Search

The elasto-plastic constitutive equation based on the concept of continuum damage mechanics is extended to cover the creep behavior for the gray cast iron as used in diesel engine parts. It employs the creep strain given by Perrin and Hayhurst and the creep damage given by Dunne and Hayhurst. Lots of material parameters in the constitutive equation are determined by using the uniaxial tensile/compressive, the creep, the relaxation, and the fatigue test results at 400°C. The identified model is used to predict the creep fatigue life with 600 sec hold-time at the maximum compressive strain. The validity of the proposed model is illustrated by comparing the calculated results with the experimental results. It is expected that the developed computational modeling can be used effectively to predict the creep fatigue life of gray cast iron machine parts.

Creep-fatigue crack propagation tests using type 304 stainless steel have been carried out under intermediate tensile load hold during an unload process in order to study the effect of the load hold position. A finite element analysis was carried out to investigate the fracture mechanics parameter. The results obtained were as follows. (1) As for the finite element analysis, the application of the fracture mechanics parameter, creep J-integral J′, was found to be effective under the intermediate tensile load hold condition as well as under the maximum load hold condition. (2) The crack propagation rate was correlated well with the fracture mechanics parameter, creep J-integral range (ΔJc), under the intermediate tensile load hold condition as well as under the maximum load hold condition.

Damage evaluation method for 316FR stainless steel based on micro crack growth behavior has been investigated to improve the life prediction method for high temperature plant. Micro-crack growth was observed in the early stage of life in the coarse grain material even at lower strain ranges. On the other hand, crack growth started in the later stage of the life at lower strain ranges in the fine grain material. The initiation life of 0.2mm length micro-crack could be evaluated by a creep-fatigue damage estimation method using the ductility exhaustion method. In this evaluation, initiation life of 0.2-mm-length micro crack in pure fatigue and creep test was used to calculate the fatigue and creep damage. Crack growth life was also able to predict by a crack growth analysis based on nonlinear fracture mechanics. The failure life of the specimen was determined by the summation of initiation and growth life and this prediction was confirmed to give a reasonable accuracy. Micro-crack initiation and growth is the main mechanism of damage in creep fatigue and lives of real components strongly depend on their allowable crack size. Therefore, the life prediction method will be rationalized by relating the life or damage evaluating in the method with micro crack length as mentioned in this study.

The effect of prior creep-fatigue damage on the creep and the fatigue characteristics was studied to investigate the creep-fatigue life evaluation procedure of 316FR stainless steel. Creep and fatigue tests were conducted at 550°C by using the specimen exposed to prior creep-fatigue cycles at the same temperature and interrupted at 1/4 Nf, 1/2 Nf and 3/4 Nf cycle. The creep and fatigue strength of the pre-damaged material showed monotonic reduction with the prior creep-fatigue damage compared with the virgin material. The creep ductility also showed monotonic reduction with the prior creep-fatigue damage. These results were evaluated by the stress-based Time Fraction Rule and the strain-based Ductility Exhaustion Method. The result showed that the application of the Ductility Exhaustion Method to the creep-fatigue damage evaluation is more promising than the Time Fraction Rule.

The effect of prior fatigue damage on the creep characteristics of 316FR stainless steel was studied to investigate the applicability of two creep-fatigue life evaluation procedures, Time Fraction Rule and Ductility Exhaustion Method, widely used in the recent high temperature structural design codes. Creep tests were conducted at 550°C by using the specimens exposed to prior fatigue cycles at the same temparature and interrupted at 1/4 Nf, 1/2 Nf and 3/4 Nf cycles. The creep strength of the pre-damaged material showed almost no reduction compared with the virgin material until the fatigue damage of 1/2 Nf cycles, and simple summation of creep damage and fatigue damage by using Time Fraction Rule was not formed. On the other hand, the creep ductility of the predamaged material showed monotonic reduction with the prior fatigue damage from the initial stage of the damage, and the relationship between creep damage and fatigue damage evaluated by the Ductility Exhaustion Method was almost linear. These results show that the application of the stress-based Time Fraction Rule is unsuitable and the application of the strain-based Ductility Exhaustion Method is promising to the exaluation of creep-fatigue interaction damage of the 316FR stainless steel.

A study was conducted to evaluate the crack propagation behavior in a surface cracked pipe at elevated temperatures. The fatigue and creep-fatigue crack propagation tests were carried out for 316FR steel pipe (base metal pipe and weldment pipe) under four point bending at 650°C. Fracture mechanics parameters. fatigue J-integral range (ΔJj) and creep J-integral range (ΔJc), were estimated by the simplified analysis method based on the reference stress approach. The estimated crack propagation behavior was compared with the experimental results. It was confirmed that the proposed method in this paper gave a good estimate.

