The propagation rate of distributed creep microcracks is influenced by the microstructure and the inhomogeneous local deformation, and it may not follow a definite law but depend on a probabilistic characteristic, i.e., the time and space scattering of crack initiation and the fluctuation of crack propagation rate. The crack density, n(t), and the distribution of crack length, f(c), which are closely related to the initiation and propagation of distributed creep microcracks, can be measured rather easily at any time. In this paper, a method for the estimation of the microcrack propagation, rate using n(t) and f(c) was proposed based on the supposition that no definite distribution of crack propagation rate exists. The crack propagation rate was estimated on 304 stainless steel in which surface microcracks of the size of more than one grain boundary facet were observed during creep in air and in vacuum at 650°C. The estimated propagation rate was close to the maximum crack propagation rate measured as far as the applied stress dependence was concerned. It is supposed that a better estimation is obtainable if the length of creep microcracks is measured more accurately at the early stage of the propagation.

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An accurate prediction of fatigue fracture at elevated temperatures depends on the elucidation of the small crack initiation and propagation behavior. Experimental studies on the small cracks, however, have been quite limited. In this study, two types of high-temperature low-cycle fatigue tests, C-P type of a trapezoidal stress waveform with tension hold and P-P type of a high-frequency triangular stress waveform, were conducted by using smooth specimens of 304 stainless steel, and the behavior of initiation and early growth of surface small cracks was observed continuously during the tests with a high-temperature microscope. Furthermore, a detailed study was made by the interrupted tests with the replicas or scanning electron microscopy. The results obtained are summerized as follows. (1) The crack of C-P type fatigue initiates at a grain boundary between two triple points as it opens uniformly and gradually. (2) The crack growth to the adjacent grain boundary in the early stage of C-P type fatigue is caused through the same process as the crack initiation, showing a stepwise and discontinuous manner. (3) The process mentioned at (1) is the process of growth and coalescence of voids on the grain boundary owing to the grain boundary sliding. (4) The crack initiation of P-P type fatigue takes place at the intersection of slip bands and a grain boundary, and most of the cracks propagate along the grain boundaries in the early stage of fatigue. (5) The crack growth rate of P-P type fatigue decreases suddenly when the crack tip comes near to the triple point. (6) The fracture mechanisms of C-P type fatigue and P-P type fatigue are quite alike to that of creep and that of low cycle fatigue at room temperature, respectively.

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Macroscopic through-crack propagation in ductile materials under creep conditions has been described successfully by the creep J-integral, J. This paper extended the creep J-integral concept based on the power law creep to the case of shorter cracks, and limitation of the use of J was examined. The propagation of surface cracks from a small semi-circular notch of about 0.6mm diameter and that from the surface grain boundaries of the smooth specimen with 40μm grain diameter of 304 stainless steel were observed during the air and the vacuum tests. When the crack grew to the length (depth) of 0.1mm or longer, the J approach became applicable, resulting in the same correlation between dc/dt and J as that for the macro-through-crack. Cracks less than 0.1mm, or two or three grain boundary length, on the other hand, showed higher propagation rates than that estimated by the straight line extrapolation on log-log plots of dc/dt-J. The growth of creep cavities by grain boundary diffusion was considered to be a dominant mechanism for the microscopic crack propagation. The time needed to make one grain facet fracture (crack initiation) at several surface grain boundaries can be well predicted on the basis of the model of cavity growth and coalescence proposed by Chen and Argon.

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It is well known that slow-fast or tension-hold strain waveforms, which are so-called c-p type waveforms, yield the shortest life of high-temperature low-cycle fatigue for 304 stainless steel. This may be attributed to an irreversible creep deformation such as an accumulation of grain boundary sliding in the tension going direction which results from the difference in strain rate between in tension and in compression in every cycle. This characteristic seems to be a fundamental nature of creep-fatigue being typical to many kinds of heat resistant steels. From this point of view, a monotonic tensile test was conducted under a constant strain rate being equal to the tension going strain rate of slow-fast waveform for a fatigue test using a 304 stainless steel smooth specimen, and the difference between monotonic tension and fatigue in the behavior of small crack initiation and early growth along grain boundaries on the surface of the specimen was examined. The crack initiation life was shorter and the crack density was higher in monotonic tension than in fatigue. All the small cracks were arrested with the blunting of the tips in monotonic tension, although some of the cracks propagated discretely to become large cracks in fatigue. It can be concluded that the easy crack initiation and the following crack coalescence due to local necking of the specimen in monotonic tension causes the shorter failure life as compared with the fatigue life.

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Ductility normalized strainrange partitioning (DN-SRP) life relations in a perfect vacuum were determined based on the SRP life relations experimentally obtained both in air and in an imperfect vacuum for three materials: the annealed, the normalized and tempered 21/4 Cr-1 Mo steels and SUS 304 stainless steel.
As the results, it is found that they can be given by either set of equations as follows: Δε_pp=0.5D_pN_pp^-0.5 Δε_pc=0.145D_pN_pc^-0.5Δε_cp=0.111D_cN_cp^-0.5Δε_cc=0.5D_cN_cc^-0.5} or Δε_pp=D_p^0.6N_pp^-0.6Δεpc=(0.192D_p)^0.6N_pc^-0.6Δε_cp=(0.078D_c)^0.6N_cp=0.6Δε_cc=D_c^0.6N_cc^-0.6}
It is also shown that these DN-SRP life relations can predict a shorter life in the large inelastic strain range than that predicted by extrapolation of air data. So they are important properties even in evaluating the life in air of highly strained structural parts such as notches.

