Two-dimensional stress waves in an elastic/viscoplastic circular cylinder were analyzed numerically, when axisymmetric impacts with various impact velocities were applied to the end face of the cylinder. For the numerical analysis, the finite difference method along bicharacteristics was employed. By obtaining the numerical results, it is demonstrated that, near the impact face, the dynamic stress-strain relation differs clearly with each point even if on a same cross section of the cylinder. Conversely, the bounds of the area near the impact face were obtained at which the dynamic behavior becomes practically uniform. It is also shown that local stress concentrating parts are apt to be formed in the cylinder where a tensil stress arises.
In the present study, a new method for measuring the time-variation of stress at the impact end in the elastic-plastic collision of a body against another body was developed and the duration of impact was discussed. Firstly, the simple method for measuring the duration of impact, called the sensing cylinder method, was proposed. The sensing cylinder was confirmed by calibration tests to be effective for measuring not only the duration of impact but also the impulsive force generated at the impact end of a body. Secondly, as an application of the measuring method, plastic impact tests were made on pure copper and aluminum specimens with several different lengths. The time-variation of stress at the impact end of the bars colliding with the sensing cylinder and the duration of impact were measured under various impact velocities. The experimental results were compared with the theoretical ones based on the strain-rate dependent theory of plastic wave propagation. Close agreement between the observed and the calculated values was obtained.
Behavior of a slightly curved bar under longitudinal impact with a heavy rigid hammer at a low velocity was analyzed numerically, and the residual deformation of the bar was investigated. Dynamic elasto-plastic deformation was discussed for the bar subjected to the plastic strain of the same order as the elastic one. Because of difficulty in obtaining a closed form of solution for this problem due to geometric and material nonlinearity, the solid bar was replaced by a lumped-parameter model, and the equations of motion were integrated numerically. The stress at each point on the cross section was evaluated by taking account of a detailed deformation history of that point. The bending moment was calculated by integrating these stresses. This method has been confirmed to give more accurate results than using the conventional method for analyzing the behavior of bars of small slenderness ratios, in which the stress distribution over the cross section is highly nonlinear. The residual deformation of the bar was evaluated by the residual axial displacement of the struck end. The effects of the following two factors on the residual axial displacement were investigated; the ratio of the mass of the rigid hammer to that of the bar, and the impact velocity of the rigid hammer. As the result, it was shown that the residual axial displacement was expressed by the product of the power function of the mass ratio and that of the impact velocity.
In this paper, through theoretical discussions and by obtaining numerical examples, the existence of a critical impact velocity is proposed when the end of a thin-walled tube is twisted suddenly, provided that the condition is the same to the case when simple waves propagate as the Kármán-Duwez solution. Its existence is also suggested even when a combined tension-torsion impact is applied to the end of a tube, and also when an incremental combined tension-torsion impact is applied to the end of the tube on which a combined loading is applied statically.
A preparation method of a complex type RAl2 magnetic system (where R denotes rare earth ions) for Ericsson type magnetic refrigerator is investigated. Previously, the authors reported on the model complex magnetic refrigerant with layer structure and concluded that this complex magnetic refrigerant shows a temperature independent magnetic entropy change ΔSM in particular temperature range, which is suitable for the Ericsson cycle, satisfying the Carnot principle. In this paper, the preparation of a complex type RAl2 magnetic mixture by a shock compression method, which is a high density mixture of its components in grain size scale, is described and its thermal and magnetic properties are discussed in respect to application to the Ericsson type magnetic refrigerant.
Slow strain rate tests (SSRT) were carried out in synthetic sea water on three kinds of high tensile strength steels (HT60, HT80 and SNCM439) being used for offshore structures. In this study, their mechanical properties were measured and the specimen surface and fractured surface were examined after the tests. Also the parameters characterizing the test results were examined. Among various parameters, the reduction in area (RA) and the true fracture stress (σf) were found to vary clearly depending upon the aggresiveness of testing environment and the kind of steels. They were lower in sea water at the cathodic protection potential (-0.80V vs. Ag/AgCl) than in sea water without it (Ecorr), and the trend was more remarkable for HT80 and SNCM439 steels which had higher tensile strength than HT60 steel. Another parameter, the maximum crack length, Δa, obtained from the quasi-cleavage fracture surface, showed the same clear difference depending upon testing environment and the kind of steels. The maximum crack length, Δa, of SNCM439 steel was 500μm at the cathodic protection potential (-0.80V vs. Ag/AgCl) in sea water. It can be concluded that the maximum crack length, Δa, is related to the susceptibility of stress corrosion cracking and is a good parameter for SCC evaluation by means of SSRT.
