at several stress levels in the overstress range. Statistical discussions on the results obtained were made by accepting the Weibull distribution as the original distribution for the fatigue life. It was revealed that there existed a distinct difference in S-N relation between the base material specimen and the friction welded butt joint specimen, when the distribution of fatigue life was taken into consideration.
It was reported previously by one of the authors that steel became brittle by “Liquid Metal Embrittlement” when it was stressed in tension in molten zinc. Cracks formed in a welded steel structure in molten zinc grow mainly in the region adjacent to the welded jotnt. Therefore, it is necessary to study the characteristics of the welded joints in molten zinc to prevent the initiation of cracks. In this study, tensile and bending tests were carried out in molten zinc using both SM50A and STK55 steel welded joints and the results were compared with those of the base metals. The welding methods used were both manual and semi-automatic CO2 arc welding. The types of joints used were butt welded and fillet welded ones. Fractographic features of the ruptured surface of the welded joints in molten zinc were examined. The following conclusions were obtained: (1) The welded joints became brittle by tensile stress and elongations were reduced in the same way as the base metal in molten zinc. The tensile strength of the butt welded joint was also reduced to 75-78% and that of the cruciform fillet welded joint became 57-68% compared with those at room temperature in air, respectively. (2) The tensile strength of the cruciform fillet welded joint in molten zinc was reduced by 7-16% compared with that of the butt welded joint because of stress concentration at the weld bead. (3) The bending angle, at which cracks initiate, for the butt welded joint without reinforcement of weld, was smaller than that of the base metal in molten zinc. (4) The intergranular facet patterns were observed in the crack initiating position of both the weld metal and the bonding portion fractured in molten zinc, as in the case of the base metal.
It is well known that thermal stresses occur in steel structural members by rapid heat transfer from molten zinc, when they are hot dip zinc galvanized, and the residual deformation remains in them. If they have notches, the stress concentration can be induced. On the other hand, it was previously reported by the author that steel was markedly embrittled when coated with molten zinc and then deformed in tension. In this study, from the point of view of fracture mechanics, fracture toughness tests were carried out in molten zinc and the nominal fracture toughness Ki of SM50A steel was obtained using the notched 3 point bend specimens. By analyzing the nominal stress intensity factor at the slit tip of the notched rectangular SM50A steel plate immersed in molten zinc, the immersing speed of crack initiation was obtained. The result of the immersion tests on the notched rectangular plate coincided well with the above analysis. The results obtained are as follows: (1) The nominal fracture toughness Ki in molten zinc of 16mm thick SM50A steel plate with 0.3mm notch width was 100kgf/mm3/2. (2) Even if there exists the stress just below the crack initiating level in molten zinc, the delayed fracture hardly occurred within 10min. (3) The maximum nominal stress intensity factor at the slit tip of the notched rectangular steel plate decreased with increasing immersing speed.
A study has been conducted to calculate theoretically the impact fracture strength of Japanese paper on the basis of the orientation of its constituent fiber. The impact strength of paper is known to be defined as the energy which can be aborbed in the paper until its rupture during impact loading. The theoretical analyses have shown that the impact strength of Japanese paper is dependent upon the cut direction of specimen sheet; the magnitude of impact strength is the greatest in the cross direction of paper which is normal to the machine direction and decreases gradually with deviating from the cross direction of paper, while the minimum is seen in the range of the cut direction of 50 to 60 degrees. The theoretical predictions were supported by the results of experimental measurements of Haku-Uchi Gami (Japanese paper).
Recently, the adhesive bonding technique has become an important problem as FRP is now being applied to large structural constructions. In the past, the studies of structural adhesive bonding were usually made on the adhesive lap joints of aluminum adherents, and the fracture behaviors of these adhesive joints were simply followed by identifying the fracture type as the cohesion fracture of adhesive layers or the interfacial fracture. However, FRP lap joints usually show very complicated fracture. So, in this report, it was attempted to investigate the fracture behaviors and the configuration of the FRP adhesive lap joints. The correlation between deformation and fracture of the lap joints was examined based on the experimental results. The results showed that the deformation of the lap joint was directly related to the joint shapes (lap length and scarf angle). Furthermore, the fracture behaviors of FRP adhesive joints were discussed and clarified.