The most stable crack initiation pattern in an infinite plate under axisymmetric tension was analyzed by using Griffith theory. The result was compared with the experimental results of flexural tests using glass and ceramic disks. The strain energy release rate Gn for nucleation or propagation of a star-shaped crack is approximately expressed as Gn=(4πσ2a/E)(1/n)(1-1/n), where a is the crack length, n is the number of cracks, σ is the applied stress, and E is the elastic modulus. The energy consideration on the basis of Griffith theory indicated that the most stable crack uncleation pattern in homogeneous and uniform brittle materials under uniform tension is a line shaped crack (n=2). In addition, even if a star-shaped crack happens to pre-exist in a brittle material, it may propagate only as a line-shaped macro-crack. This theoretical deduction was confirmed by the flexural tests on glass and ceramic disks.
A better understanding of the debond and pull-out processes is needed because they affect the fracture toughness of fiber reinforced composites. In this paper, a new stochastic model is proposed to predict the probability distributions of debond and pull-out lengths of fibers in unidirectional fiber reinforced composities. The stress distribution in a debonded fiber was determined under existence of friction between the fiber and matrix. By using this stress distribution and a weakest-link model, the probability distributions of debond and pull-out lengths were predicted. When the debond stress is assumed to be scatterd, the computational results predicted by the new stochastic model have good agreement with the experimental data by Beaumont et al.
The influence of internal oxides on the fatigue strength of carburized steel is strongly affected by the amount of retained austenite (γR) and the nonmartensitic microstructure near the surface. Rotating bending fatigue tests were carried out on three types of carburized specimens that have the same depth of internal oxides but contain 20% and 46% γR (peak value), and 20% γR with nonmartensitic microstructure. Internal oxides reduced the fatigue strength about 30% on the specimen with 20% γR, whereas the specimen with 46% γR has a comparable fatigue strength as the one without internal oxides which was prepared by electropolishing the surface. It is also shown that the specimen with nonmartensitic microstructure has higher fatigue strength than that with only internal oxides. These three types of specimens exhibited almost the same fatigue strength when the internal oxides were removed from the surface.
In general, some carburized or decarburized layer is formed at the bolt surface by heat treatment. It has been already known that the carburized and decarburized surface layers of the steel affect the characteristics of the delayed fracture and the fatigue fracture. But the specimen shape used usually in the experiments is different from the bolt, and the degree of carburizing and decarburizing is greater than that of bolt. Therefore, it is necessary to examine the effect by using practical bolt specimens. In this study, to investigate the above problems, the delayed fracture test and the fatigue test were performed on the bolt specimens having carburized or decarburized surface laver. It was found that decarburizing at the surface layer of the bolt had a good effect on the characteristic of delayed fracture but no effect on the fatigue fracture.
It is expected that a da/dN-ΔK relation measured under a high stress ratio becomes the most conservative design curve for fatigue crack growth at stage 2 in steels. Based on this thought, fatigue crack growth tests with high stress ratios were carried out on steels at room temperature in laboratory air. A few kinds of steels whose yield stresses are from 400MPa to 1500MPa were used in these tests. A predictive method of the design curve for steels was discussed from both the experimental results and the literature survey, and it was found that the design curve at stages 2a and 2b might be predicted by using the yield stress.
Statistical fatigue crack growth tests were carried out under axial loading by using 2017-T3 Al alloy plate specimens having a single defect or double defects through thickness. Theoretical derivations were attempted on the fatigue life distributions of a single-defect and double-defect specimens. The former theoretical distribution was derived based on the empirical distribution of the observed crack initiation lives and Paris' law of crack propagation. By using the same assumptions, the latter theoretical distribution was derived on the basis of crack coalescence probability assuming that the failure occurs immediately after the coalescence of cracks. These results were in good agreement with the experimental trends.