Hydrogen permeation experiments were performed continuously over 45 hours on iron and steels (0.007-0.19%C) by an electrochemical method (nickel-plating method). The hydrogen was charged cathodically with a constant current density, 40A/m2, in two buffer solutions. The blisters were observed optically and electron scanning microscopically. The hydrogen diffusivity (D) and hydrogen content (C0) were kept constant during hydrogen permeation when an acetic buffer solution was used. However, D decreased and C0 increased with permeation time, when a boric buffer solution was used. In the latter, many blisters were formed in the specimens during the permeation, but there was no blister in the former because C0 was less than 0.1ppm. Blisters were formed at nonmetallic inclusions, Al2O3. These hydrogen permeation behaviors are discussed by the hydrogen trap effect at nonmetallic inclusions and at blisters around nonmetallic inclusions.
Electrochemical study is necessary to know the hydrogen distribution near a crack tip in hydrogen embrittlement type stress corrosion cracking (HESCC). In this study, hydrogen absorption at a slit tip in high tensile strength steels was analized by an electric field equation and the hydrogen distribution was computed by the two dimensional finite differential method based on this boundary condition. As the result, the criterion for HESCC initiation was found to be described by the hydrogen activity and hydrostatic stress parameter. It was made clear that HESCC initiation was able to be predicted by using the criterion.
Heat Checking is a very important problem limiting the service life of aluminum die casting dies. In this paper, the relationship between the surface residual stress, which varies with die casting operation, and the initiation and propagation of heat checking is described. Measurement of residual stress and observation of heat checking were carried out on a cover and an ejector production die made by SKD61 (AISI H13). The surface residual stress was measured by X-ray and the heat checking was observed in detail by SEM after every prescribed shots from the beginning to the end of the die service life. The mechanism of residual stress variation and the effects of the position and the cavity shape of dies on the residual stress variation and the initiation and propagation of heat checking were clarified. The following conclusions were drawn from this investigation. (1) The surface residual stress, which was compressive before die casting operation, changed immediately toward tensile side and saturated in the early stage of operation, say at only 50 shots. This fact is considered to result from the following process; the surface layer of dies is rapidly heated by molten aluminum. The thermal expansion due to this rapid heating is restrainted by the substratum and thus the surface layer is subjected plastically to compressive strain. After cooling, the surface residual stress changes toward tensile side and saturates at some tensile value in the early stage of die casting operation. (2) The value of saturated tensile residual stress depended on the position and the shape of die cavity. This experimental result is caused by the magnitude of thermal load and restraint. (3) On the gate side of die cavity, in which the saturated tensile residual stress is large because of large thermal load, microcracks initiated in the saturated period. But on the position apart from the gate, in which the saturated tensile residual stress is small because of small thermal load, the residual stress did not decrease and microcracks did not initiate even at the end of die service life. (4) In the later stage of crack propagation on the gate side of die cavity, heat checking was observed by the naked eye and the residual stress decreased to zero.
The effects of Co content and WC grain size on the fatigue strength of WC-Co hardmetals have been investigated. Alternating tension-compression (stress ratio R=-1) fatigue tests and compression fatigue tests (R=-∞) were carried out on WC-Co hardmetals whose Co contents are 6%, 12% and 20%, and their WC grain size ranges from 1.1μm to 3.0μm. For tension-compression fatigue tests (R=-1), all the materials tested have shown about the same values of fatigue limit, regardless of Co content and WC grain size. In the region of finite life, WC-12%Co (WC grain size 1.4μm), which is the material with medium Co content, has shown the minimum life and WC-6%Co, which is the material with smallest Co content, has shown the longest life. It has been confirmed that the minimum life of the material with 12%Co (1.4μm) is due to the largest size of the initiation crack zone formed by the cleavage of hcp Co and WC grains. The residual tensile strength of the fatigued specimens under cyclic compression has been investigated as a function of the number of load cycles. The results have shown that the materials with lower Co content (or smaller Co mean free path) show higher residual tensile strength. Though, in the material with 20%Co, fracture by tensile loading was initiated from the shear mode cracks formed during the cyclic compression loading, in all other materials tested, fracture by tensile loading was initiated from the grooves formed by the dropping of the surface layer during the cyclic compression loading, in the same way as in the material with 12%Co 1.4μm WC grain which was reported in the previous reports.
