Age hardening was significantly promoted at a weld joint of a conventionally heat-treated Ti-15V-3Cr-3Sn-3Al alloy. This local increase in hardness resulted in a decrease in fracture toughness of the weld joint. So a new heat treatment method has been developed to achieve its high fracture toughness. On the basis of our experimental results, the age hardening behavior of the weld joint has been evaluated by X-ray diffraction. Back-reflection Debye patterns were observed in the weld-metal (WM), heat-affect-zone (HAZ) and base-metal (BM) of welded materials. The Debye patterns of WM and HAZ became wide in comparison with BM. Micro-lattice-strain and total misorientation increased in WM and HAZ. Furthermore, in WM and HAZ, sub-boundaries were observed within a recrystallized β grain. These results may suggest that sub-structures markedly affect the age hardening of the weld joint.
The stress generated as a result of the difference in coefficient of thermal expansion between matrix and fiber was measured during the heating and cooling processes of a γ-alumina fiber-reinforced Al-5%Cu alloy. In order to understand the thermally induced residual stress better, an automated device was used by which in-situ X-ray diffraction measurements at elevated temperatures could be performed. The composite with 50 vol% of 17μm diam. Fibers was fabricated by a squeeze casting method, and was given an aging heat treatment which consisted of heating the composite to 800K for 4hr, water quenching and reheating to 463K for 6hr. At room temperature (RT) the matrix of the water quenched composite showed a tensile thermal residual stress of 80MPa. In-situ measurements on the composite heated up to 463K showed a constant matrix compressive stress of -50MPa during the whole aging period of 6hr. In the subsequent thermal cycling process of the aged composite, the matrix stress changed elastically between -50MPa at 463K and 100MPa at RT. On heating back to RT after cooling the composite to RT from 463K and then to liquid nitrogen temperature, a reduction of the residual stress was observed. On further heating of the composite from RT, plastic deformation of the matrix was observed.
Fatigue tests were carried out under various load controlled conditions using a Ni-Cr-Mo steel, JIS SNCM 439 equivalent to AISI4340, tempered at three different temperatures. The residual stress was measured by an X-ray diffraction method on the fatigue fractured surface. The relation between the residual stress and the stress intensity factor was discussed. The main results obtained were as follows: (1) The residual stress on the fatigue fractured surface was separated into the tensile and compressive components and the relations between these components and the stress intensity factor were established quantitatively for the three tempering temperatures. (2) The fatigue crack propagation rate could be estimated by using the results (1). (3) The tensile residual stress component was higher for lower tempering temperature. (4) The maximum value of the tensile residual stress component was nearly equal to the yield stress of the material. (5) The absolute value of the compressive residual stress component was higher for lower tempering temperature. (6) Under the positive load ratio, the residual stress on the fatigue fractured surface was not affected by the contact of the fractured surfaces due to the crack closure. (7) The effect of roughness on the shape of the estimated curve could be ignored if the roughness was nearly constant through out the whole range of stress intensity factor.
The residual stress distributions of bonded dissimilar materials were determined by the two- and three-dimensional (2-D and 3-D) thermoelastoplastic stress analyses using FEM. A silicon nitride plate was brazed to a carbon steel plate, a thin copper plate being sandwiched between them to eliminate the high residual stress generation by brazing. Two kinds of specimens of different sizes, 30×110×14mm and 30×44×5mm, were prepared. The residual stress on the surface of the silicon nitride was also measured by X-ray diffraction. In the 2-D FEM, the stress was calculated by assuming the plane stress state. In the interfacial region, the stress distributions of the two specimens were similar independently of the size of the specimen. In the central region on the surface of the specimen, the stress σx, in the direction perpendicular to the interface, calculated by the 2-D FEM changed slowly, taking tensile and compressive values in the steel and silicon nitride, respectively. On the other hand, the 3-D FEM gave sharp compressive and tensile stress peaks in the steel and silicon nitride, respectively. As the distance from the interface increased, the stresses calculated by the 2-D and 3-D FEM agreed each other and vanished at a longer distance from the interface. The maximum tensile stress σx appeared at the upper and lower corners near the interface in the silicon nitride. In the interfacial region, the stress σx calculated by the 3-D FEM changed rapidly in the cross section of the specimen, although in the 2-D FEM the stress was assumed to be unchanged through the thickness of the specimen. These results suggest the need for the 3-D stress analysis for calculating the residual stress of the bonded dissimilar materials.
