In order to investigate the influence of residual stress due to surface-treatments on the hydrogen sensitivity and delayed fracture strength, static bending tests and cantilever bending delayed fracture tests were performed on Cr-Mo high strength steel. The methods of paper polish, sandblast and shotblast were adopted in this experiments and the results of these specimens were compared with those without surface-treatment. The residual stress in the surface layer was measured by X-ray diffraction technique. The conclusions obtained were summarized as follows; The sensitivity of hydrogen embrittlement for the blasted specimens decreased by the presence of compressive residual stress in comparison with that for the specimens without surface-treatment. It was found that there was a good correlation for the surface-treated specimens between the variation of maximum stress in static bending tests and that of residual stress corresponding to the change in hydrogen charged time. The delayed fracture life for the surface-treated materials also depended on the residual stress distribution near the surface, and the crack initiation time in delayed fracture tests was largely prolonged in the case that high compressive residual stress was created from the surface to the inner part such as the case of shotblasted specimen. Therefore, it was considered that the application of blast-treatment to the material surface was much effective for the improvement of not only the adhesion of coating but also delayed fracture strength.
Dehydration process of gypsum in waste sulfuric acid from the sulfate manufacturing process of titanium oxide was studied. When Calcium sulfate dihydrate changed into unhydrite, two different dehydration processes were observed according to the experimental conditions. In one process, dihydrate directly changed into unhydrite and in the other, hemihydrate appeared as the intermediate. By the optical microscope examination, it was observed that the crystals of both hemihydrate and unhydrite were formed through solution mechanism. When the crystal seeds of unhydrite were added, the induction period of the reaction decreased with increasing amount of the crystal seeds. In the case where the crystal seeds were fine, different kinds of dehydration process was observed.
A supersaturated Cu-26Ni-8Fe alloy was deformed by cold rolling and the changes occurring on annealing at 750°C have been follovred by hardness testing and optical and electron microscopy observation. It was found that a modulated structure formed prior to recrystallization of a supersaturated solid solution. In the case of the heavily deformed sample, the iron nickel-rich phases grow anywhere including grain boundaries or deformation bands, and the recrystallization can start in the area free of the modulated structure around the coagulated iron nickel-rich phases. Therefore, the recrystallized structure consists of fine equiaxed copper-rich and iron nickel-rich grains termed as the microduplex structure. The structure has a good strength and ductility at room temperature. The structure also has a suitable high temperature stability and so appears capable of undergoing superplastic deformation at 750°C. The strain rate sensitivity index of flow stress m of about 0.5 was obtained and the specimen maintained a relatively fine equiaxed grain structure even after large elongation at 750°C.
Push-pull fatigue tests at a constant stress amplitude were carried out on mild steel specimens which were annealed and pre-strained in tension to 1%, 5% and 10% strains. Some relationships between the dislocation structure and the plastic strain range Δεp or Vickers hardness HV were investigated during the fatigue process. (1) The dislocation structure induced previously by the prestrain εpre was recovered and rearranged with stress cycling, and became a typical cell structure by fatigue in the later stage. (2) The annealed and the 1% pre-strained specimens in which the total dislocation density ρt and HV increased during the fatigue process showed cyclic hardening. On the other hand, the 5% and the 10% pre-strained specimens in which they decreased showed cyclic softening. (3) The pre-strained specimens showed higher values of Δεp than that of the annealed one in the initial stage. But, as the amount of εpre increased the value of the Δεp became smaller and the number of cycles to the maximum of the Δεp became larger. (4) The variations of ρt for the annealed and the pre-strained specimens during the fatigue process were expressible by the same equation derived from dislocation dynamics. (5) During the fatigue process, the relationship between HV and ρt was expressed as a simple relation, HV ∝ √ρt. (6) A mean cell size Lm at a fatigue failure point did not depend on εpre, but on the stress amplitude or the number of cycles to failure.
