日本機械学会維持規格の極限荷重評価に対する有限要素解析の適用方法

抄録

A procedure for application of finite element analyses (FEAs) for the limit load assessment was investigated for the fitness-for-service code for nuclear facility by the Japan Society of Mechanical Engineers. The current code prescribes the equations to obtain the limit load for pipes. FEAs were carried out for various geometrical conditions and their results were compared with the limit load obtained by the equations. It was shown that the FEAs derived unconservative limit loads for pipes with a circumferential crack. The stress triaxiality near the crack tip, whose effect is not considered in the limit load equations, increased the failure load. On the other hand, the limit load equation for pipes with an axial crack predicted conservative burst pressure because the equation was originally developed for wall-thinning. The comparison with the failure tests using cracked pipe revealed that FEAs predicted accurate bust pressure for pipes with an axial crack whereas they made unconservative prediction of failure bending load for pipes with a circumferential crack. It was suggested, for plastic collapse assessment of pipes under bending load, the flow stress for the elastic-perfect-plastic stress-strain curve should be multiplied by 0.84 to the average of yield strength and ultimate strength, which is used for the current code. On the other hand, the flow stress prescribed in the current code is applicable to the assessment for pipes subjected to internal pressure or tensile load.