The X-ray stress measurements were carried out on 18Cr-8Ni austenitic stainless steel specimens with different half-value breadths by the fixed ψ0 and ψ0 oscillation methods with CrKβ γ(311) diffraction. The peak position of the diffraction profile was determined by several methods, such as the half-value breadth method, the parabola fitting method and the center of gravity method by a computer and the accuracy of stress measurement was examined. The results obtained were as follows: (1) It is not desirable to determine the peak position of diffraction profile by the center of gravity method, from a viewpoint of the accuracy of the computation. (2) When the data of X-ray stress measurements obtained by the fixed ψ0 and ψ0 oscillation method were treated by using the half-value breadth or the parabola fitting method with corrections for background and an absorption factor, the error was as small as that of the graphic half-value breadth method. (3) The corrected value of the absorption factor obtained from the normal distribution curves instead of the diffraction profies was not necessarily the same with the experimental value.
The fatigue lives of metals up to crack initiation have been investigated with a stochastic model based on accumulation of fatigue damage. In the model, it is assumed that the fatigue damage is accumulated randomly inside the metal in the form of dislocations under the repeated stress and that a microscopic crack occurs when the strain energy due to local piling-up of dislocations exceeds a specific value. The size of the initiated crack is on the order of microscopic structure (cell) corresponding to the scale of dislocation piling. The cell size is determined from probabilistic arguments. The growing process from a cell-sized crack to a grain-sized one is treated with some assumptions. Our theoretical results are compared with the experiments using three kinds of low carbon steel S20C with different grain sizes, and the consistency of the model is examined.
A total strain-time parametric method representing long-time stress relaxation data with different total strains at specified temperatures has been proposed. Parametric analysis was made using the observed stress relaxation data up to about 10000hr for the total strains ranging from 0.10 to 0.25 percent on 1Cr-0.5Mo-0.25V, 12Cr-1Mo-1W-0.25V and 19Cr-9Ni-1.4Mo-1.4W-Nb high temperature bolting steels. The proposed total strain-time parameter (PT) is as follows: PT=εT-b0tb1εTa0+a1logt where εT is the total strain, t is the time, and a0, a1, b0 and b1 are constants. The present parameter was deduced by the following analyses: (1) the plastic strain (εP) could be expressed by means of the equation, εP=kεTm (m and k are coefficients), because a linear correlation of the data points in the coordinates of log εP and logεT was obtained; (2) both of the coefficients m and k in the above equation depended on time, and they were approximated by means of the respective equations, m=a0+a1logt and k=b0tb1. The observed stress relaxation data for each total strain, including the data of which the relaxation curves showed an inflection of a convex nature in a certain time range, were combined into a single correlation curve with the coordinates of the residual stress (σr) and the new parameter (PT). The regression equation up to second degree, σr=c0+c1PT+c2PT2 (c0 is a constant, c1 and c2 are the regression coefficients), was applied for the master relaxation data. The estimated relaxation curves for the specified total strain calculated from the regression equation showed good agreement with the observed ones.
The purpose of this study was to clarify the mechanism of the thermal deformation and stress build-up of stator windings as well as their failure mechanism under thermal cycling. In this study, the thermal elongation of stator windings was measured over a wide range of temperature variations. By comparing the experimental results with the theoretical ones, the mechanism of thermal deformation and stress build-up was discussed. Thermal cycling tests were also carried out on six kinds of stator windings to clarify the failure mechanism of the stator windings under thermal cycling. In this test, the failure was detected by measuring the flatwise thickness and the tangent delta characteristics of the stator windings. Moreover, the breakdown voltage characteristics were measured at the last stage of the test. The failure mechanism of the stator windings was discussed in conjunction with thermal stress. From this study, tape separation, mica slipping-out phenomena and bonding failure in mica splitting were revealed. Moreover, the relationship between the mechanical failures described above and the tangent delta and breakdown voltage characteristics was clarified.
The reduction in low-cycle fatigue strength due to the notch geometry was studied on carbon steel (SFVV1) and austenitic stainless steel (SUS 304). Since the calculated low-cycle fatigue strength reduction factor (Kf) varies depending upon a given estimation method of nominal strain at the notch root area, a new evaluation method of nominal strain was developed from the experimental data on the circumferentially grooved specimens. The Kf value thus determined was shown to be consistent well with the FEM analysis of strain concentration factor, but was considerably lower than those specified in the ASME Code Sec. III above the 3Sm level.
