The X-ray elastic constants of two-phase stainless steel and four kinds of 18-8 stainless steel tensile pre-strained at low temperatures were measured by the ψ0 oscillation method with CrKβ γ(311) diffraction and CrKα α(211) diffraction, and the accuracy of the measurements was examined. The results obtained are summarized as follows. (1) When CrKβ γ(311) diffraction plane was used, the value of constant KX was not much influenced by the amount of α-phase. The K-value (KX=37.26kg/mm2/deg) of the austenitic stainless steel reported in the joint research by the Committee on the Mechanical Behaviour of Materials was considered suitable for X-ray stress measurement of two-phase stainless steel such as ferrite-austenitic stainless steel and strain-induced metastable austenitic stainless steel. (2) On the CrKα α(211) diffraction plane, it seemed that the value of constant KX for the two-phase stainless steel differed from the K-value for ferritic steel (KX=-32.44kg/mm2/deg). (3) In the X-ray stress measurement of the two-phase stainless steel with the CrKβ γ(311) diffraction intensity curve influenced by CrKα α(211) diffraction, the measured stress value increased up to 4kg/mm2 by the influence of α(211) profile under the present experimental conditions.
As the first step of the study on non-destructive evaluation of material damage and residual life for martensitic stainless steel machine parts used at high-temperatures, three methods of non-destructive material testing were adapted for the detection of micro-structural changes caused by long-term heating in 12% Cr-Mo-W-V steel. The results obtained are summarized as follows: (1) As the material deterioration caused by long-term heating, softening was dominant under the condition of high temperature and long heating time. Before the occurence of softening, embrittlement was observed. (2) The ratio of hardness to the initial hardness showed a simple relation with Larson-Miller parameter for normally heat treated 12% Cr-Mo-W-V steel. Thus the hardness test is effective for the estimation of service history of the machine parts. (3) The eddy current and X-ray diffraction tests were sensitive to the micro structural changes during long-term heating. Thus, it is possible to use them as a method of non-destructive evaluation of material deterioration such as embrittlement appearing before softening.
For the purpose of stress measurements in a particular grain of metallic materials or in a few grains of grain oriented materials, the strain conditions in individual grains were analyzed in terms of crystallography. The analytical results were examined experimentally by X-ray stress measurements which were carried out on the grain oriented silicon steel sheets being in the state of uniaxial plastic deformation. In the case that the silicon steel sheets were stressed in parallel to the rolling direction, it was found experimentally that the stress in each grain could be regarded as constant and nearly equal to the applied stress. This tendency was also recognized in the analytical results of the stress in grains calculated on the assumptions of the Sachs' and Taylor's models, and the stress in both models agreed with those measured by the X-rays. In the case that the tensile axis was selected coincident with the direction perpendicular to the rolling one in the plane of the sheet, the measured values indicated a large amount of scattering due to the dispersion of the stress in each grain. Although the results from the Taylor's model showed a similar tendency, the amount of scattering from this model was larger than the measured one. On the other hand, no scattering was recognized in the results from the Sachs' model. In this case, the stress of the specimen in bulk could be obtained from the averaged values of the lattice strain over several times of measurement.
Quantitative evaluation of stress intensity factors during brittle crack propagation and at crack arrest was made by applying the X-ray diffraction technique to the fracture surface of compact specimens of a pressure vessel steel SA533B Cl. 1. The stress intensity factor determined using the half value breadth of the X-ray diffraction intensity curve from the fracture surface decreased gradually as the crack propagated, and then it oscillated before arrest. The stress intensity factor at crack arrest was not necessarily the minimum value obtained during the crack propagation. The stress intensity factor at crack arrest was in good agreement with the crack arrest toughness KIa determined by the static analysis, while it was smaller than KID by the dynamic one.
In the residual stress measurement with X-ray, the measured lattice strain εψ is not proportional to sin2ψ when a steep stress gradient exists in the surface layer, because the X-ray penetrating depth becomes shallower with increasing ψ. This means that the traditional sin2ψ method, which is based on the linearity of εψ-sin2ψ diagram, is hardly adoptable for such cases. The purpose of the present work was to find out a method of analysing such a state of residual stress precisely. Two methods were used to analyse the diffraction profile: The first is the Warren's method including the“powder pattern power theorem”and the theory of“small coherent domains and strains”. The second is the sum-up method of each sub-profile coming from thinly divided layers and having a normal distribution pattern of which location and standard deviation relate to macro- and micro- strains, respectively. From these analyses, it was found that the diffraction profiles from the layer with steep gradient of residual stresses became more asymmetric with increasing gradient and with decreasing breadth of diffraction line. It was noticed further that the strains calculated from the peak positions and the medians of half width of profiles began to deviate essentially from those calculated from the centroids as the stress gradient increased. The physical meaning of the weighted averaging method was studied in detail and it was concluded that in this method a more precise result would be obtained when the position of centroid was used instead of the peak or the median of half width of the diffraction profile.
