Kenji Hatanaka

A measurement of three-dimensional strains around a creep-crack tip

A new method for measuring the strain around the high-temperature creep-crack tip is proposed in the present paper. A grid pattern was described in a space of about 30 μm with a diamond stylus in the area ahead of the precrack of the specimen. Then, the distortion of the grids and the change in specimen thickness which were induced by the creep deformation were measured by means of a photo-microscope and a roughness-measuring system, respectively. The three-dimensional components of the strain were calculated using the Lagrangian equation, into which the above measurement was introduced. The route of the creep-crack extension was examined in association with the local strain measured by the proposed method.

Tensile Creep of Sintered Silicon Nitride Ceramics at Elevated Temperatures

A good understanding of high temperature creep properties of engineering ceramics is greatly necessitated to promote its application to high temperature-machine components. Our knowledge of high temperature creep of this material, however, is quite limited at present. One of main reasons for this is in difficulty in measuring displacement which is generated in ceramics sample loaded at elevated temperatures above 1000°C.

An Estimation of Mean Stress in Circumferentially Notched Components of Annealed Copper Subjected to Cyclic Loading.

Circumferentially notched components of annealed copper were low-cycle fatigued under load-controlled condition being accompanied with mean load. In addition, their cyclic stress-strain responses were calculated under the corresponding loading conditions by means of elastic-plastic finite element method, where the constitutive equation proposed by the authors in their earlier paper was employed. The calculated nominal stress-diametral strain response at the minimum cross-section was in quite good agreement with the experiment, showing the validity of the present computation. The compressive mean stress generated at first one cycle at the notch-apex shakes down to be attenuated during the following cyclic loading under the condition of the load ratio Rp = -1.0, while this changes very slightly in the same direction during the further cyclic loadings at the Rp = -1/3 and 0. The stress ratio at the notch root Rσ is generally smaller than Rp, and the difference between them is much greater at Rp = -1/3 and 0 than at Rp = -1.0, in the sharper notch than in the duller notch, and under smaller applied cyclic loading than under the greater one. Incidentally, the tensile mean stress was yielded in the center region at the minimum cross-section of the notched components.

Crack Opening/Closing Behaviors under Biaxial Cyclic Loadings in Carbon Steel

Abstract. The crack opening/closing behaviors were calculated for the crucifor m specimen with through-thickness pre-crack, which is subjected to biaxial cyclic loa dings at various biaxial stress ratios, by means of three dimensional elastic-plastic finite element method. The calculation showed the followings: The crack opening displacement i ncreases as biaxial stress ratio decreases under the cyclic loadings at the biaxia l ratios in the range of -1.0<T<0, while this is almost remains constant under the cyclic loading conditions in the rang of 0<T<1.0, describing well the experimental results. The crack opening displacement is much greater under the cyclic loading condition of -1.0 ≤T≤0 than under the one of 0 ≤T≤1.0, and the crack closing load point is much more delayed under the latter than under the former. Meanwhile, the crack opening load poi nt is around at the unloaded point, irrespective of T.

AFM observations of grain boundary step in low-cycle fatigue in copper

The push-pull low cycle fatigue test were performed under the strain-controlled condition and the step height formed at grain boundary, which is precursors of crack initiation, was measured in annealed copper using AFM. Then the step growth behaviors in cyclic loading process were compared with the one obtained from fatigue test performed under stress-controlled condition. In the early fatigue process, grain boundary step was not formed under strain-controlled condition. In the early fatigue process, grain boundary step was not formed under strain-controlled condition, whereas steps with 20-50nm in height were generated under stress- controlled condition. The critical step height at grain boundary at crack initiation Hc was quite large under strain controlled condition as compared with the one under stress controlled condition.

Fabrications and Mechanical Properties of Sintered SiC Fiber-Bonded Ceramics.