Crystal orientation and residual stress development in AlN films deposited on BLC glass and quartz substrates were investigated by an X-ray diffraction method. The deposition was made by a planar magnetron sputtering system with two facing targets under the nitrogen gas pressure of 0.39Pa in the substrate temperature range between 373K and 553K. The measurement of diffraction intensity from (00·2) plane showed that the c-axis orientation of AlN films was improved when deposited at higher substrate temperatures than 523K. Large tensile residual stresses, 0.9-1.4GPa for the BLC substrate and 1.4-2.7GPa for the quartz substrate, were found in the films deposited at low substrate temperatures (≤523K). Especially, the residual stress in the film on a quartz substrate showed a large increase with substrate temperature, whereas that on a BLC substrate showed a small increase. A very small stress was found in the film deposited at high substrate temperatures (>523K) for both substrates. The role of intrinsic and thermal residual stresses on resultant stress development was discussed.
Boiler feed water pump shafts are subjected to severe thrust loads and cyclic bending moments. Half-value width and residual stress measurements by X-ray diffraction methods, which are sensitive to fatigue damage, are employed for the fatigue life assessments of pump shafts. In order to establish a damage detection system, a portable, light-weight X-ray diffraction apparatus with a rotating receiving slit and X-ray detector and a small diameter X-ray tube has been developed. Consequently, the residual stress can be measured within 20 minutes which is about half the time required using a conventional apparatus. The X-ray apparatus has been installed in a system with a specially designed mechanical scanner for a pump shaft, and as a result, the fatigue damage can be automatically measured on the pump shaft surface. The system has been applied to more than 150 actual pump shafts in a factory during in-service inspections.
A new method was proposed to estimate nondestructively the steep distribution of residual stress in the surface layer of ground ceramics with the use of the glancing incidence X-ray diffraction method. In the estimation, we assumed an exponential decrement of the residual stress near the surface, and derived a formula for the strain measured by X-rays as a function of sin2ψ. Experimentally, we obtained the strain for a very wide range of sin2ψ from heavily ground silicon nitride by the glancing incidence X-ray diffraction method. A strong nonlinearity was found in sin2ψ diagram at very high ψ-angles, that is at very shallow penetration depths of X-rays. From the analysis of the nonlinearity by our new method, the stress distribution in the surface layer was determined. The residual stress took the maximum compression of about 2000MPa at 0.4μm from the surface, and then diminished at about 10μm in depth.
In order to design the toner particles of thermoplastic resin coated with charge contorl agents (CCA), a dry coating method was applied to its compounding process. Experiments were carried out with a high speed-high shear mill by changing operation time. The compound states of the particles obtained were evaluated by EPMA mapping analysis and other methods. Then a relation between the compound state and triboelectric charging was studied by investigating the variation of specific charge (charge-to-mass ratio) with the mixing time of composite particles and carrier. The results obtained are summarized as follows. The whole coating process can be divided into three regions. The variation of specific charge shows that the CCA-Coating fraction on the surface of core material influences both the maximum specific charge and the charging speed. A good correlation was recognized between the binding strength of CCA and the charge stability. Based on these facts, it is possible to design the CCA-coated thermoplastic resin particles which have more excellent triboelectric charging characteristics than the conventional particles obtained by kneeding and pulverizing.
