In order to establish a life prediction method for gas turbuine blades, fatigue tests using intermittent strain waveforms which simulated the service loading of gas turbine blades were carried out for SUS 316 stainless steel and two Ni-base superalloys (IN 738 LC and Inco. 713 LC) at elevated temperatures. The fatigue behavior and fatigue strength in these tests had been reported in the previous papers by the present authors. In this paper, several methods of damage evaluation using intermittent strain waveforms were examined on the basis of a linear cumulative damage concept, and the following conclusion was obtained; The failure life (Nes) in the intermittent strain cycling can be predicted by the equation below, which takes the acceleration of crack propagation rate due to the superposed small strain cycling into consideration. Nes·(1/Nh+η·n2/N2)=1 where Nh and N2 are the fatigue lives observed under trapezoidal and constant strain waves respectively. η is the ratio of crack propagation rate under superposed small strain cycling to that under constant strain amplitude cycling, which is represented by the following equations. R≤0;η=2α/(1-R)αR>0;η=2α·(1+R)/(1-R) where α is the Paris's exponent and R is the stress ratio calculated by using the cyclic stress-strain relationship.
The present paper deals with temperature and stress in a solid cylinder due to limited heating. Thermomechanical coupling was taken into account in the analysis. Numerical calculations were performed for various heating time and thermomechanical coupling parameters. The results obtained are summarized as follows: (1) The coupled temperature at the surface after a limited heating time becomes higher than the uncoupled one. (2) A long heating time causes large axial tensile stress at the surface after the limited heating time. The coupled axial tensile stress is smaller than the uncoupled one. (3) Any maximum axial tensile stress at the surface after a limited heating time does not become larger than the maximum axial compressive stress at the surface during the heating period.
Creep crack growth behavior of 21/4 Cr-1 Mo steel at 550°C was investigated by energy rate line integral, C* under conventional constant load control and deflection rate control. The same relationship of da/dt versus C*-integral was obtained by both of the control methods. The increasing deflection rate control test proposed in this paper was found to be a very efficient method because a wide range of creep crack growth data can be obtained stably on a single specimen. Various formulae to experimentally determine C*-integral of CT specimens were compared. Equation (3) considering the axial force as well as the bending effect agreed with the analytical results by the finite element method in the range of a/W≥0.5. The result of C*-integral estimation using the fully plastic solution, agreed with the experimental results in steady state creep conditions, when the plane strain solution was applied. Although the C*-integral approach with the fully plastic solution seems prospective for practical creep crack growth prediction, more investigations are needed under various conditions.
In order to investigate the effect of cyclic stress change on creep crack propagation, deformation tests on smooth specimens and crack propagation tests on notched specimens of 0.16% carbon steel were carried out at 673K (400°C) under a time-dependent fatigue condition. The crack propagation rate in time-dependent fatigue was much faster than that in static creep. It was owing to the acceleration of creep strain rate in the vicinity of crack tip because the dynamic recovery took place in the compression period. In these tests, the effect of the transition from small scale creep to large scale creep was small. The crack propagation rate had fairly good correlation with the experimentally evaluated creep J-integral range, Delta;Jc, which was equivalent to the analytically evaluated creep J-integral range based on the deformation property in time-dependent fatigue. In detail, however, the crack propagation rate in time-dependent fatigue was rather slower than that in static creep for an equal Delta;Jc value. It was concluded to be attributable to the existence of the incubation time of crack propagation during the tensile stress hold time in time-dependent fatigue. The equivalent creep J-integral range, (Delta;Jc)eq, which was the effective component of Delta;Jc for the creep crack propagation rate under a fatigue condition could be evaluated by the following equation; (Delta;Jc)eq=Delta;Jc/γ, γ=1+C11-R/C2+τH, where C1 and C2 are constants, R is the stress ratio, τH is the nondimensional tensile stress hold time.
