Stability of a stiffened toroidal sector under uniform external pressure

This paper present the results of a stability analysis on a sector of the FTU (Frascati Tokamak Upgrade) toroidal vacuum vessel. FTU is an experimental machine, now under construction, mainly devoted to the study of the effects of lower hybrid radiofrequency heating on plasmas in reactor relevant conditions. Its vacuum chamber is a stainless steel structure completely welded with a major radius of 0.935 m and a minor radius of 0.335 m consisting of 12 toroidal thin sector joined together by thick ribs. This structure is loaded by comprensive electromagnetic forces both in toroidal and in radial direction that can create instability conditions. One of the major problems in its design is to determine the points where this phenomenon is likely to occur in order to avoid dangerous buckling situations. Theoretical analysis of the stability behaviour of one of these sector has been conducted by means of the ABAQUAS finite element code. The critical load has been determined by a classical algorithm and by the modified Riks methods. Both methods have given similar results in an elastic analysis. Furthermore the second one has been applied also using an elastoplastic model of the material to determine the critical load and the post buckling behaviour of the structure. Experimental tests have been conducted on a full scale model of the toroidal sector. The model has been placed in a large tank filled with water where the pressure has been gradually increased up to the collapse of the structure. The theoretical and experimental results have been compared and a good agreement has been found between them