Aluminum alloy (A2017-T3) compact tension specimens were tested under repeated two-step load conditions, and the crack growth rate and crack closure behavior were observed. The higher and lower stress levels, KH and KL, were selected in the region II of the crack growth rate curve of the material obtained by ΔK increasing tests. It was found that the crack growth rate da/dN and crack opening stress intensity Kop were affected by the stress cycle ratio KL/NH; namely, da/dN was decreased and Kop was raised by the increase of NL/NH. The estimation of the crack growth rate for lower stress level KL by the linear cummulative law shows that the estimated relation of (da/dN)L versus (Keff)L was in better agreement with the fundamental crack growth rate curve when the crack closure only in the repetition of KL was taken into account, than when the closure at both stress levels of KH and KL were considered.
Plane bending fatigue tests of steel sheet specimens corroded with salt-water were carried out in order to examine the relation between the feature of the corroded surface and the fatigue strength of the corroded specimens. The material used was quenched and tempered spring steel sheet SUP6 (Hv470). The main factors affecting the degradation of fatigue strength due to corrosion were the reduction of sectional area and the increase in surface roughness, as is the case of 0.075%C and 0.15%C hot-rolled steel sheets. The following equation for estimation of the bending fatigue strength σwap of the corroded specimens, proposed in our previous report, has been ascertained to hold also for a high strength steel such as spring steel SUP 6. σwap=(Zc/Z0)·(1-ηΔdmax0.4)·σwo Where Z is the section modulus, η=0.45 for 0.075%C steel, 0.65 for 0.15%C steel and 1.1 for SUP 6 (Hv470), and Δdmax is the maximum depth of roughness (mm). The value of η increased as the hardness of steel sheet became higher, and was well related to the notch sensitivity of the material.
Bending fatigue tests were made for VC coated steels which were coated by immersing in a molten borax bath and for hardened steels which were quenched and tempered, in order to clarify the effect of VC coating. The results obtained are as follows; (1) The endurance limit of VC coated steels was a little lower than that of hardened steels. It is considered to be mainly due to the decrease of hardness in the substrates. Accordingly, the endurance limit recovered almost to the level of hardened steels by an additional diffusion treatment. (2) The initiation point of fatigue fracture of VC coated steel in reversed bending was on the substrate just under the VC layer. Hence, the endurance limit is correlated to the hardness of this part. (3) However, there is a considerable scatter in this relationship and the endurance limit of VC coated steels was a little lower than that of hardened steels with equal hardness. These results suggest that the fatigue strength of VC coated steels is determined not only by the hardness but also by other factors. For example, the residual tensile stress in the substrate just under the VC layer is one of the factors besides hardness.
Fatigue tests were conducted on the moulded joints of aluminum and an epoxide adhesive material, and the debond growth behavior at its moulded interface pretreated in two different ways was observed. As the basic data, the fatigue crack propagation characteristics of the adhesive itself were experimentally obtained. The debond growth behavior at the interface by fatigue was predicted under the following conditions; (1) The stress intensity factor of an interface crack is approximately equal to that of a crack in homogenious material as derived from the no-slip model proposed by Mak et al.1). (2) The debond growth rate per cycle is equal to the crack propagation rate of the adhesive material. (3) The residual stress on the moulded interface estimated by a photoelastic experiment and FEM analysis is taken into account as an influential factor for the stress ratio of fatigue load. Fairly good agreement between the experimental and analytical results was found for the interface pretreated with sand-blast and chromic-acid. However the debond growth rate for the interface with no chromic-acid pretreatment was faster nearly by a factor of one order of magnitude than the prediction. Microscopic analyses of the debonded interfaces were conducted with SEM and X-ray microanalyzer and the debond growth mechanism was discussed.