A sintered SiC fiber-bonded ceramic was synthesized by hot-pressing the plied sheets of an amorphous Si-Al-C-O fiber. Here we describe the microscopic structure and mechanical properties of the two-directional sintered SiC fiber-bonded ceramic synthesized from three kinds of starting fiber with different concentration of excess carbon and oxygen and different fiber diameter. The desirable sintered SiC fiber-bonded ceramic has been found to show a perfectly close-packed structure of the hexagonal columnar fibers with a very thin interfacial carbon layer. Furthermore, the interior of the fiber element was composed of sintered β-SiC crystal without an obvious second phase at the grain boundary and its triple points. Its mechanical properties at high temperatures are closely relate to the microscopic structure. The strength and the fracture behavior are strongly dominated by the uniformity of the interfacial carbon layer and a densified structure of the fiber element. A reduction in the excess carbon and oxygen included in the starting fiber resulted in the improved mechanical strength of the fiber-bonded-ceramic. Furthermore, it was found the highest four-point bending strength (-500MPa) could be obtained by the use of a thinner starting fiber (8μm) with low concentration of carbon and oxygen. It was concluded from these facts that the improvement in mechanical strength was strongly related to the formation of the uniform interfacial carbon layer and the most densified structure of the fiber element.

Dynamic Creep in Sintered Silicon Nitride Ceramics at Elevated Temperature.

Dynamic creep tests were performed for the specimen with projections of sintered silicon nitride ceramics at test temperature, T=1300°C, where triangular stress wave was imposed at stress rates σ=0.1, 0.5 and 20 MPa/s. Then dynamic creep displacement was measured between the two projections by means of the laser beam type displacement measuring system. Cyclic strain range decreased monotonously with increase in the number of stress cycles in dynamic creep test and this attenuation was much more enhanced at the higher cyclic stress, suggesting that cyclic deformation resistance increases during dynamic creep at 1300°C. The equivalent stresses σep.1 and σep.2 were assessed to examine the relationships between the dynamic and static creep strain rates, and the dynamic and static creep life times to failure. Then minimum creep strain rate ecmln and creep life time to failure tf were plotted against the equivalent stresses in the dynamic creep. It was shown that the relationships between ecmln and σep.1, and tf, and σep.2 were in quite good agreement with those in the static creep.

An Interpretation of Surface Fatigue Crack Growth in Pre-tensioned Notched Cylindrical Component of Mild Steel Based on Elastic-Plastic Stress Analysis.

A stress raiser such as notches, holes and structural discontinuity is commonly contained in machine components, which reduces the fatigue life time. The extension of the fatigue life time is quite significant problem in notched components from the viewpoint of engineering. The pre-tensile loading has been extensively known as one of useful means for improving fatigue strength. The fatigue tests, however, showed that pre-tensile loading hardly influenced the fatigue crack growth rate in the notched cylindrical specimen. The stress/strain analysis was conducted for the notched cylindrical component using the elastic-plastic finite element method, where cyclic softening and variation of mean strain were taken into account. The stress/strain analysis showed that compressive residual stress attenuated around the notch root in the fatigue process. Almost no influence of pre-tensile loading on fatigue crack growth seems to be induced by such a redistribution of residual stress occurring during cyclic loading.

An Elastic-Plastic Finite Element Method Analysis of Cyclic Stress-Strain Response in the Vicinity of Crack Tip in Annealed Copper under Load-Controlled Condition.

A through-thickness center-cracked specimen of fully annealed copper was cyclically deformed under a load-controlled condition. Then cyclic stress-strain response in the vicinity of the crack tip and the crack tip opening displacement were calculated using the elastic-plastic finite element method in which the constitutive equation derived on the basis of dislocation dynamics is used. The distributions of the axial and shear strains along with changes in them and in the crack tip opening displacement during each half of a load cycle were calculated ; the results corresponded well with the measurements. In addition, the calculated stress-strain response showed that the axial strain range and the mean strain decreased to a much greater extent closer to the crack tip as the cyclic loading process progressed.

Optimum Design of Sintered Silicon Nitride Ceramics Specimen with Projections for High-Temperature Creep Test.

A tensile creep specimen with projections was designed to measure the creep displacement by means of the laser beam displacement meter at temperatures elevated beyond 1000°C. Stress/strain concentrations, however, occur around the root of projections in this specimen. The high-temperature creep strain was determined form the measurement of tensile displacement between the two projections by correcting influence of the projections. The procedure used to determine the steadystate creep constitutive equation of Si3N4 ceramics was proposed by combining a high-temperature creep test using the speciment with projections and the finite-element method (FEM) calculations for the specimen. Furthermore, the method for designing the optimum creep specimen with projections was proposed via the FEM calculations using the steady-state creep constitutive equation.