Buckling of thin cylindrical shell subject to uniform external pressure G. Forasassi, R. Lo Frano Department of Mechanical, Nuclear and Production Engineering, University of Pisa, Via Diotisalvi, n°2-56126 Pisa (Italy) Tel: +39-050- 836688 Fax: +39-050-836665 Email address: g.forasassi@ing.unipi.it , rosa.lofrano@ing.unipi.it Abstract -The buckling of cylindrical shells under uniform external pressure loading has been widely investigated. In general, when tubes are subjected to external pressure, collapse is initiated by yielding, but interaction with instability is significant, in that imperfections associated with fabrication of shells reduce the load bearing capacity by a significant amount even when thickness is considerable. A specific buckling analysis is used to predict collapse failure of long pressure vessels and pipelines when they are subjected to external over-pressure. The problem of buckling for variable load conditions is relevant for the optimisation of several Nuclear Power Plant applications as, for instance, the IRIS (International Reactor Innovative and Secure) LWR integrated Steam Generator (SG) tubes. In this paper, we consider in addition to the usual assumptions of thin shell, homogeneous and isotropic material, also the tube geometric imperfections and plastic deformations that may affect the limit load. When all those conditions are considered at present, a complete theoretical analysis was not founding the literature. At Pisa University a research activity is being carried out on the buckling of thin walled metal specimen, with reference to several geometries and two different stainless steel materials. A test equipment (with the necessary data acquisition facility), suitable for carrying out many test on this issue, as well as numerical models implemented on the MARC FEM code, were set up. In this report, the results of the performed analyses of critical pressure load determination with different numerical and experimental approaches are presented. The numerical results obtained are compared with the experimental results, for the same geometry and loading conditions, showing a good agreement between these two approaches. I. INTRODUCTION The stability of circular cylindrical shells under uniform lateral pressure has been widely investigated. This type of buckling analysis is used to predict collapse failure of long thin shell and pipeline subjected to external pressure. Many researchers have studied buckling of circular cylindrical shell under external pressure and more accurate solutions of the present problem were obtained for short cylindrical shells and for anisotropic shells, respectively. However, actual calculations seem to be confined to some special ranges of the shells geometries, boundary conditions and loading conditions. 1 Timoshenko and Gere gave the classical buckling solution with uniform thickness, in the case of uniform lateral pressure. 2 The general equation of deformation must be considered in calculating the intensity of lateral pressure at which buckling occurs. This classical approach refers to the thin walled cylindrical shell, while in real structures “thin shells” can have different thicknesses and diameter values, taking into account also the fluctuations introduced during the production processes. When thin shells are subjected to external pressure, the collapse is initiated by yielding, which is often the dominant factor, but the interaction with the instability is meaningful. In fact, the presence of imperfections reduces the load bearing capacity by an amount of engineering significance; so the classical elastic approach is not adequate. The major factors that affect the collapse pressure of pipes are the diameter-to- thickness ratio D/t, the Young's modulus and yield stress of the material in the circumferential direction, and initial imperfections in the form of ovality and wall thickness variations. 3-4 The present paper deals with the buckling issue of a thin circular cylindrical shell as the steam generator’s tubes considered for an integrated PWR as the IRIS Reactor. The IRIS reactor preliminary design is currently under way; its vessel houses not only the nuclear core and control rods, but also all the major reactor coolant system components including pumps, steam generators, pressurizer, control rod drive mechanisms and neutron reflector. Therefore the IRIS integral vessel is larger than a comparable traditional PWR pressure vessel, but the size of the IRIS containment is a fraction of the size of the reactor having the same number of loops, resulting in a significant reduction in the overall size of the reactor plant. 5 The IRIS SG’s are of the once through type, with the primary fluid outside the tubes. The buckling analysis is used to predict failure of long pipelines, subjected to external over-pressure. A possible reference configuration is used to perform the different analysis as for the theoretical as for the computational ones. Further, the results of each analysis are subjected to a comparison with the experimental ones.