An investigation has been carried out concerning the effect of water absorption on the tensile and fatigue properties of a Technora (HM50, Teijin)/epoxy and a Kevlar49 (Dupont)/epoxy composites. The effect of water on the fracture mechanism was observed using AE measurements and SEM microscopy. The weight gain for the HM50 specimens immersed in distilled water at 70°C levels off at about 2% after two months. For the Kevlar49 this is about 3.5%. Moisture does not appear to have any influence on the tensile strength. For the HM50 the elastic moduli of the dry and wet specimens are the same at the start. After a certain point the dry specimen stiffens, whereas the wet one first yields, and then stiffens. The Kevlar49 specimens do not show any yielding behavior unlike the HM50. For both composites the specimens in water exhibit a longer fatigue life than in air, though the Kevlar49 has a slightly bigger increase in life than the HM50. In water the AE signal total is much lower than in air. The HM50 composite has a lot of matrix attached to the fiber after failure, while the Kevlar49 shows a very clean pullout of the fiber. The fracture of the fiber itself is also quite different in the two types of composites, and the HM50 fiber shows a more ductile behavior than the Kevlar49 fiber.
The fatigue fracture mechanism of class A-SMC which has been developed for the outer panel in automobiles was studied by determining the parameters of acoustic emission (AE) detected during its fatigue fracture process. The results obtained are as follows: (1) The macroscopic fatigue fracture behavior is accurately reflected by the AE event count rate and the total AE energy, which are the most proper AE parameters to represent the macroscopic fracture pattern of composite materials. (2) It is confirmed that the fatigue fracture mechanism can not be explained by the AE amplitude, even if by using the results of the macroscopic and microscopic observations of fatigue fracture surface. (3) The microscopic fracture mechanism during the fatigue test can be clarified by observing the fracture surface microscopically and applying the relation among the AE frequency, the load and the power spectrum obtained on the static tensile test to the relation among the AE frequency, the number of cycle and the power spectrum obtained in the tensile fatigue test. It is found that the most proper AE parameter to represent the fatigue fracture behavior of constituent elements in the composite materials is the AE frequency spectrum.
Since the anisotropy of ultrasonic SH wave velocities in plates was shown available to the nondestructive measurement of residual stress, much attention has been paid to the direction-dependence of propagation characteristics of ultrasonic waves. In this paper, it is intended to extend the acoustoelastic relation in slightly orthotropic materials to an arbitrary propagation direction of plane elastic waves. At first, the perturbation method was applied to the eigen value problem of the general acoustical tensor, and the formal solutions were obtained within the first-order of stress and the second-order of elastic anisotropy. Then these solutions were applied to the case where the effects of stress and anisotropy are of equal degree, and the velocities and polarization directions were explicitly expressed within the first-order of stress and anisotropy as the functions of the angles θ and φ specifying the propagation direction with respect to the principal axes of orthotropy. From these expressions, which are approximate but practically sufficient, the results previously obtained by Iwashimizu et al., King et al., and Thompson et al. were rederived by specializing the propagation direction. Finally, the simplification of the general relations was considered by imposing those restrictions on the anisotropic parameters, which are suggested by the theoretical analysis of macroscopic elastic constants of textured polycrystalline aggregates.
The present research is concerned about how different the W/C ratio and the pore volume are between the upper paste and the lower paste of coarse aggregate in concrete. The value of color difference in hardened concrete was measured with a color difference meter. Each specimen contained a certain amount of red ink. Since the value of color difference in hardened concrete was proportional to the quantity of water with red ink in it, it was confirmed that the value of color difference can be used as an index which shows how much of water transfers as the concrete hardens. Firstly, the results showed that the W/C ratio increased partially near the lower surface of coarse aggregate in concrete. Secondly, the comparison of the volume of hardened paste near the upper surface of coarse aggregate with that near the lower surface by using a porosimeter showed that the total pore volume of hardened paste was larger near the lower surface of coarse aggregate than near the upper surface.