Influence of Local Imperfections on the Collapse Strength of Domed End Closures

Abstract

The influence of localized shape imperfections on the elastic buckling of torispheres is discussed in this paper. Perfect and imperfect injection-moulded torispheres were manufactured from ABS plastic and tested under vacuum. The models were made in five batches of thirty and all had the same basic geometry. One series was near-perfect and the other four had single localized increased-radius (‘flat patch’) imperfections in them. The centres of the imperfections were all located 12° from the axis of symmetry and the four series differed only in the size of the imperfection. This specific positioning of the imperfection was influenced by some unusual results which were published recently (1). The agreement between the experimental results and the numerical predictions, based on the BOSOR 4 and ABAQUS codes, was very good. In particular, the validity of the lower-bound curve of collapse pressure versus size of the imperfection was confirmed experimentally. The predicted buckling strength was also not influenced by the location of the flattening (that is whether on-axis or 12° off-axis) and the numerical results were confirmed by experiment. A finite element case study of an imperfect hemisphere with an on-axis and a 12° off-axis flat patch was also carried out. Once again, the location of the localized imperfection did not affect the buckling strength of the shell. This finding is at variance with a prediction made recently by other investigators of this problem.