The impact transition characteristics and fracture behavior were investigated on friction welded joints with structure softened and grain refined with annealing treatments. After manufactured from 0.46%C hot-rolled bar steel, the friction welded joints tested in this study were annealed at 800°C for 60 minutes in a vacuum furnace. The impact specimens were taken from the central portion of bar joint, and a V-notch was given along the weld interface of the joint. The impact tests were carried out at various temperatures, using the instrumented Charpy Impact machine. The testing temperature was controlled in liquid nitrogen and heating oil. The main results obtained are as follows; (1) The absorption energy of the annealed joint was in all regions extremely larger than the absorbed energy of the as-welded joint and was restored in the same degree up to the level of the hot-rolled base metal with the exception of the upper shelf energy. (2) The energy transition temperature was about 27°C in the annealed joint and agreed closely to the temperature estimated from fracture surface. This transition temperature was lower in comparison with those of both the as-welded joint and the hot-rolled base metal. (3) In the case of the annealed joint in the transition region, the crack propagation resistance was raised more remarkably than that of the as-welded joint. (4) The impact bending strength, which was calculated by the formula of beam subjected to three-point bending from the maximum impact load obtained in the instrumented tests, increased in the order of the as-welded joint, hot-rolled base metal and annealed joint. They became maximum around -20°C, where 100% brittle fracture occurred.
The effect of the interaction between crack-tip slip band and grain boundary on plasticity induced crack closure was analyzed. On the basis of the method proposed by Budiansky and Hutchinson, the crack opening stress and the crack opening displacement were calculated for the constant and proportional residual stretch models in which the residual stretch was assumed to be constant and proportional along the crack surfaces, respectively. The crack opening stress and the crack opening displacement range decreased with increasing crack length. Then the simulation method developed by Newman was applied to crack growth. The crack opening stress increased with increasing crack length. The distribution of the residual stretch was geometrically similar to the change of the maximum crack opening displacement with crack length. The relation between the crack opening displacement range and the effective stress intensity range was almost unique irrespective of applied stress and boundary condition.
A cumulative fatigue damage rule has been proposed for random stress sequences from a view point of internal stress and effective stress. Applied stress was devided into two parts through practical measurements and simulation; internal stress as a backward stress for the deformation and effective stress as a drag due to plastic flow. For the fatigue damage evaluation, a cumulative damage rule like Miner's was operated with the help of ES-N diagram and effective stress measurements during fatigue process. The former was obtained by fatigue tests where effective stresses were kept constant during the tests and the latter was made by frequent monitoring of effective stress during the tests, which resulted in an experimental expression of formulae. It was found that the internal stress at a given cycle can be expressed in a regressive form in terms of the stress level and the internal stress of the latest stress cycle. That is, σi(n)=F(σa(n-1), σi(n-1), σa(n)) where subscripts (n-1) and (n) are the latest number of cycle and the current number of cycle, respectively. The damage rule and experimental expression of effective stress were examined on two materials with or without strain-ageing hardenability, namely S35C steel and Al-1.1 mass% Mn alloy. Agreement between the experimental results and the simmulations was found to be fairly good.
Fatigue tests under plane bending and pulsating tension were carried out the notched FRP plates for a wide range of notch-root radii and stress amplitudes. An attention was focussed on the fatigue damage development near the notch root of specimens. The luminance at the limited spot near the notch root was measured successively during fatigue tests to evaluate the fatigue damage. Closer observation by means of a scanning electron microscope revealed that the initiation of microcracks at the notch root was accompanied with a decrease in luminance near the notch root. The experiment shows that the number of cycles to fatigue damage initiation is determined by both the maximum elastic stress at the notch root and the notch-root radius. On the basis of the concept of linear notch mechanics, the experimental results can be clearly elucidated and a criterion of fatigue damage initiation is determined in terms of a combination of the maximum elastic stress, σmax, notch-root radius ρ and the number of cycles to fatigue damage initiation Nd. The criterion is expressed as: σmax·(Nd)m=C(ρ), where m is the material constant. The parameter C is the material constant, which is governed by the notch-root radius only and is independent of other notch geometries and specimen size. By applying the criterion derived here, it is possible to make an accurate estimate of the fatigue life for notched FRP plates.
