Nobuhiro Isobe

Life prediction of 316FR stainless steel under creep–fatigue loading with elastic follow-up

Abstract Creep–fatigue tests of low-carbon and medium-nitrogen type 316 stainless steel including elastic follow-up were carried out and the effect of elastic follow-up on the creep–fatigue life was discussed. Crack growth was observed throughout the life and it was faster in elastic follow-up tests than in strain-controlled tests. Creep–fatigue life prediction was made by estimating inelastic strain behavior during displacement holding. The ductility exhaustion rule gave good life prediction, whereas the time fraction rule was non-conservative as the failure life became longer. The ductility exhaustion rule is recommended for estimating creep damage under conditions of creep–fatigue loading with elastic follow-up.

Experimental Investigation of Strain Concentration Evaluation Based on the Stress Redistribution Locus Method

Evaluating the local strain in structural discontinuities is an important technology in high temperature design of fast reactor components because the failure mode in high-temperature fatigue or creep fatigue damage usually results from the crack initiation and growth from such locally high strained areas. A rationalized method of evaluating strain concentration that was experimentally studied is discussed. The stress redistribution locus (SRL) method had been proposed to improve the accuracy with which local stress and strain can be evaluated in the structural discontinuities. This method is based on the concept that the locus of stress redistribution from an elastic to an inelastic state, or that during relaxation, strongly depends on the structure, and the locus almost coincides with the locus obtained through elastic-creep analysis. High-temperature fatigue tests of circumferentially notched specimens were conducted accompanied by the measurement of local strain carried out with a capacitance-type strain gauge. The measured strain was compared with the predictions made with SRL, and the methods accuracy was evaluated. SRL improved the accuracy of inelastic strain estimation while remaining reasonably conservative in comparison with Neubers rule, which is used in high temperature design codes.

Life Management System for Hot-Gas-Path Components of Gas Turbines

Recently the number of gas-turbine-powered combined-cycle plants has been increasing because of their efficiency and environmental compatibility. Gas turbine operating conditions are severe, especially for hot-gas-path components. To improve the reliability of such components and to extend their life, we have developed a life management system based on a residual-life-assessment method. The system makes possible integrated residual-life-assessment based on numerical analyses, material destructive-tests, nondestructive inspections, statistical analyses of field machine data, and the use of a database. To develop the system, the primary damage mechanism for each component is clarified and material degradation is evaluated. For nozzles, the system describes a method of predicting the maximum surface crack growth. The validity of the methods is verified by assessment of the inspection data. This paper also describes optimization of operating cost and RAM (reliability, availability and maintainability).Copyright

Multiaxial Low Cycle Fatigue for Ni-Base Single Crystal Super Alloy at High Temperature

This paper presents multiaxial low cycle fatigue lives of YH61 Ni-base single crystal superalloy at elevated temperature. Tension-torsion low cycle fatigue tests were performed using hollow cylinder specimens with aligning the specimen axis to [100] direction at 1173 K. Biaxial tension-compression low cycle fatigue tests were also performed using cruciform specimens whose x and y axes were aligned to \(\langle 110\rangle \) directions. The effect of strain multiaxiality on the multiaxial low cycle fatigue lives was discussed. Couples of multiaxial stress and strain parameters were applied to the experimental data for correlating the multiaxial low cycle fatigue lives of both types of the specimens. The strain parameters yielded a large scatter in the correlation and Mises equivalent stress was only the parameter to give a satisfactory correlation of multiaxial low cycle fatigue lives of the superalloy.

Fuzzy Logic Combined Logistic Regression Methodology for Gas Turbine First-Stage Nozzle Life Prediction

Significant aspects of intelligent maintenance include the abilities to diagnose impending failures, prognose the remaining useful lifetime of the process and schedule maintenance operations so that uptime is maximized. Prognosis is probably the most difficult of the three issues leading to total intelligent maintenance. This paper describes a fuzzy logic combined logistic regression method of fatigue severity assessment and remaining useful life prediction of gas turbine hot components. Logistic regression method is proposed to derive fuzzy logic rule base using historical maintenance running records and engineers’ experience. Implementation of the prognostic methodology presents a great opportunity to significantly enhance current engine health monitoring capabilities and risk management practices.

High Temperature Multiaxial Creep-Fatigue Life Prediction for YH61 Nickel-Base Single Crystal Superalloy

This paper discusses an evaluation method of creep-fatigue lives of YH61 single crystal superalloy under multiaxial loading at high temperature. Three types of creep-fatigue tests were performed using three types of the single crystalsuperalloy specimens at 1173K. They were push-pull tests using solid bar specimens, tension-torsion tests using hollow cylinder specimens and biaxial tension-compression tests using cruciform specimens. Anisotropic strain and Mises stress in combination with frequency modified fatigue equation were applied for evaluating the creep-fatigue lives in the three types of tests. The former parameter gave a relatively large scatter but the latter parameter a small scatter in the correlation.