Torsional fatigue tests and creep tests were carried out on a 60Sn-40Pb solder. In both tests three temperatures (303∼333K) were used, and in the fatigue test five cycling frequencies (0.001∼5.0 Hz) were used. An evaluation method for the solder fatigue life under relatively high temperature or low cycling frequency was proposed from the test results, noting that the characteristics of fatigue under the above mentioned conditions were similar to the creep characteristics. Furthermore, this evaluation method was applied to the estimation of the crack initiation life of through-hole-type joints. By comparing the estimated values with the experimental ones, it was clarified that this method was valid for the assessment of the crack initiation life of solder joints.

Creep-fatlgue tests were conducted using 316FR stainless steel at 650°C in air under a variety of strain time programs including symmetric continuous, unsymmetric continuous and strain hold cycles. By comparison of the results with test data obtained in vacuum, it was found that the life reduction which occurred for a slow tension-fast compression strain wave and the tensile strain holding wave was smaller in air than in vacuum, and that the symmetric continuous cycle gave the largest life reduction from the data obtained in vacuum. Following the creep-fatigue tests, fracture surfaces were investigated by SEM. When time/rate-dependent life reduction occurred, the fracture mode in vacuum was intergranular, but in air, striations were observed and integranular fracture was rarely observed.

The major factor dominating creep and creep-fatigue failure at high temperature for SUS304 stainless steel is the nucleation and growth of creep cavities on grain boundaries. In this study, previously proposed cavity growth models for creep conditions were expanded to give models for creep-fatigue condition. The analytical cavity growth rate was compared with experimental data obtained in our prevlcus study. As a result, it was found that the cavity growth process under stress-controlled creep-fatigue conditions continuously observed by the SEM could be explained using the cavity growth rate equation consisting of the sum of constrained diffusion-controlled and deformation-controlled growth rates. Also, a cavity growth mechanism map, which can be applied to various load patterns, was developed based on the models for cavity growth in creep conditions.

Structural components operating at high temperatures in power plants are subjected to interaction of thermal fatigue and creep which results in creep-fatigue damage. In evaluating the life of those components, it is important to understand microscopic damage evolution under creep-fatigue conditions. In this study, static creep and creep-fatigue tests with tensile hold-time were conducted on a SUS 304 stainless steel by using a high-temperature fatigue testing machine combined with a scanning electron microscope (SEM), and cavity growth and coalescence behaviors on surface grain boundaries were observed continuously by the SEM. Quantitative analysis of creep cavity growth based on the observations was made for comparison with theoretical growth models. As a result, it was found that cavities nucleate at random and grow preferentially on grain boundaries in a direction almost normal to the stress axis. Under creep condition, the cavities grew monotonously on grain boundaries while remaining an elliptical shape. On the other hand, under creep-fatigue conditions, the cavities grew due to the effect of the local strain distribution around the grain boundary due to cyclic loading and microcracks of one grain-boundary length were formed by coalescence of the cavities. Also, cavity nucleation and growth rates under the creep-fatigue condition were more rapid than those under the static creep condition and the constrained cavity growth model coincided well with the experimental data for creep.

A series of creep-fatigue tests was conducted with Modified 9Cr-1Mo steel at 600°C in 0. 1 mPa vacuum to assess the accumulation of creep-fatigue damage. In these tests, each test specimen was subjected to prior creep-fatigue loading followed by subsequent fatigue loading or prior fatigue loading followed by subsequent creep-fatigue loading. The linear summation of cumulative damage of fatigue and creep life fraction was smaller than unity for the former case, and larger than unity for the latter case. The damage model based on the overstress concept was applied to the test results to verify its applicability. The accuracy of the life evaluation was not enough because of the nonlinear interaction between creep and fatigue.

Life prediction methods of creep fatigue were investigated to consider applications to Hastily XRs at very high temperatures where materials would be damaged severely by creep deformation. The ductility exhaustion rule, coupled with a creep constitutive equation consisting of primary and secondary stages, showed a good prediction of creep-fatigue life as well as the damage-rate equations. The strain range partitioning method was also discussed regarding its possibility for life prediction. The results of these methods were compared with the time fraction rule in conjunction with the Miner's rule. In these experiments, creep tests were done under constant stress condition in air, and low. cycle fatigue tests were carried out with and without a dwell period in vacuum. Creep-fatigue damage was evaluated in the life of low-cycle fatigue tests with a dwell period. All specimens were fabricated from the same hot-rolled plates to reduce scatter in different heat treatments.