In X-ray residual stress measurements for uni-directionally deformed surfaces such as ground, planed or rolled, a non-linearity of d-sin2ψ diagram, so called ψ-splitting (ψ-Aufspaltung), has been observed recently. It has been known that this is caused by the residual shear stress induced in the deformed layer by external forces. To gain a further understanding of the ψ-splitting, d-sin2ψ relations were measured under various combinations of the method of working, the material worked and the diffraction plane. The strain distributions were measured not only for a matrix phase but also for a carbide phase. The results obtained are summarized as follows: (1) The ψ-splitting was observed when the measurements were taken in the working direction but not in the perpendicular direction. The extent of ψ-splitting became larger as the degree of working increased. (2) The ψ-splitting was observed in pure iron as well as in steels containing carbide phase. The extent of ψ-splitting increased as the volume fraction of carbide phase increased. (3) Although the appearance of ψ-splitting did not depend on the selection of diffraction plane, the behavior of the ψ-splitting, upon changing the sign of ψ, for the matrix phase and that for the carbide phase were mutually reversed. This means that the sign of the residual shear stress in the carbide phase is opposite to that in the matrix phase. On the basis of these experimental facts, a model of dislocation arrangement in the unidirectionally deformed layer is proposed by which the appearance of ψ-splitting can be easily understood. It is concluded that the ψ-splitting is caused by a dislocation structure within the X-ray penetrating layer which is composed by cell structure and by dislocation loops around carbide particles.
The problem of a cylindrical flat-ended projectile impinging on a flat rigid anvil has been analyzed by many investigators in order to assess the dynamic stress-strain relation at high strain rates. Though impact speed in the method described above is higher than those of other tests such as Split Hopkinson Bar Method, the temperature rise caused by plastic work cannot be neglect in this method. Such temperature rise creates nonhomogeneity in the loading region and this nonhomogeneity is maintained almost constant in the unloading region. In this paper, the simulation of elasto-plastic waves propagated in a finite bar, which impinged on a flat rigid anvil, with steady temperature gradients was made by a computer. The temperature distribution was assumed to be an exponential function of the axial distance of the bar. A constitutive equation used in the analysis was a rate-dependent type and it was assumed that the static stress was a linear function of absolute temperature. The problem was formulated and solved numerically by the method of characteristics with supplement rectangular net. The numerical results show that the temperature distribution has influence upon stress and strain. Especially, it has considerable influence upon the deformation resistance and unloading wave from the free end.
The relation between microcracks and transition temperatures was examined on a spheroidized 0.7% carbon steel. The spheroidization anneal was achieved by repeated heating and cooling between 710 and 735°C after being quenched in oil. The specimens were stretched to fracture under a constant speed of crosshead at various temperatures. The four kinds of fracture regions divided by three transition temperatures were determined from the temperature dependence of tensile properties. At each transition temperature range, the fractured specimens were observed under an optical microscope and a scanning electron microscope. In the temperature range below the transition temperature TC, at which fracture stress took a maximum value below the nil-ductility transition temperature TB, microcracks were initiated at the boundary between the cementite particle and surrounding ferrite perpendicular to the direction of tension, and propagated by going around particles. In the temperature range from TC to TB, crack initiation occured at the cementite particles and its propagation occured by cleavage of the ferrites. In the temperature range from TB to the ductility transition temperature TD, microcracks initiated at ferrites by their slipping and also at cementite particles and propagated by cracking cementite particles. Many microcracks of void type were observed above TD.
This paper was intended to investigate the influence of the welded heat input on the polarization behavior and the susceptibility to stress corrosion cracking (SCC) of the welded joint of commercial SUS 316 type austenitic stainless steel in 42% boiling MgCl2 solution. The measurements of the polarization curves and the potentiostatic constant load SCC tests were carried out under the applied stress of 25kgf/mm2 on the specimens welded with various heat inputs from 14 to 46kJ/cm, and the crack distribution in the cross section of specimens was observed by an optical microscope. The pitting potential in the polarization curve of the deposited metal zone shifted remarkably toward less noble potential with the increase of heat input, while those of the base metal zone and the heat affected zone shifted toward the other way. The SCC susceptibility of the welded specimens depended remarkably on the applied potential irrespective of the amount of heat input. The susceptibility to SCC of each part of the welded joint was influenced by the heat input, and this agreed with the SCC susceptibility estimated from its polarization behavior.
Investigations were made on the fatigue properties of satin woven glass cloth FRP with special emphases on the effects of input wave type and programmed stress pattern. Fatigue tests were conducted under both the constant stress and the programmed block type variable amplitudes by a hydraulic fatigue testing machine. It is well known that a Gaussian narrow band random process has a probability density distribution of peak amplitudes that is close to Rayleigh distribution. Therefore, an amplitude-time relationship in the actual program load was determined in such a way that the envelope of peak amplitude of the sinusoidal excitation has a probability density function of Reyleigh distribution. The test results showed that the effects of input wave type (sinusoidal, triangular and pulse) and stress pattern (High-Low, Low-High, Low-High-Low, Random) could be explained fairly well to some extent with the aid of R.M.S. peak stress value. Furthermore, in clarifying the fatigue properties of the material under service loads, the effect of peak stress of low frequency would be one of the interesting subjects which must be dealt with. Hence, the associated investigation was incorporated in the present study.
