耐熱ばね用として開発された窒化珪素の静疲労試験は平滑材および表面き裂材を用いて, 1200℃, 大気中で3点曲げ試験により行われた。 また, 室温および高温 (1100-1200℃) での破壊応力のき裂寸法依存性を調べた上, 本供試材へのプロセスゾーン寸法破壊基準の適用性について検討した。 その結果, 室温はもちろん高温においても, 有効であることが判明した。
本実験において, 微細構造の制御により高強度の, 柱状粒子によるき裂の偏向と架橋効果により高じん性窒化珪素セラミックスが得られた。 試験片表面での半円き裂長さが650μmの時静疲労限度は120MPaであった。 その時の疲労寿命は10⁷秒であり, 報告されているそれより長かった。 このような静的試験および静疲労試験の結果, 本窒化珪素は耐熱用ばね材料として有用なものと判断された。

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The remaining life evaluation is conducted on PP tested and CP tested 316LC steel by using the two procedures previously proposed for Mod.9Cr-1Mo steel by the authors. The creep-fatigue damage rules determined by the authors for 316LC steels are used, where the crack initiation period cannot be neglected in CP type straining as well as in PP type straining.
As the results, it is found that both the proposed procedure 1 and 2 can estimate the remaining life, the material damage and the applied inelastic strain range with better accuracy in case of 316LC steel than in case of Mod.9Cr-1Mo steel. Among the two the procedure 1 is superior in prediction accuracy to the procedure 2, where the former needs the measured surface crack length at a given total number of cycles while the latter the measured surface crack growth rate. Especially satisfactory results are obtained when the procedure 1 is adopted and the measured surface crack length is 300μm or longer. That is, the ratio of the predicted value to the actual one is ranged below 1.1 for the remaining life and from 0.9 to 1.3 for both the material damage and the applied inelastic strain range.

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The test technique for measuring Young's modulus for ceramics at elevated temperature was studied. (1) Test technique for measuring Young's modulus at elevated temperature was developed by using the impact sound. (2) The eigenvalues increased monotonically with an increase in slenderness ratio. The difference in eigenvalues due to the slenderness ratio, was more remarkable in higher modes. The eigenvalues were not affected by the Poisson's ratio. (3) The Young's modulus of the SiC and Si₃N₄ did not depend upon the elevated temperature. The Young's modulus of Al₂O₃, AlN, ZrO₂, CrC and TiN decreased with an increase in temperature. The Young's modulus of the composite ceramics Si₃N₄/BN increased slightly with increasing temperature.

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The idea and procedures are presented for evaluating the remaining life based on a new creep-fatigue damage rule. The proposed method needs only information about crack length at a given number of cycles or crack length change during a given cycles interval and F_ij, the ratio of the partitioned inelastic strain range to the total inelastic strain range. It is shown that the material damage and the applied inelastic strain range as well as the remaining life of IJ tested Mod.9Cr-1Mo steel can be evaluated when the difference between the measured crack length and the hypothetical initial crack length or the measured crack length change is greater than 30μm, and that the respective accuracy in predicting each of the remaining life, the applied inelastic strain range and the material damage is within about factor of 3-4, 2.5-2.8 and 1.5-3.

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A method for dynamic strength evaluation of ceramic beams containing small flaws was studied. (1) The critical frequency range was obtained with evaluating the strength of beams on the basis of the static-fatigue limit. (2) The critical frequency range increased slightly, with an increase of the equivalent crack length. (3) With an increase in the ratio of the breadth and depth, the eigenvalues increased, except for the fundamental eigenvalue. The eigenvalues increased monotonically, with an increase in the slenderness ratio and the stiffness of support. (4) For an increase in the damping ratio, the critical frequency range was constant, and then decreased. The fundamental critical frequency range was not affected by the damping ratio.