The strain range partitioning (SRP) life relation was experimentally determined at 550°C both in air and in vacuum (<2× 10-6mmHg) for the annealed and the normalized and tempered 21/4 Cr-1 Mo steels, and the effects of air environment and heat treatment on the SRP life relation of 21/4 Cr-1 Mo steel were examined. All of Δεij-Nij properties were found to be sensitive to the environment in the strain range lower than (Δεij)cr while not in the strain range higher than (Δεij)cr. The value of (Δεij)cr was larger in the annealed material than in the normalized and tempered one. The effects of heat treatment on Δεpp-Npp and ΔεNcc-Ncc properties were not so remarkable for the both materials, whereas Δεpc-Npc and Δεcp-Ncp properties of the annealed material were superior to those of the normalized and tempered one. The tendency that in the annealed material Npc was smaller than or equal to Ncp in air can be fully attributed to the environmental effect, because the present vacuum data of both materials show that Ncp tends to be smaller than Npc independent of the value of the inelastic strain range. The present vacuum condition (<2× 10-6mmHg) was not a perfect one for obtaining the SRP life relation in vacuum for 21/4 Cr-1 Mo steel. This conclusion was derived from the result that all theΔεcc-Ncc data in the present vacuum condition gave shorter lives than those predicted by Manson's equation Δεcc=Dc0.6Ncc-0.6. Based on these results and the facts reported in the literatures, a method to obtain the SRP life relation in perfect vacuum from both air data and imperfect vacuum data was proposed. The SRP life relation in perfect vacuum thus obtained was found to be useful to explain quantitatively the effects of air environment and heat treatment.
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: Δεpp=0.5DpNpp-0.5 Δεpc=0.145DpNpc-0.5Δεcp=0.111DcNcp-0.5Δεcc=0.5DcNcc-0.5} or Δεpp=Dp0.6Npp-0.6Δεpc=(0.192Dp)0.6Npc-0.6Δεcp=(0.078Dc)0.6Ncp=0.6Δεcc=Dc0.6Ncc-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.
Cr-Mo-V cast steel is being used widely for steam turbine castings, since it possesses superior elevated temperature properties. Recently, however, the degradation of material properties of these castings has been noticed after long-term service at high temperature and high pressure environments. In this study, the metallurgical and mechanical characteristics of long-term degraded Cr-Mo-V castings were investigated, and a method to estimate the degree of deterioration was examined. The main results obtained are summarized as follows: (1) Reduction in mechanical strength after long-term service is considered to be caused by the carbide growth and the recovery of microstructure. (2) Although grain boundary is subjected to reversible temper embrittlement after service, the toughness can be restored by the de-embrittlement procedure. (3) The long-term degraded material showed low creep rupture strength and low ductility, because creep cavities were easily formed. The de-embrittled material showed the same behavior. (4) Anodic polarization measurement by which SACM can be observed was found to be a suitable method to detect the material property change after long-term service.
The effect of service loading on the fatigue behavior of a steam turbine rotor steel with large inclusions (1Cr-1Mo-1/4V steel with non-metallic inclusions of about 0.5mm in diameter) was investigated at 550°C. The service loading was simulated by a multiple two-step straining (MTSS). Under the MTSS tests, the life of the defect-free materials was reduced remarkably (1/10-1/100) as previously reported by the authors, and the reduction was even larger for a material containing large inclusions. However, the inclusions had little effect in reducing the life as far as a constant strain range was applied. Therefore the effect of inclusions was effectively evaluated through variable strain testing, not by conventional low cycle fatigue tests. An estimation of fatigue life was carried out based on the assumption of linear cumulative fatigue damage law in terms of the measured plastic strain range. This method tended to overestimate the life by a factor of two for the defected material under programed loading, though it effectively predicted the life for the defect-free ones. This was attributed to the crack initiation period reduced by inclusions. In the case of low cycle fatigue tests the crack initiation life was relatively short (-20%) even for the defect-free materials and therefore little effect of inclusions on the total life was noticed. For a conservative designing of a turbine rotor which might contain defects, it would be of significance to apply a fracture mechanics approach on an assumption that the total life is predominated by the crack growth period.