The purpose of this study was to test the weighted averaging method experimentally. A plane lapping of an annealed bearing steel SUJ-2 generated a steep gradient of residual stress within its shallow surface layer. X-ray strain measurements on the lapped and successive electropolished surfaces revealed that the strain εψ was not linearly dependent on sin2ψ, but curved parabolically on sin2ψ diagram independent of the measured directions. It was also found that the stresses calculated from the peak positions or medians of half width of diffraction profiles began to deviate clearly from those calculated from the centroid positions when the stress gradient was larger than 2-3kg/mm2/μm. The weighted averaging method gave a residual stress pattern continuously distributing from the surface to depth when the principal stress σ3 was considered to exist in the direction normal to the surface within the X-ray penetrating depth. The experimental plots on sin2ψ diagram showed that the strain free direction ψ* deviated from sin2ψ=2ν/(1+ν) appropriate for the state of plane stress, depending on the sign and magnitude of σ3. This means that the tri-axial stress analysis must be considered when the mechanically deformed surfaces such as lapped, polished, cut and ground are treated.
Wide flanged rolled beams are important structural elements and are used in many engineering fields. From the standpoint of economical use of the structural element, the allowable stress is approaching close to the yield stress so that the knowledge of the residual stress in the beam is becoming of great importance to the design engineers. A existing non-destructive method of measuring residual stress in a structural metal element, such as the X-ray method, is limited its usage in the sense that only the surface layer is surveyed. Recently, acoustoelasticity has been paid attention as a technique of non-destructive stress analysis. Acoustoelasticity, as an analogy to photoelasticity, makes use of stress-induced acoustic birefringent effect to measure the stress distribution in metallic materials. It is very attractive because we need not use a transparent plastic model as in photoelasticity. However, much should be done before this method is established as a standard non-destructive technique of stress analysis. The most important is to separate the stress-induced birefringent effect from the texture-induced one. But for only special cases, such as axisymetric stress distribution and the case in which stress-free region is known a priori, the residual stress distribution can be evaluated non-destructively. In this paper, the residual stress distribution was measured in a wide flanged rolled beam by use of a T-type transducer which has been developed recently and the results were compared with the conventional destructive method of residual stress measurements. Also, the distribution of residual stress introduced in the cooling process after hot roolling was analyzed numerically and compared with the experimental results.
Repeated stress-relaxation of NaCl single crystals has been studied at room temperature. It has been found that the work-hardening during repeated stress relaxation can be analyzed in terms of two processes. One appears at the early part of each cycle and corresponds to the work-hardening that proceeds fast. This process depends on the time interval for relaxation, Δt, and the initially applied stress, σ0, suggesting that the process is related to the relaxation. This process may be interpreted as follows: The immobilization and grouping of the dislocations proceed during the relaxation and the grouped dislocations are dispersed during the reloading process, causing the hardening at the early stage of the cycle experiment. The other process is independent of Δt and corresponds to the work-hardening that proceeds slowly with the cyclic stress. This type of hardening is related to the active straining rather than the relaxation. During this process the dislocation density and inter-dislocation reaction increase with straining. This process may be explained by using the assumption that the hardening coefficient, θr, during the relaxation and the so-called rising coefficient, K, at the beginning of the restraining are constant.
It has been well established that the propagation process of through cracks under repeated stress can be treated by fracture mechanics. It has been also reported, however, that the propagation law of small fatigue cracks is quite different from that of through cracks and especially that in some materials the growth rate of small cracks cannot be treated as a function of stress intensity range. In this paper, the growth law of small fatigue cracks was investigated on the basis of a dislocation multiplication model. A new law was proposed on the analogy of the growth law of small creep cracks. The modified J-integral J(N) (fatigue J-integral) which corresponds to the creep J-integral J' in the case of creep was defined, and it was shown that the growth rate of small fatigue cracks was proportional to J(N).
The effect of loading history on the crack closure behavior was investigated during K-increasing and K-decreasing processes in fatigue crack propagation tests. It was found that the crack opening point, Kop, which had been once raised by the prior loading did not easily decrease in the following K-decreasing process, so Kop in the K-decreasing process was higher than that in the K-increasing or K-constant process, which resulted in to give the low crack propagation rate and the high threshold condition for crack propagation in the K-decreasing test. This is the reason for the fact that the Kth determined by the conventional K-decreasing test is usually higher than that calculated from the fatigue strength of a cracked specimen. From the above mentioned testing results and discussion, it may be concluded that it is dangerous to estimate the criterion whether a flaw will grow or not based on the Kth determined by the K-decreasing test and it should be determined by the K-increasing method using an ideally pre-cracked specimen.
