This paper presents results of an experimental study on the buckling of circular cylindrical shells under pure bending. In the experiment, emphasis was placed on the effect of the length of the cylinder. A high precision loading device was developed, by which the torque can be applied equally on both ends of the cylinder. Test specimens made from polyester films have a length-to-diameter ratio ranging from 1 to 20 and a diameter-to-thickness ratio of 156, 200, 234, or 300. Large deformation accompanied by the flattening of the cross section was observed before buckling in long cylinders. For those having the length-to-diameter ratio greater than 15, the buckling torque was about 60% of the classical value calculated on the basis of the classical buckling stress under axial compression. The pre-buckling deformation was hardly observed in short cylinders. For those having the length-to-diameter ratio from 1 to 3, the buckling torque approached the classical value.
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