Fatigue crack growth tests of silicon nitride Si3N4 were carried out under four point bending load using the square bar specimens of 12×16×40mm at room temperature. A pre-crack was introduced by the bridge indentation (BI) method. K-decreasing tests under cyclic load with the stress ratio R=0.1 and 0.6 and also under static load were first carried out, and after observing the cease of the crack growth, K-increasing tests were performd excepting the case under static load. Crack closure was detected on most specimens by the elastic compliance method, and furthermore, SEM observations of crack paths were made to see what had happned during the crack growth. The threshold and the region of steady crack growth were observed more clearly under cyclic load, and the effect of load cycling obviously existed, which became more evident when the maximum stress intensity factor Kmax approached the threshold. The wedge effect caused by fine fragments left on the crack surface played an important role to the crack closure behavior of each specimen and it was suggested that the crack growth rate was controlled by both of the effective stress intensity range ΔKeff and the effective mean stress intensity factor Km, eff' at least as a first approximation.
The effect of stress waveform on the cyclic fatigue behavior of Y2O3 PSZ was investigated and the following conclusions were obtained. (1) The cyclic fatigue behavior was independent of frequency in the range from 30Hz to 550Hz under the same stess waveform. (2) By evaluating the stress waveform effect on the basis of an empirical equation of subcritical crack growth rate, the predicted line was found to agree well with the experimental one under the modified rectangular pulse wave and sinsuoidal wave for the case of stress amplitude ratio R=0.8. (3) In the case of R=0.7, however, the experimental line shifted toward longer life than the predicted one. The shift magnitude was independent of frequency and dependent on stress waveform.
By using reaction injection molding of rigid polyurethane including a blowing agent in its raw materials, an integral-skin foam (referred as RIM molding) consisting of high density skin and low density foamed core layers can be obtained in one injection processing. By considering this RIM molding as a sandwich structure, the relation between the layer construction and flexural fatigue properties of RIM molding was investigated. It was found that the fracture modes in the fatigue test were classified as: (1) the tensile fracture-type and (2) the shear fracture-type, depending upon the layer construction. Moreover, a remarkable difference in the degree of decrease in flexural strength due to fatigue between two fracture modes was observed. That is, the fatigue strength at 107 cycles in the tensile fracture mode was about 20% of the static strength, while the one in the shear fracture mode was about 30% of that. The results obtained here can provide an important guide to determine the optimum layer construction concerning flexural fatigue properties in the structural design of moldings, when RIM moldings are applied to thermal-insulated housing.
The stress-strain behaviour of fuel-cell reformer tubes used in a creep-fatigue condition was analyzed by a finite element method. By applying Manson's strain-range-partitioning method to the stress-strain hysteresis loop obtained in this analysis, the fatigue life was estimated for the tubes made of low C-HP-Nb (0.15C-25Cr-35Ni-Nb steel) and HP-BST-M (0.49C-25Cr-35Ni-Nb-Ti steel), which have excellent creep-fatigue properties. Also, the cost efficiency (life/cost) was compared between these materials and a low cost alloy, HK40 (25Cr-20Ni steel). The main results obtained are as follows; (1) The plastic strain range, Δεp, is negligibly small in these tubes. Therefore, the total strain range, Δεt, is nearly equal to the sum of the elastic and creep strain ranges, Δεe+Δεc. (2) The total strain range, Δεt, is almost constant for a variety of creep constitutive equations. The creep strain range, Δεc, therefore, can be decreased by using materials with high creep strength, such as HP-BST-M. (3) A longer tube life can be expected by using HP-BST-M because of its smaller Δεc and better Δεij-Nij properties than low C-HP-Nb. (4) The HP-BST-M is more cost-efficient than low cost HK40.
Creep-fatigue and oxidation properties of Zr-free and Zr-containing HK40 and HP centrifugal heat-resisting alloys were studied. The mechanism of improving these properties by Zr addition was discussed through microstructural examination of materials. The following results were obtained; (1) A small amount of Zr addition markedly impoves the low-cycle-fatigue life under creep conditions, creep rupture strength and ductility, and oxidation resistance of HK40 and HP. (2) The improvement of creep-fatigue properties is thought to be brought about by the following two effects; (i) Zirconium gives an excellent resistance to the localized oxidation of eutectic carbides and/or grain-boundaries, which can be fatigue crack initiation sites. (ii) A number of small Zr-carbides (ZrC) are precipitated homogeneously in grains at elevated temperatures. These carbides suppress the formation of sub-boundaries and the precipitation of coarse carbides (M23C6) on sub-boundaries, which can be fatigue crack propagation paths.