The crack initiation behaviour during stress corrosion cracking (SCC) of SUS 310S stainless steel in boiling 35% MgCl2 solution has been studied by the constant potential method. The constant load type SCC testing apparatus capable of measuring the dissolution current and elongation of the exposed specimens during the test was used in the present study. The crack initiation morphology during SCC was dependent upon the potential range; pitting and then general corrosion occurred as the potential was shifted to more noble values. The potential dependence of its morphology was in close agreement with the fact that the cracks at open circuit potentials initiated at the bottom of pits. The SCC in the pitting potential range occurred in three steps, i. e., pitting growth, micro-crack growth from the pit-bottom, and macro-crack propagation. These steps were closely related to the time variation of dissolution current during the test. Based on the results mentioned above, the crack initiation behaviour at the bottom of pits in the pitting potential range was discussed.
In recent years, many investigations have been carried out to evaluate fatigue life of the components of combustion chamber on marine diesel engines, such as piston crown, cylinder head etc. It seems, however, fairly difficult to evaluate accurately the fatigue life of these components in service conditions, since the scattering of material strengths and loads is pretty large and the service condition usually contains various factors which can not be realized in laboratory experiments. Therefore, the fatigue life evaluation of these components should be executed based on the probability of failures in service, similar to the way applied on the fatigue life design for air-plane, caraxle, etc. In this first report, as a fundamental study for fatigue life design of the components of combustion chamber, the failure data of piston crowns in service were analysed statistically. It was confirmed that the type of failure distribution in service conditions was the logarithmic normal distribution, and the data of mean life, standard deviation, fluctuation coefficient were obtained. Also, the stress distributions on some components were analysed by FEM calculation and stress measurements.
B- and SiC- reinforced-aluminum alloys were consolidated under the diffusion bonding condition without using the vacuum bagging process. Metal cylinders reinforced with the filaments in the hoop direction were consolidated under a high bonding pressure introduced by the difference in thermal expansion between the inner and the outer dies heated to the consolidating temperature, while reinforced metal plates were consolidated with a hot press. B- and SiC- aluminum alloys were shown to have high strength and high tensile modulus not only in the direction of reinforcing filaments but also in the direction perpendicular to reinforcement. The strengths of B- and SiC- aluminum alloys were about 85 and 75% of the values calculated by the rule of mixtures, respectively, while the tensile moduli coincided rather well with the predicted values for both materials. The ratio of the fatigue strength to the tensile strength for B- and SiC- aluminum alloys was large in contrast with the small ratio for an aluminum alloy reinforced with SUS 304 filament. The hoop strength and the tensile modulus of B- and SiC- aluminum alloy cylinders coincided relatively well with those of the corresponding reinforced plate specimens. A high speed rotor made out of a B- aluminum alloy cylinder with a diameter of 200mm and a length of 300mm was shown to have a stable properties and endured the circumferential speed of 675m/S (64200rpm).
Dilatant flow behavior was studied with a capillary viscometer over a shear rate range of 102-106sec-1 for the aqueous suspensions of titanium dioxides which showed pseudoplastic flow in lower shear rates (10-2-102sec-1). The dilatant flow of these suspensions was found to depend not only on the amount of sodium pyrophosphate but also on the diameter of suspended particles. From the sedimentation tests, the effects of both the amount of sodium pyrophosphate and the particle size on the dilatant flow were revealed to be closely associated with the degree of dispersion of suspended particles. With decreasing the degree of dispersion, the dilatant flow appeared in lower shear rates accompanied by an increase in the limiting viscosities at both low and high shear rates. This may be resulted from an increase in the effective volume fraction of particles including a part of dispersion medium entrapped between the flocculated particles. The degree of dispersion was dependent on particle size and showed maxima at different amounts of sodium pyrophosphate. From the measurement of the amount of adsorption of dispersing agent, however, it was proved that the degree of dispersion became maxima at the same equilibrium concentration of pyrophosphate ions irrespective of particle sizes. These results were successfully explained in terms of DLVO theory. The effect of the particle size on the relationship between the degree of dispersion and the equilibrium concentration of pyrophosphate ions may be attributed to the difference in the number of particles in unit volume.
Since brittle lacquer coating of air-drying type displays viscoelastic behaviour, brittleness of the lacquer coating is markedly dominated by loading time and test temperature. When the technique of brittle coating is applied in the strain measurement, it is necessary to compare the coating cracks on the calibration bars with those on the engineering structures to be inspected, although these two type of specimens have been subjected to different load application time. Besides, the relaxation method is often utilized to analyze the compressed regions on the structure. Few studies have been made on the mechanical properties of the coating by taking viscoelasticity into account, because it is too difficult to make mechanical tests on a coating film of high brittleness separately. In this study, therefore, the influence of both loading time and test temperature on the strain sensitivity of some brittle lacquer coatings were investigated on the calibration bars by using a new loading device controlling the load application time. The results obtained are as follows. The influence of loading and relaxation time on the strain sensitivity was clarified at various test temperatures. It was confirmed that the strain sensitivity scarcely changed with time up to about 10 seconds. The sensitivity master curves presenting the combination of the loading time, the test temperature and the acid value of basic ingredients were composed successfully by applying the reduction principle.