Fatigue crack growth data obtained in the simulated BWR water environment were analyzed to establish a formula for reference fatigue crack growth rate (FCGR) of austenitic stainless steels in BWR water. The effects of material, mechanical and environmental factors were taken into the reference curve, which was expressed as: da/dN=8.17×10−12s˙Tr0.5s˙ΔK3.0/1−R2.121≦ΔK≦50 MPam where da/dN is fatigue crack growth rate in m/cycle, Tr is load rising time in seconds, ΔK is range (double amplitude) of K–value in MPam, and R is stress ratio. Tr=1 s if Tr<1 s, and Tr=1000 s if Tr cannot be defined. ΔK=Kmax−Kmin if R≧0.ΔK=Kmax if R<0.R=Kmin/Kmax. The proposed formula provides conservative FCGR at low stress ratio. Although only a few data show higher FCGR than that by proposed formula at high R, these data are located in a wide scatter range of FCGR and are regarded to be invalid. The proposed formula is going to be introduced in the Japanese Plant Operation and Maintenance Standard.
Fracture strength of cracked pipes made of moderate toughness materials is derived using J-integral value, which represents the driving force for fracture, and J−R curve, which corresponds to the material strength. However, in practical applications, it is not easy to obtain the J-integral value and J−R curve for the material of interest. Then, in the JSME Rules on Fitness-forService for Nuclear Power Plants (Codes for Nuclear Power Generation Facilities), the fracture strength is derived using the limit load divided by the Z-factor, which is given by equations in the code. Although Z-factor is largely influenced by crack depth, the current Z-factor equations do not include the effect of crack depth. Recently, new Z-factor equations, which took the effect of crack depth into account, were proposed. In this report, the validation of the proposed Z-factor equations was examined through a benchmark analysis for sample cases. The analyzed model was pipes with an axial or circumferential surface crack. The fracture stress due to ductile instability was calculated for an internal pressure or a bending load for various pipe and crack geometries. The analysis results derived by four organizations showed good agreement with the proposed equations and did not exhibit significant scattering. However, the new Z-factor equations were not conservative for all cases because the equations were derived by best-fit regressions. Then, in this report, revision were made on the new Z-factor equations so that the equations predict conservative fracture strength of cracked pipe of various geometries.
This paper shows the technical basis of revision to the JSME Fitness-for-Service Code (the FFS code) for flaw evaluation methods of pipes which have a very shallow circumferential flaw. When a flaw in a pipe is very small, the allowable stress between the FFS code and the JSME Design and Construction Code (the design code) is mismatched. Fracture strength of a pipe with small flaw depends on the tensile strength accompanied by large deformation. Therefore, fracture mechanics is not applicable in such a case. This mismatch has been resolved for an axial crack assessment by improving definition of flow stress for shallow crack. In this study, the authors investigated this mismatch in the allowable stress in the flaw assessment for a pipe with a circumferential crack. Some past fracture test results of pipes showed that flawed pipes did not fracture at the flaw section when the circumferential flaw size was small and they failed by oval deformation or plastic buckling. Allowable stress for such behavior has been incorporated in some existing design codes as a restriction for plastic collapse. Through the reevaluation of the existing piping fracture test results, the applicability of fracture evaluation methods defined in the FFS code was examined for the case that flaw size was very small. As a result, the fracture evaluation method based on flow stress was found not to be applicable when flaw size was very small, and the failure criterion in this case depended on the collapse strength accompanying with ovalization. Revisions of the FFS code reflecting these examination results were proposed in this paper.
Fatigue tests were conducted on the notched specimens of Ni-base-alloy NCF600 in high temperature water. Notch root strain was analyzed by finite element method (FEM) to calculate the notch root strain rate and the environmental fatigue correction factor Fen. Relationship between notch root fictitious stress amplitude and corrected fatigue life in water FenNwater are compared with the fatigue data of smooth specimen in air (i.e. best-fit curve) and it was found that the corrected fatigue life data in water shows a little shorter but almost agree with best-fit curve in air for the specimen with Kt = 1.4A. For the specimen with Kt = 3, corrected fatigue life in water is longer than that in air and the difference between both lives becomes larger with decreasing stress amplitude. The longer fatigue life in sharp notched specimen than that in dull notched specimen at the same notch root stain amplitude is thought to be dominated by the difference in the crack propagation life since the stress distributions on the cross section are decisively different. It is concluded that the fatigue life of notched specimen in high temperature water is adequately predicted using the modified rate approach method when the notch root strain is appropriately estimated e.g. by FEM analysis even though it gives excessively conservative prediction for sharp notch at low stress amplitude. The aspect of environmental fatigue of notched specimens are summarized on carbon steel and stainless steels with Ni-base alloy studied in Japan Nuclear Energy Safety Organization (JNES) Environmental Fatigue Test (EFT) project.
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