Creep-fatigue tests of dissimilar metal electron beam welded joint between A387 Gr. 22 steel and SUS316 steel (A387-SUS316) were carried out under strain-controlled cycling over the welded joint at a temperature of 873K in air. The creep-fatigue life of A387-SUS316 was shorter than those of base metals. In order to discuss the creep-fatigue fracture behavior of A387-SUS316, the change of strain distribution with strain cycling was investigated by using the specimen on which heat-resisting Moiré grid was inscribed. A higher strain was produced on SUS316 side at the early stage of fatigue life, and the strain concetration occurred on A387 side during the subsequent strain cycling. It was also found that the hardness distribution was one of the most important measures on which the strain distribution was reflected. Furthermore, a new prediction method for creep-fatigue life of dissimilar metal welded joint was proposed by combining the inelastic analysis and strain range partitioning approach. The predicted creep-fatigue life approximately agreed with the experimental results.
Older thermal power plants are increasing in number today which are to be used beyond their design lives. In order to keep their high performance and to extend their useful lives, it is quite necessary to fully understand the degradation phenomena of the component materials during service operation and to make more accurate life assessment. The embrittlement, one of the typical material degradations, was investigated for the CrMoV steam turbine rotors retired after long time service at high temperatures in thermal power plants, as well as for the laboratory aged rotors. The V notch Charpy impact tests showed a remarkable increase in the fracture appearance transition temperature (FATT) in some rotors. The Auger electron spectroscopy analysis indicated that this increase was attributable to the phosphorus segregation to the grain boundary. The electrochemical polarization (ECP) test was applied to the rotor samples and the value of ECP minimum current density, which was corrected for the grain size obtained by the metallographical investigation of replica taken from the ECP test surface, was found to be a very good measure of the embrittlement. Based on these informations from ECP test, replica observation and rotor production record, a nondestructive method for evaluating in-service toughness degradation of CrMoV steam turbine rotors was presented. According to this method, the degraded FATT of a CrMoV rotor could be estimated within±20°C.
A new method has been developed for imparting electric conductivity to ZnO particles. The conductive particles, which are used as the antistatic agents for the protection of electronic machinery against disturbance by static electricity occurring in plastic and rubber materials, were obtained by treating pure ZnO particles with propylene at a temperature of-or higher than 300°C and depositing the electric conductive film. The properties of the products deposited on ZnO particles were investigated by ESCA, X-ray diffraction and IR analysis. The electric conductivity, carbon content and whiteness levels of this powder were measured. The products on ZnO were mostly carbon and formed a 1nm thick film. In comparison with the conventional technique of Al doping to ZnO, it has been ascertained that the conductivity is achievable by this new technique while the particles remain small in size. These conductive particles are comparable regarding conductivity, particle size and whiteness with the Sb-doped conductive SnO2 particles currently in market. In addition, they are advantageous in that they can be produced at a much lower cost due to the simplified production process and lower material costs.
An image processing system using a personal computer was applied to the stress measurement by copper electroplating. The microscopic TV image of the plating structure of an electroplated specimen after cyclic loading was directly analyzed by the image processing system, and the area of each grain grown in the plated layer was measured automatically. The relation between the area of grain and the number of stress cycles was examined under various stress amplitudes and the relation between the grain growth rate and the area of grain was obtained. It was found that this relation depends on the amplitude of stress. Therefore, the surface stress can be estimated from the grain growth rate based on this relation. As the grain size is in the range from 0.01 to 0.1mm, stress in a very small region can be measured based on the growth of each grain. Stress measurement was actually carried out by this method. The result showed that the error was about 1.0% and this method is sufficiently accurate.