An Evaluation Method for Plastic Buckling of Cantilever Type Pipes Controlled by Displacement Loads: Part 1—Proposal of the Estimation Method and the Criterion

In Fast Breeder Reactor, dominant stresses in the piping system are secondary, which are induced by constraint of the thermal expansion of components and pipes. Therefore, the structural design rule should essentially prevent the buckling caused by displacement controlled loads. In this study, the evaluation method of the buckling criterion of cantilever type pipes subjected to lateral displacements is proposed. We define the criterion of the deformation controlled buckling based on bending strain at local buckling portion. Then finite element analysis (FEA) is performed for estimating the displacement and bending strain at local buckling portion during the displacement controlled buckling. Those defined criterion and results of FEA are availed to investigate the evaluation method of the displacement controlled buckling criterion. In those FEA, material properties of Mod.9Cr-1Mo steel are applied, because the material is a candidate for primary and secondary heat transport system components of JSFR (Japan Sodium cooled Fast Reactor), and those of 316FR (type 316 stainless steel modified for FR) are also used to compare the FEA results of Mod.9Cr-1Mo steel. Results of FEA suggest that buckling behaviors strongly depend on the distribution of bending moment and strain hardening behavior of the material. Therefore, those features must be considered in construction of the evaluation method of displacement controlled buckling. To consider the distribution of bending moment, two indexes are defined. One indicates nonlinear bending displacement of a straight pipe. The other indicates the configuration of the pipe and material properties. Relationship between those indexes, which including criterion of displacement controlled buckling, is formulated by the enveloped line considering the effect of the strain hardening behavior of the material. As a result, an equation which represents the criterion for displacement controlled buckling of cantilever type pipes was proposed. This equation consists of two indexes as mentioned above, and can estimate criterion of displacement controlled buckling by material properties and pipe configuration. The proposed equation can be applied to the pipes made of Mod.9Cr-1Mo steel and 316FR.