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It is expected from the previous researches that the growth of many distributed cracks from the initiation to the size of several grains is the main process of creep fracture in heat-resisting alloys. Such a study on intergranular microcrack growth behaviors is important to clarify the characteristics of creep fracture.
In this study, a continuous observation of intergranular microcrack growth at the surface of smooth specimens of 304 stainless steel was made during creep tests at 650°C in air and in vacuum. It was found that the effect of environment was different depending upon the stress value. That is, at high stress, the rupture time was shortened by intergranular oxidation, and on the contrary at low stress, it was lengthened by the formed oxide film which has protective and restorative actions. Also it was shown that the microcrack growth behaviors in creep were explainable on the basis of Weibull distribution of crack length.

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The authors have reported that the fatigue crack propagation behavior at elevated temperatures can be classified into cycle dependent- and time dependent-types and that there exists an independent fracture mechanics law between the crack propagation rate and the J-integrals for each type. In this study, the fatigue life law for smooth specimens was derived by integrating the crack propagation law. They are formularized as follows;
(1) cycle dependent fatigue, ΔW_f^mfN_f=D_f
(2) time dependent fatigue, ΔW_c^mcN_f=D_ct_o^1-mc where ΔW_f and ΔW_c are strain energy parameters, N_f; number of cycles to failure, m_f, m_c and D_f, D_c; constants, t_o; tension time (see Fig. 1). Here, it should be emphasized that ΔW_f and ΔW_c are functions of not only the plastic or the creep strain range but also the stress range, the maximum stress and the elastic strain range.
The validity of the above fatigue life law was discussed using the elevated temperature fatigue test data of three alloys; 2¼Cr-1Mo steel in which the effect of strain waveform on the fatigue life is known to be small, 304 stainless steel which shows the life dependence on dynamic aging, and Inconel 718 which is one of the low ductility superalloys. As a result, the fatigue life was correlated well with ΔW_f for the cycle dependent fatigue and with ΔW_c for the time dependent fatigue for all the materials.

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In order to improve creep-fatigue life prediction technology based on the crack growth of 2¼Cr-Mo steel casting, an investigation was carried out to predict the cycle-to-failure of smooth specimens with the fatigue-crack-growth-rate data obtained with CT specimens, as well as to comprehend the crack growth behavior with fatigue tests and creep-fatigue tests.
The results obtained are as follows:
(1) The micro-crack-growth-rate property of the casting material can be evaluated with the growth-rate data obtained by the macro-crack-growth tests with CT specimens.
(2) There is good correlation between the behavior of the main crack growth and the life ratio for the surface crack growth behavior. In addition, the crack initiates at a relatively early stage of N/N_f=0.15 and the most part of the life is occupied by crack growth.
(3) The calculated growth curves of the micro-surface-cracks correspond comparatively well with the observed crack growth behavior. Thus, even when the crack propagation life is considered to govern the total life, the cycles-to-failure of the specimen level can be predicted accurately within a factor of 2.

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In order to investigate the behavior of small crack initiation and growth in creep-fatigue at high temperature, c-p type fatigue (slow tension and fast compression) tests were carried out using smooth specimens of Type 304 stainless steel, 21/4Cr-1Mo steel and Ti-17 titanium alloy. Their average grain sizes were 50, 20 and 5μm, respectively. The results obtained are summarized as follows:
(1) Multiple small cracks initiated and grew along the grain boundaries on the specimen surface.
(2) The increasing rate of crack density showed stronger dependence on the strain range than the crack growth rate.
(3) Remarkable fluctuation of the crack growth rate, which is characteristic of microstructurally small cracks, was found for the cracks of the half length being smaller than 4 or 6 times of the average grain diameter.
(4) The average growth rate of the small cracks coincided with the rate extrapolated from the large crack growth law.
(5) The normalized relation between crack growth rate and crack length, being available for representing the region of microstructurally small cracks, was successfully derived on the basis of large crack growth law and average grain diameter of materials. This relation was independent of material and strain range.

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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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The surface small crack growth behavior of SUS304 stainless steel under fast-fast and slow-fast strain controlled low-cycle fatigue conditions was investigated at 873K with the specimens having different grain sizes. It was shown that the cumulative probability of crack length distribution in both fast-fast and slow-fast cyclings could be represented by a Weibull distribution as a function of crack length in the early stage of fatigue life. In the stage after the half fatigue life, the cumulative probability of crack length distribution showed a knee, and a small number of main cracks grew predominantly. The knee occurred when the crack length was comparable to one grain size. It was also shown that the surface small crack growth rate showed the minimum value when the cracks arrived at the grain boudaries. The above behavior resulted from that the grain boundaries temporarily impeded the small crack growth. Furthermore, the small crack growth rate was compared with the macroscopic crack growth rate. The law for the macroscopic crack growth was applicable to the small crack growth in slow-fast cycling, up to the crack length being one grain size. On the other hand, the small crack growth rate in fast-fast cycling was about ten times as large as the macroscopic crack growth rate in the region where the crack length was smaller than or comparable to the grain size.

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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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この研究の目的は歯牙喪失状態が情動行動に及ぼす影響を検討することである.研究方法は抜歯老齢ラットと対照としての非抜歯老齢ラットを用い日内活動性試験および情動性試験を行った.対照群に比べて臼歯喪失させたモデルである抜歯群では日内活動量が有意に減少した.また情動性試験により抜歯群に情動性の変化が観察された.以上臼歯抜歯老齢モデルにおいて夜行性行動の障害, 自発性の低下, 不安の増加などの痴呆の周辺症状が観察されたことから長期の歯牙喪失状態が痴呆の危険因子の1っとなる可能性が示唆された.

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