The frequency effect on the notch sensitivity in low-cycle fatigue life was studied at an elevated temperature. The material used was SUS 316 austenitic stainless steel and the nominal total strain range controlled tests were performed at 600°C in atmosphere with the frequency range from 0.001 to 1Hz. The elastic stress concentration factors Kt employed were 2.6, 4.2 and 6.0. From the tests, the following conclusions were obtained. The number of cycles to failure of the notched specimen decreased with an increase of Kt. But the rate of decrease in fatigue life with an increase of Kt decreased with decreasing frequency. The above results were discussed from the viewpoints of applied stress cycling and of a model analysis on crack propagation. Also discussions on the life prediction rule of notched specimens were presented in connection with ASME Boiler & Pressure Vessel Code Case N-47. An empirical formula of Nf=Nf0Ktm νl was proposed and shown to predict accurately the life of notched specimens at any condition of Kt and ν. Furthermore, the background of the empirical formula was discussed from the viewpoints of Neuber rule and the frequency modified fatigue life equation. The same type of the formula was derived by combining the frequency modified fatigue life equation with Neuber's rule.
This paper deals with the stress and strain fields and the modified J-integral near a crack tip under the condition of incipient creep deformation. It was assumed in this analysis that (1) the total strain can be divided into the elastic and creep strains, (2) the crack does not grow (a stationary crack), (3) the elastic strain prevails except for a region near the crack tip where the creep strain is predominantly large compared with the elastic strain, and (4) the definition of the modified J-integral, J', is valid in this region. Analytical expressions for the stress and strain fields and the J' evaluation near the crack tip were derived for the Norton-type steady-state creep, the time-hardening creep and the strain-hardening creep hypotheses. The validity of the proposed expressions was confirmed numerically by using the finite element analysis. It was found that the values of stress, strain and J' were very high in the vicinity of the crack tip during incipient creep deformation. For example, under constant loading the J' value near the crack tip was inversely proportional to loading time. J' approached to its steady-state value J'st as the creep deformation increased. As a measure of this transition, a characteristic time ttr was defined, which is proportional to Jel/J'st where Jel denotes the elastic J-value. This transition time ttr was evaluated for a type 304 stainless steel and a Cr-Mo-V steel using the data available in the literature. It was shown that this transition should be taken into account for predicting the initial phase of creep crack growth of low ductility Cr-Mo-V steel. The effect of plastic strain was also discussed and it was suggested that the plastic J-integral Jpl divided by J'st could be taken as another characteristic time.
The effect of a two- or three-dimensional platelike defect on the crack propagation behavior of fatigue or delayed failure was examined, and the results were analyzed by using the single large dislocation model. The three-dimensional platelike defect which lies on the same plane as that of the main crack accelerates the propagation of the main crack, while the one which lies perpendicular to the main crack decelerates the propagation of the main crack. The platelike defect lying with an angle of 45° against the plane of the main crack slightly increases the crack propagation velocity and makes the twist and step on the crack propagation path. The interaction behavior between a propagating crack and a two-dimensional or three-dimensional defect can be explained qualitatively by the single large dislocation model.
Fatigue crack propagation behaviors of explosively clad steel plates were studied by making fatigue crack propagation tests under a constant stress intensity factor range and by carring out a FEM elasto-plastic analysis of fatigue crack growth. The effects of the initial residual stress, hardening layer and yield point of each original material on crack propagation were examined. It was concluded from these examinations that for explosively clad materials which had a clad interface, residual stresses and a hardening layer near the interface, the fatigue crack propagation rate was not related to the stress intensity factor range ΔK, but closely to the maximum strain range at the crack tip, Δεymax.
The purpose of this paper was to make clear the effect of specimen dimension on surface damage under an action of pulsating impact contact load. The dynamic stress produced by a falling weight on a rod specimen was studied at first, and the dynamic strain was measured. In pulsating impact experiments, two series of commercial steel S25C and S45C were used. The surface damage was defined by the first appearance of a pitting on the surface and the pulsating number Np till to the appearance was discussed. The results obtained are summarized as follows. (1) A concise equation for the dynamic stress in a rod specimen produced by a falling weight was formulated. (2) The dynamic strain showed a good correspondence to the theoretical result and the distribution of the maximum shearing strain was given. (3) The surface hardness HV increased with an increase of number of impact. But no clear correlation was found between HV and Np. (4) Np was smaller for a shorter length specimen and larger for S45C than S25C. The increase of surface hardness by heat treatment caused an increase in Np. (5) Before an appearance of a pitting, several visible cracks were seen on the surface of a specimen. These results were explained by taking into account the effect of the superposition of stress waves, which were produced by an impact load, in a specimen. The effect depended on the dimension of a specimen.