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A rod type and a cylinder-rod type load supports are developed for superconducting magnets of magnetic levitation vehicles. The load supports act as thermal insulators and, at the same time, are required to sustain vibrating loads. The supports are made of Al₂O₃FRP and have joints contacting with metal sleeve. The material of rod is unidirectional Al₂O₃FRP, while that of cylinder is satin woven laminating Al₂O₃FRP. The results obtained in this study on the mechanical behavior of the load supports are as follows:
(1) The unidirectional Al₂O₃FRP rod and the metal sleeve can be tightly joined by pulling them to the opposite directions, each of which has a matching taper face with solid lubricant before assembling.
(2) The impregnation resin of Al₂O₃FRP should have a high glass transition temperature, i.e. 453K, for reverting a decrease in joint force due to stress relaxation of the resin at room temperature. By giving a sufficiently large circumferencial elastic strain to both the rod and the sleeve than the cooling thermal strain of Al₂O₃FRP, high joint force remains even at a cryogenic temperature and the joint can sustain high vibrating loads.
(3) The replenishment resin, which forms a wedge shape at both ends of cylinder in the wedge shape groove of the metal sleeve, should have the glass transition temperature of 398K, so as to prevent the joint from stress relaxation at room temperature. By curing the replenishment resin in the cylinder-rod type load support, the stress relaxation in the rod is kept as low as 10% because of high enough glass transition temperature of rod resin.

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Low-cycle fatigue tests were carried out on copper alloy and SUS 304 steel, to observe the initiation of fatigue cracks in smooth specimens and the subsequent growth of the micro-cracks. Their surface crack lengths were examined using cellulose acetate replicas. The results obtained were as follows:
(1) Initiation of a main crack which causes the failure of smooth speimens occurred at the early stage of their lives. The crack initiation cycle ratio N_c/N_f was small for SUS 304 steel in the low-cycle regime, but in the high-cycle regime for copper.
(2) The crack initiation site was almost near the grain boundary triple point and the subsequent growth rate was high at the early stage. The initiation at the slip bands was rarely observed and the crack length did not change for a while.
(3) The experimental crack growth rate was not well correlated in fracture mechanics fashion by ΔK or ΔJ. The crack growth rate at the stable stage was propotional to the crack length, so the nondimensional fatigue crack growth rate k=1/a_e·da/dN represented the data well in this study.
(4) The nondimensional fatigue crack growth rate k was closely connected with the failure lives N_f of smooth specimens, in such a way as k=(9-10)/N_f.
(5) An expression for the damage in low-cycle fatigue was derived on the basis of micro-crack growth behavior.

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To assure high-temperature component safety and reliability, a life prediction method has been developed. The method is based on the micro-crack growth rate and the microcrack distribution.
Strain controlled low-cycle fatigue tests were carried out on the plain specimens of SUS 316 at 923K in air and the initiation and growth behavior of surface micro-cracks were observed.
The results obtained are summarized below.
(1) The creep-fatigue life is governed by micro-crack growth.
(2) A linear relationship is observed between the logarithms of main surface crack length and the life ratio, N/N_f, for various strain loading conditions.
(3) The distribution of micro-crack length obeys the Weibull distribution.
Moreover, the maximum crack length in an area larger than the sample area can be estimated with a help of the statistics of extremes. Therefore, the remaining life of high-temperature components may be accurately predicted by measuring the distribution of surface micro-crack length in critical parts.

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A life assessment method based on micro-damage mechanisms for high temperature components is presented. This method is emphasized on distributed micro-cracks and micro-voids behavior which nucleated and grew up to macro-crack initiation in the components. A growth law of these micro-damage and a statistical treatment for the distributed micro-cracks are also considered in this life assessment procedure. microvoid density as creep damage is investigated for non-destructive damage detection methods. A replication technique using the grain boundary etching method as NDE of material embrittlement is researched and developed.
CrMoV steels including a long term used materials are investigated on micro-crack and micro-void nucleation and growth behavior. Based on the experimental results and the damage mechanisms, a residual life assessment method is discussed.
These results verified the damage of the components would be accurately assessed by the method based on the micro-damage mechanisms.

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In order to clarify the validity of the previously proposed creep-fatigue life prediction model based on the strain range partitioning concept, the applicability of the proposed model to type 316LC steel was examined. Two-step variable PP type straining (high-low and low-high) tests were conducted at 700°C, and the experimental results were compared with those predicted by the proposed model. In both high-low and low-high tests, 1.0% was chosen as the higher strain range, and 0.8%, 0.4% and 0.3% were chosen as the lower strain ranges. Furthermore, PP type small crack growth behavior in smooth bar specimens was observed at 700°C under constant strain range by the surface replica technique and compared with that predicted by the proposed model.
The high-low and low-high test results could be predicted by the linear damage rule (LDR) when the lower strain range was 0.8%. On the other hand, when the lower strain range was 0.4% or 0.3%, the deviation of experimental data from the LDR prediction was found. That is, the sum of the life ratio of the first and second step straining was smaller than unity in the high-low tests, whereas it was larger than unity in the low-high tests.
These experimental results were well explained by the proposed fatigue life prediction model, in which it was assumed that the crack initiation life is not negligible when the inelastic strain range is smaller than the critical strain range, (Δε_pp)_cr, and it was found that the proposed model was successfully applied to type 316LC steel. The value of (Δε_pp)_cr was 0.23%. The experimentally obtained small crack growth curves were well predicted by the proposed model using the correction factor, with which the macro through crack growth rate in the CCT specimen was converted into the small surface crack growth rate in the smooth bar specimen.

