Second order buckling load of underwater composite cylinder

In this paper, a novel nonlinear second-order buckling load computational method is proposed for underwater composite submersibles using variational principle while solving the effects of nonorthogonal anisotropic constitutive relationship and all linear and nonlinear terms in the discrete Lagrangian strain. For 20 medium-length experimental specimens in the range of 14.11∼68.70 radius-to-thickness ratios, the maximum prediction error of this method is -15.235 %, the minimum prediction error is 0.90 %, and the average prediction error is 6.17 %; the absolute prediction errors are within 10 % for 16 specimens, and within 15 % for 19 specimens. For the 22 experimental specimens in the range of 8.33 ∼ 68.70 radius-to-thickness ratios with length-to-radius ratio increasing up to 24, the maximum prediction error of the method is -18.85 %, the minimum prediction error is 0.90 %, and the average prediction error is 6.98 %; the absolute prediction error is within 10 % for 16 of the specimens, and within 15 % for 20 of the specimens. In addition, this method has good convergence, stability, and angle order immunity. The method improves the prediction accuracy and reduces the prediction error fluctuation range compared with analytical methods and commercial FEM software. The method solves the difficulty that commercial FEM software is unable to predict the buckling loads under classical simply supported boundary condition. Among the total 22 specimens, the prediction results about 19 of them shows conservative results, with the maximum degree of conservatism of -18.85 %, which is within the engineering acceptance range. This is conducive to providing a more adequate safety margin of buckling load for the design of underwater composite submersibles in offshore engineering.