A Comparative Study of Negligible Creep Curves for Rational Elevated Temperature Design

In Japanese elevated temperature standard, creep considering design is required for all ferrite steels applied over 375°C and all austenitic stainless steels applied over 425°C regardless of the operating time. On the other hands, ASME Sec.III Subsection NH, RCC-MR and R5 provide the additional rules to determine the negligible creep range. In those standards, each material is evaluated as non-creep considering design region, although there are varieties of applicable materials and the rules to settle the negligible creep range in each standard. 316FR and Mod.9Cr-1Mo are candidate materials of Japan sodium-cooled fast reactor (JSFR), and those high creep resistant properties extend the negligible creep damage area over the conventional temperature limits. Extension of non-creep design area widens design windows and simplifies the creep analysis procedure. To reply those requirements, authors already proposed original negligible criterion and discussed about it. In this paper, we recall the backgrounds of the negligible creep criterion which have already been proposed. Then the negligible creep criterion and relating property in each standard were compared. For estimating the evaluation procedure of each criterion, the common material properties used in “Elevated Temperature Structural Design Guide for Commercialized Fast Reactor (FDS)” were applied to each standard’s criteria. All standards have the negligible creep curves/regions for type 18Cr-8Ni steels and type 18Cr-12Ni-2.5Mo steels, although ASME Sec.III Subsection NH defines just the criteria of negligible creep for the rule of inelastic strain limits. On the diagram of temperature-negligible creep time, the negligible creep curves of 316L(N)(1S) in RCC-MR and R5 exist between those of SUS316 and 316FR in FDS. The negligible creep regions defined in all standards are similar for austenitic stainless steels, although those criteria are different. Comparison of the negligible creep curves by each criterion with FDS’s material properties indicated that the criterion in FDS provides the most conservative curve. In case of Mod.9Cr-1Mo steel, FDS and R5 provide relationship between temperatures and time for estimating the negligible creep time. ASME Sec. III Subsection NH provides only procedures and has no practical allowable values, and RCC-MR doesn’t have the negligible creep curve. Comparison of the negligible creep curves in each criterion with FDS’s material properties indicated that FDS’s criterion allows the longest negligible creep. The negligible creep criteria in ASME Sec.III Subsection NH, RCC-MR and R5 are not practicable for Mod.9Cr-1Mo. On the other hands, FDS criterion raises the temperature limits from conventional 375°C to about 425°C even when the components designed lifetime is 60years. Sensitivities to the difference of criteria and material properties were discussed and concluded that negligible creep curve is strongly dependent on the combination of criteria and material properties. Some evaluations proved that the negligible creep curves in FDS are moderately conservative and practicable.Copyright

Development of High Chromium Steel for SFR in Japan and Creep-Fatigue Assessment of the Welded Joint

This paper describes the provisional material specifications of the high chromium (Cr) ferritic steel for the Sodium cooled Fast Reactor (SFR) and development of creep-fatigue assessment procedure for the welded joint made of the steel. Based on the test results, it was revealed that tungsten (W) should be diminished to achieve better creep-fatigue strength and toughness after long term aging at elevated temperature. Metallurgical examinations using a scanning electron microscope showed that W precipitated on the grain boundaries as “Laves phase” during aging process. The toughness of the steel which contained much W might be degraded by such coarse precipitations on the grain boundaries. As a result, provisional specifications of the high Cr ferritic steel for SFR pipes and tubes were proposed. Creep-fatigue strength assessment procedure for the welded joints made of the steels was also investigated. An assessment procedure using 2-element model was proposed and verified by comparing with some creep-fatigue test results. The creep-fatigue lives observed in the experiments were well predicted by the proposed assessment procedure, but the failure of the welded joints really occurred in the heat affected zone (HAZ) in some creep-fatigue tests. Since the HAZ was not taken into account in the procedure, there were obviously some rooms for improvement. Creep-fatigue failure mechanisms of the welded joint must be investigated and the characteristics of the HAZ must be formulated for more precise creep-fatigue strength assessment.Copyright

A Rational Identification of Creep Design Area Using Negligible Creep Curves

For extension of non-creep design area and simplification of design procedures, a rational identification method of creep design area by negligible creep (NC) curves was studied. NC curves of six kinds of austenite stainless and ferrite steels for fast reactors were determined based on domestic material data. NC curves provide the relation between temperature and time that does not induce damageable creep strain under the constant stress 1.5Sm (Sm: design stress intensity). In existing Japanese design guides, non-creep design area is severely restricted by constant upper temperature limit for austenite stainless steel and ferrite steel. In the case of 316FR steel and SUS410J3, which are candidate materials of Japanese commercialized fast reactors and have excellent material property, this limit can be extended by NC curve concept considering the duration of high temperature operation. NC curves under secondary stress considering stress relaxation were also studied. However, rationalization effect was insufficient whereas evaluation process was too complex. Therefore, at the present stage, NC curves at constant stress level 1.5Sm were adopted to identify creep design area. The concept of NC curve was introduced into the interim structural design guide for commercialized fast reactors in Japan to simplify the creep design of fast reactor systems. Utilizing these curves, non-creep design becomes possible for components operated at comparatively lower temperature in normal condition.Copyright

Micro-crack growth behavior in weldments of a nickel-base superalloy under biaxial low-cycle fatigue at high temperature

Abstract Tensile-torsional-combined biaxial low-cycle fatigue tests on welded tubular specimens of the Ni-base superalloy Hastelloy-X were carried out and the micro-crack growth behavior in the weldments was investigated with the aim of improving life assessment methods for high-temperature components. Welded hollow cylindrical specimens of two types were prepared: one welded in the axial direction and the other welded in the circumferential direction. Fatigue lives of the welded specimens were about half that of the base metal. In both weld and base metal, the initiation of micro-cracks was observed in the early stage of life, but the initiated length of the micro-cracks in the weld metal was about 0.5 mm while the equivalent figure for base metal was about 0.1 mm. The crack growth life from 0.5 mm to failure in the base metal specimen almost coincided with the failure life in the welded specimen. The maximum principal strain was confirmed to be a good parameter for evaluating crack growth rates for both weldments and base metal. These results show that the reduction in fatigue strength is not due to the strain concentration at the weld. The fatigue life of weldments of Hastelloy-X is affected by the initiated lengths of micro-cracks affect.