In this study, strain-controlled low-cycle fatigue tests at 650°C were carried out in an air environment in order to examine the effect of strain waveform on the fatigue fracture mode. Four different strain waveforms producing four basic inelastic strain ranges (i. e., PP, PC, CF and CC types) given in the strain range partitioning method by Manson et al. were sellected in these tests. The surface and internal cracking modes of the failed specimens were observed by means of a scanning electron microscope as well as an optical microscope. The results obtained in this study were as follows; (1) The cracking mode under PP and PC strain waveforms was of transgranular type, and striation was observed on both of the fracture surfaces. (2) The cracking mode under CP and CC strain waveforms was intergranular. A lot of internal grain-boundary cracks were also observed in the CP type fatigue, but few in the CC type fatigue. (3) The failure life in the PC type fatigue was shorter than in the PP type fatigue at the same inelastic strain range. This difference may be interpreted mainly as a difference in life to crack initiation between the two types.
The effects of both vacuum environment and shape of stress wave of “cp” and “cc” as shown in Fig. 2 on push-pull cyclic creep fracture of SUS 316 stainless steel were examined experimentally in both vacuum of 0.1Torr and 1 atmosphere at 600°C. (1) The cyclic creep failure time of the material tested was remarkably dependent on environment and shape of stress wave. The time to failure under cp stress wave with tensile creep hold in 0.1Torr was shorter than that in air. On the contrary, the failure time under cc stress wave with both tensile and compressive creep hold in 0.1Torr was larger than that in air. The latter fact was also revealed under pc stress wave with compressive creep hold. (2) The above behavior under cp wave resulted from both early crack initiation and fast crack propagation in tension cycle in 0.1Torr, and the latter effect under cc wave resulted from late initiation and slow propagation of crack in 0.1Torr. (3) The crack propagation rate of the material tested was well represented by a single curve on the basis of modified J integral, independent of stress levels, shapes of stress wave and environments. (4) The variations of both crack initiation time and crack initiation time and crack propagation rate of the metal under vacuum environment of 0.1Torr seemed to result mainly from the variation of macroscopic creep rate of the specimen. It was considered that the increase of creep rate under cp wave in 0.1Torr resulted from the increase of dislocation density as in the case of static creep of the metal in 0.1Torr, and also that the decrease of creep rate under cc wave in 0.1Torr was analogous to the deformation and fracture behavior of high-temperature low-cycle fatigue of the metal.
With the purpose of investigating the fatigue life under combined bending and torsional stresses acting at different frequencies, a testing machine was constructed, and tests were carried out on S45C carbon steel specimens. The ratio of the frequency of reversed torsion to that of reversed bending was in the range of 0.19 to 4.9. In the previous paper, the method to estimate the fatigue life under a monofrequency in-phase combined stress was proposed. In this paper, in order to extend the method to the bifrequency case, the following assumption was made: -if the torsional component of a bifrequency combined stress is changed to a different torsional stress which would induce the same damage as that of the original one after the same number of bending cycles, the resultant combined stress can be considered as same as the monofrequency case, and the preceding method should be applicable. The experimental results obtained supported this conception, especially in the high-stress region.
Using Triethyrenetetramine (TETA), m-Phenylenediamine (m-PDA) and polyamide (Versamide-140, V-140) as the hardners, three kinds of epoxy resin adhesives were prepared to examine the effect of constitution of resin-hardner systems on the bonding properties of epoxy resin adhesives for wood. The physical and mechanical properties of cured epoxy resin adhesives were affected by the kind and amount of hardners added. The bonding properties were also remarkably influenced by the mechanical properties and water absorbability of cured epoxy resin, and the excellent bonding strength and durability of adhesives having higher Tg and lower absorbing capacity were obtained when m-PDA hardner was used. On the contrary, in the case of TETA and V-140, the mechanical properties of adhesive films changed remarkably at 100°C and poor wood failure was observed. However, the bonding strength was retained well after boil and dry cyclic exposure.