The fracture behavior during tensile tests on unidirectional flawed samples of carbon-reinforced nylon 6 at angles of 0°, 45° and 90° between fiber and loading direction has been studied with simultaneous AE monitoring. It was observed that the direction of crack propagation was along fiber/matrix interface in each case of 0°, 45° and 90° samples. For on-axis loading, the fracture energy was large, the crack propagated rapidly, and the fracture surface was smooth, while for off-axis loading these were contrary. Furthermore, the fiber/matrix debonding, which is the main failure mode in unidirectional materials, has been investigated in detail from the results of AE parameters, crack propagating behavior and SEM observation. An obvious difference was found in AE amplitude as well as in the distribution of power spectra between on-axis and off-axis loadings.
As a first step in the study on the fatigue mechanism of fiber and/or particle reinforced aluminum alloys, we carried out, in this study, a series of fatigue tests and fatigue crack growth tests on an alumina short fiber reinforced aluminum alloy in push-pull load conditions. The results indicated that the fatigue failure of this fiber reinforced aluminum alloy was governed by two main mechanisms: One was a debonding of the fiber and the matrix induced by the localized plastic strain, depending on the mismatch of elastic constants, and another was the failure of the matrix induced by the accumulation of fatigue damage around the bond interface. In the higher applied stress range, and at room and relatively lower temperatures, the former mechanism is predominant, but in the lower stress range, and consequently in longer life region, the latter mechanism plays a main role and this trend becomes more dominant with each increase in temperature.
MgO and MgO-ZrO2 sintered bodies with different grain sizes were prepared by the pressureless sintering. Four sides of the specimens were machined parallel to the eventual tensile axis of bending tests with three types of tools; lapping machine, grinding machine and emery paper. Then the strength was measured by 3-point bending to examine the effects of grain size and surface roughness on the bending strength of MgO sintered body. We propose a model of single edge crack length, as, that is controlled by the grain size (D)) and the maximum surface roughness (Rmax). In this model, an inherent crack which is dependent on grain size is assumed to situated at the bottom of the surface roughness curve with depth of 0.8Rmax. Therefore, as is expressed by the equation, as=as0+0.8Rmax, where as0 is defined as the crack length controlling the strength of smooth-faced specimen. In the case of MgO sintered bodies, as0 increased with an increase in grain size according to the equation, as0=4.48+0.25D-1.38×10-3D2. Consequently, the bending strength, σ, was expressed by the equation, σ=KIC/[Y(4.48+0.25D-1.38×10-3D2+0.8Rmax)1/2], where Y is a shape factor.
The effect of high temperature oxidation on bending strength of Si3N4 containing AlN-Y2O3-HfO2 and of SiC containing B-C additives was studied at room and elevated temperatures up to 1673K. The surface of all the oxidation specimens was covered by a uniform layer after oxidation. The three-point bending strength of Si3N4 after oxidation was significantly decreased at room temperature but it was nearly equal to that before oxidation at elevated temperatures at which oxidation was performed. On the other hand, the bending strength of SiC after oxidation was not decreased at both room and elevated temperatures. No crack was observed on the oxide layer formed at Si3N4 surface after oxidizing at elevated temperatures and cooling down to room temperature, while many fine cracks were observed on the oxide layer formed at SiC surface. From the residual stress calculation by FEM after the oxidizing and cooling procedures, it was confirmed that the decrease in bending strength of oxidized Si3N4 at room temperature is caused by the residual stress generated by the difference in thermal expansion between the oxide layer and the base ceramic body. It was also considered that no-decreased in bending strength of SiC at room temperature is due to the relaxation of such residual stress by layer cracking.
This paper proposes an inverse analysis procedure to estimate unknown thermal properties and heat transfer coefficient in transient heat conduction. The Laplace-transform boundary element method and the standard optimization technique are used to analyze the inverse problem. It is assumed that the time-series data of temperature measured at some selected points can be used for inverse analysis. The inverse problem is reduced into an optimum problem in which a set of parameters describing unknown values should be estimated. Numerical experiment is carried out to investigate the influence of measurement errors on the estimation results. The usefulness of the proposed method of inverse analysis is demonstrated from numerical simulation for several examples. It is revealed that the estimation results are greatly influenced by errors included in the measured data.