Z-factor equations for elastic-plastic fracture mechanics analysis prescribed in the JSME rules on fitness-for-service for nuclear power plants

Abstract

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-for-Service 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.

Keywords

• fitness-for-service code
• elastic-plastic fracture analysis
• z-factor
• structural integrity
• surface crack
• nuclear power generation