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Strain controlled time-dependent fatigue tests were carried out on plain specimens of 304 stainless steel at 550°C in air and the initiation and propagation behavior of main cracks were observed. The strain wave shapes were widely varied, including one day hold at tensile peak strain.
The slow strain rate and long hold time in tensile strain enhanced both crack initiation and propagation, while those in compressive strain did not affect them, because the time-dependent fatigue in the test condition was dominated not by creep damage but by environmental oxidation. A good correlation existed between the period of tensile straining in one cycle τ_t and the fatigue-oxidation interaction characteristics under various strain wave shapes. The number of cycles to initiate 0.05mm long crack N_c0.05, the number of cycles to failure N_f and the fatigue crack growth rate were proportional to the power of τ_t when τ_t was longer than about 100sec.
The value of N_c0.05 was frequently less than 10% of N_f and the crack propagation behavior governed the failure cycles. Although the fatigue crack growth rate da/dN was large when the crack was smaller than a grain diameter, 0.05mm, it was proportional to the effective crack length a_e for large crack sizes at each test condition. The normalized fatigue crack growth rate k[=(da/dN)/a_e] can be obtained from N_f of a plain specimen through the relation expressed as k=10/N_f.

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In order to clarify the validity of the previously proposed creep-fatigue damage rule, in which small crack growth curves are predicted by the crack growth rate equations based on the strain range partitioning concept and used as the damage accumulation curves, the correspondence between the predicted small crack growth behavior and the experimentally obtained behavior was examined. The small crack growth behavior of smooth bar specimens in PP, CP, PC and CC tests was observed for Mod. 9Cr-1Mo steel at 600°C by the surface replica technique, and compared with that predicted by the proposed damage rule. The tests were conducted under constant strain ranges, where 1.0% was chosen as the high strain range and 0.3%-0.5% were chosen as the low strain ranges.
The predicted small crack growth curves well corresponded with the experimentally obtained relationship between the maximum crack length and the life ratio except for the CP, PC and CC type low strain ranges. The deviation between the prediction and experiment for the CP, PC and CC type low strain range was found to be decreased by modifying the initial crack length in the proposed damage rule and by taking the crack initiation life into consideration like the case for the PP type low strain ranges.

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Creep-fatigue rupture life and its elastic stress concentration dependence for circumferentially notched 1CrMoV steel bars have been investigated. The elastic stress concentration K_t dependence of creep-fatigue life under 30 minutes tensile hold cyclic loading at 550°C for the long term used material was different from the unused material. The notch strengthening behavior was observed on the both materials as in the case of creep loading conditions. The maximum rupture life was observed at K_t=1.9 for the unused material, while it was at K_t=2.6 for the long term used material.
The distribution of hardness across the minimum notch section corresponded to the distribution of cavity density. Many cavities were observed at the softening part. The hardness of the long term used material was lower than the unused one.
The fracture strain of the notched specimen under creep-fatigue loading had an inversely proportional relationship with the rupture life. The high triaxial stress state specimen of the unused material (K_t=2.6 or 3.5) exhibited low fracture strain in the intermediate stress range, about a half of the strain of others. The fracture strain of the long term used material was higher than the unused one. In the lower stress range, the fracture strain of the notched specimen was relatively low for both of the materials. Bridgman effective stress which considers the stress triaxiality seemed useful to well evaluate the rupture life of circumferentially notched specimen under creep-fatigue conditions for both the long term used and unused materials.

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A test technique for loading the arbitrary stress and a method for strength evaluation were studied to develop a reliability testing method. (1) The test technique for loading arbitrary stress was developed by using thermal stress. (2) The thermal stress reached a maximum immediatly after cooling, and then decreased monotonically. (3) The maximum thermal stress increased with an increase of both temperature and thickness of specimen. (4) The strength of brittle materials was evaluated by using a relationship between temperature and the maximum stress.

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