The evolution and healing process of creep-fatigue damage are discussed on the basis of the experimental observation of cavities and small cracks. Multiple cavities and small cracks are formed along grain boundaries inside a smooth specimen of a Cr-Mo-V steel in creep-fatigue with slow-tension and fast-compression cycling at the relatively high temperature of 923 K. The cracks begin to appear at the middle stage of life, while the cavities initiate in the very early stage. The failure is brought about by the coalescence of cracks in the late stage. It is also found that these cavities and cracks caused by the diffusive creep in tension disappear during successive compression creep in creep-fatigue with fast-tension and slow-compression cycling. The failure life of the specimen can be extended by the healing cavities and cracks.

Microstructural randomness of materials causes the fluctuation of initiation lives and growth rates of small cracks during high-temperature creep and creep-fatigue. The crack initiation life is especially strongly dependent on the grain-boundary inclination to the stress axis. Taking into account the effect of random grain-boundary inclination, the basic probabilities of initiation are analyzed by a convolution integral method on the basis of the three-dimensional stochastic model. Any probability of crack inclination and initiation life can be calculated from the basic probabilities. Excellent coincidence is obtained from the comparison between the analyses and experimental data of Type 304 stainless seeel under the creep-fatigue condition.

The creep-fatigue life prediction method based on the overstress concept is exteded for life evaluation under the biaxial creep-fatigue condition. The time-independent damage parameter is described by two geometically different damage parameters, one of which represents the fatigue damage in terms of a resolved overstress, and the other represents the same in terms of the overstress mormal to it. The time-dependent damage parameter is described by the maximum principal overstress. A strain rate effect is also introduced into the multiaxial creep-fatigue damage model in order to evaluate the test results with an environmental effect. The presented theory is applied to the biaxial creep-fatigue test results which were previously reported by the authors, and it is shown that the theory has a great applicability to predict the biaxial creep-fatigue life with an environmental effect within the predicability of a factor of 2.

The accurate prediction of the fatigue fracture at elevated temperature will depend on the elucidation of the small crack initiation and propagation laws. In this study, smooth specimens of a 304 stainless steel were subjected to the slow-fast cyclic strain of 1% range at 650°C in air and in a vacuum (about 10-2 Pa), respectively. All small cracks on the fixed surface of a specimen were observed with an optical microscope, and their propagation curves and propagation rate were mesured in detail. The surface length and the depth of small cracks covered a wide range, from 10μm to 1 mm. The propagation behavior was classified into two types : propagating cracks with coalescence ; and non-propagating cracks blocked by grain boundary triple points. On the other hand, the statistical treatment of the crack propagation rates showed that the mode of the distribution of their rates agreed with the through macro-crack propagation law, but that their upper and lower limits largely deviated beyond the bounds of the factor of two on the macro-crack propagation rate.

This paper describes the accuracy of FEM analysis for the estimation of local strain at the notch root in creep-fatigue conditions, comparing the local strain calculated by FEM with the measured strain using high temperature strain gages. The specimens are commercial pure aluminum notched plates with both side grooves whose elastic stress concentration factors are 0.5 and 2.0. The test was made at 473 K in air. The following results are obtained. In no-hold time fatigue tents, FEM analysis accurately estimates the local strains at the notch root. In no-hold time fatigue tents, FEM analysis accurately estimates the local strains at the notch root. In the hold time test, on the other hand, FEM analysis estimates a smaller strain range compared with the actual strain range occurring at the notch root. The reason for the disagreement between FEM analysis and the actual strain results from the change in material response due to creep loading. In the creep-fatigue condition, so-called plastic-creep interaction, the FEM analysis does not take account of, occurs. The local strain calculated from conventional methods, Neuber's rule, Stowell-Hardrath-Ohman, Koe's methods are also compared with the strain measurement for no-hold time tents. All the methods overestimate the strain concentration at the notch root.

2・1/4 Cr-1 Mo鋼(550℃), SUS 304鋼(600℃)を対象として切欠試験片のクリープ疲労試験を行いき裂発生寿命を求めた.併せて,平滑材の疲労およびクリープ破断試験を行うとともに,切欠試験片の非弾性解析を実施して,き裂発生寿命の推定を試みた.その結果,推定寿命と実験寿命は良好な対応を示し,本研究で採用した非弾性解析手法,解析に使用した材料特性,および寿命評価手法の妥当性が確認できた.