The elastic torsional buckling strength has been determined experimentally for thin-walled cylinders fabricated with glass/epoxy, boron/epoxy, and graphite/epoxy composite materials and composite-reinforced aluminum and titanium. Cylinders have been tested with several unidirectional-ply orientations and several cross-ply layups. Specimens were designed with diameter-to-thickness (D/t) ratios of approximately 150 and 300 and in two lengths (L), 10 in. (25.4 cm) and 20 in. (50.8 cm).

The results of these tests were compared with the buckling strengths predicted by the torsional buckling analysis of Chao. In this analysis, the instability loads of heterogeneous anisotropic cylinders are calculated with Timoshenko's equilibrium equations. The computer program associated with the analysis seeks the solution with the lowest buckling strength by iterating on the number of circumferential buckling waves. For the cylinders considered (L/r = 3.3 and L/r = 6.1), the experimental buckling torques were approximately 85 percent of the torques predicted by the Chao analysis. In the cross-ply laminate cylinders, the stacking sequence of the plies was found to have a marked effect on the elastic buckling torque.

Reversal of the stacking sequence of the cross-plied cylinders resulted in buckling torques which differed by a factor of 2. Similar results were obtained by reversal of the direction of twist on the original stacking sequence. This observation may be important in applications where reversal of loading can occur. The size effect of linear scaling was investigated for cylinders whose dimensions were different but whose L/r and D/t ratios were the same. As expected from the analysis, the experimental buckling stress remained constant.