This paper describes damage evaluation and estimation of remaining life of SUS 304 stainless steel in creep, low-cycle fatigue and creep-fatigue at 873K in air. Creep, fatigue and creep-fatigue damage curves were drawn by the method proposed by D.A. Woodford and the relations between these damages and non-destructive parameters, i.e., microvickers hardness and quantities obtained from X-ray diffraction, were discussed. From these tests, the following conclusions were obtained. (1) Constant damage lines in the diagram of remaining lives in creep and fatigue could be drawn by changing load levels during the tests. Constant damage lines in creep-fatigue were also made by a linear damage rule using both static creep and fatigue damage curves, which agree well with the experimental data in creep-fatigue. (2) Microvickers hardness and half-value breadth in X-ray diffraction are appropriate parameters to evaluate creep damage but are not proper to evaluate fatigue damage. Particle size and microstrain obtained by X-ray profile analysis are good parameters to evaluate both creep and fatigue damages.
To establish an evaluation method of elevated temperature strength for weldments, a series of investigations1)-5) had been performed on the low-cycle fatigue strength. In this study, the suitable evaluation methods of creep and creep-fatigue strengths for weldments were proposed and were applied to the life prediction of FBR weldments. The following results were obtained. (1) In the evaluation of low-cycle fatigue strength for the weldments, the accuracy of life prediction by the Method (I), Method (II) and FEM was determined mainly by the difference in cyclic deformation resistance between the base metal and the weld metal. The Method (I), Method (II) and FEM were effective for the weldments which have small difference in cyclic deformation resistance such as stainless steel and 21/4 Cr-1 Mo steel with PWHT. But for the weldments which have large difference in cyclic deformation resistance such as the 21/4 Cr-1 Mo steel without PWHT, the Method (I) predicted the life overconservatively and FEM overestimated the life inversely. (2) In the evaluation of creep strength for the weldments, the minimum creep strain rate of the 21/4 Cr-1 Mo steel weldment was predicted by the Independent Method which paid attention to only the weakest part of the weldment and also by the Interaction Method which considered the interaction between the base metal and the weld metal. Both of them were found to be effective for the 21/4 Cr-1 Mo steel weldments. (3) In the evaluation of creep-fatigue strength for the weldments, a new method was proposed which was an extension of the fatigue evaluation method (I) based on the linear damage summation rule. It could predict the life of the 21/4 Cr-1 Mo steel weldments within a factor of two on life.
High Temperature Gas-cooled Reactor (HTGR) systems should be designed based on the high temperature structural strength design procedures. On the development of design code, the determination of failure criteria under cyclic loading and severe environments is one of the most important items. By using the previous experimental data for Ni-base wrought alloys, Inconel 617 and Hastelloy XR, several evaluation methods for creep-fatigue interaction were examined for their capability to predict their cyclic loading behavior for HTGR application. At first, the strainrange partitioning method, the frequency modified damage function and the linear damage summation rule were discussed. However, these methods were not satisfactory with the above experimental results. Thus, in this paper, a new fracture criterion, which is a modification of the linear damage summation rule, is proposed based on the experimental data. In this criterion, fracture is considered to occur when the sum of the fatigue damage, which is the function of the applied cyclic strain magnitude, and the modified creep damage, which is the function of the applied cyclic stress magnitude (determined as time devided by cyclic creep rupture time reflecting difference of creep damages by tensile creep and compressive creep), reaches a constant value. This criterion was successfully applied to the life prediction of materials at HTGR temperatures.
D.C. electrical potential technique has been successfully used to monitor crack extension in CT specimens at elevated temperatures. An electrical potential measurement system with various features was constructed. A simple graphite paper method was used to obtain the calibration curve for crack length ratio (a/W) vs. normalized electrical potential (Va/Va0). Polynomial equations (1) (2) were proposed for electrical potential calibration of CT specimens. These equations agreed well with elastic fatigue crack growth, elastic-plastic fatigue crack growth as well as creep crack growth test results for carbon steel, low alloy steel and austenitic stainless steel in the range of RT to 650°C. A convenient procedure to convert electrical potential into crack length was also proposed as shown in Fig. 6. These proposed calibration curve and conversion procedure are useful for standardization of electrical potential technique in future.