In order to study the effect of the loading time of impact tensile loads on the crack propagation rate, impact fatigue tests were carried out on circumferential V-notched specimens of 0.2% carbon steel by using a repeated impact tensile loading apparatus. Based on the experimental data, the crack initiation life and the crack propagation life for circumferential V-notched specimens were estimated empirically. The results obtained were summarized as follows; (1) The relation between the striation spacing (S) and the stress intensity factor (ΔKI) is given empirically by, S=C0TnsΔKIms where C0, ns, and ms are experimental constants, and T is the loading time of impact tensile loads. (2) The crack propagation life (Np) and the crack initiation life (Nc) can be estimated by the following formulas and the estimated values were found to agree with the experimental results. Np=∫afaida/C0TnsΔKIms Nc=(D*/σ)1/m*T-n*/m*-∫afaida/C0TnsΔKIms where D*, m*, and n* are experimental constants, ai is the initial crack size, and af is the final crack size.
The present investigation has been carried out to make clear the effect of strain-induced α'-martensite on stress corrosion cracking (SCC) susceptibility of austenitic stainless steel. The specimens were prepared from SUS 301 stainless steel sheets with metastable austenite, and were given the prestrain of 10, 20 and 40% at the temperature -80 to 80°C. Then the SCC susceptibility of these specimens was evaluated in boiling 35% MgCl2 solution under the applied stress of 35kg/mm2. The change of α'-martensite content depending on prestrain and prestraining temperature gave a remarkable effect not only on the mechanical properties but also on the SCC susceptibility. It was found that the SCC susceptibility of prestrained specimens extremely decreased when α'-martensite content was within a specific range: e.g., about 10∼90% on 20% prestrained specimens. On the other hand, the fracture surface morphology was considerably influenced by the change of α'-martensite content. In the case of small α'-martensite content, the fracture mode changed from transgranular cracking with typical fan-shaped pattern to mixed intergranular-transgranular cracking. In the case of large α'-martensite content, however, transgranular cracking was observed, which seemed to be caused by the dissolution of martensite-lath. Based on these results, the possibility of hydrogen cracking was discussed in the SCC behavior of 20% prestrained specimens under anodic and catholic polalization.
In this study, the resistance of concrete to seawater such as durability against sulphates and chlorides which must be examined prior to practical use at the seaside or offshore has been experimentally investigated. It was aimed to make clear the relationship between the reaction products in concrete with seawater and the deterioration process of concrete. The X-ray diffraction analysis method was applied to identify the materials produced in concrete under seawater environment, and from the strength and tendencies of the diffraction peaks the reaction process and kinds of product were investigated. Three kinds of cement, namely the normal portland cement, blast furnace slug cement and seawater resisting cement developed for the sea grauting, two kinds of mixing water, fresh and sea water, several curing period and immersing conditions were selected. The main results obtained were as follows: The products in normal portland cement were ettringite and calciumchloroaluminate, while those in the compound cement were calciumchloroaluminate and gypsum. It was also found that in the compound cement the content of alumina in slug should be limited to a certain amount, because alumina contributes to produce calciumchloroaluminate.
Recently, it has become important to estimate the strength of materials as many mechanical structures are being used under severe conditions. X-ray stress measurement is the only method which can be applied to analyze residual stress without destruction, and thus it is regarded as an effective means for investigating material strength. In the present study, an automatic method of X-ray stress measurement was introduced and the data compiling and processing system for evaluation of stress was developed. The system was compared with the conventional method. In this system, the diffracted X-ray data were sampled at a certain time interval with the continuous scanning method and were smoothed by the parabolic approximation of 9 points. After correcting the background, the stress was calculated by the 9 point's parabola fitting method and the half value breadth method. The results showed no difference between the value obtained by this system and that by conventional method. However, by making use of computer, it was possible to eliminate completely such human mistakes as drawing a chart and reading the diffraction angle from the chart, and to achieve the automatic and high efficient operation on X-ray stress measurement.