Study On Initial Geometric Imperfection Model Adopted In Numerical Simulation Of Roll-Welded Cylindrical Shells Under External Pressure

Cylindrical shells under external pressure are sensitive to initial geometric imperfections which were produced in the manufacturing process. The buckling pressure will decrease dramatically even if the geometric imperfection is small. Buckling is one form of the failure of cylindrical shells under external pressure. Safety factor is used in current design code in dealing with the effect of geometric imperfections. With the development of numerical simulation technology, finite element analysis is adopted to calculate the buckling pressure of cylindrical shells. But there is no consensus about how to describe the initial geometric imperfections in present simulation. In this paper, a model was presented to depict the initial geometric imperfections of roll-welded cylindrical shells, based on measured data. Furthermore, accuracy and reliability of the model were validated by buckling experiments.The profile data of the cylindrical shells were obtained by a laser scanner. Fourier series was adopted to describe the initial geometric imperfections of the cylindrical shells and the parameters in the Fourier series were determined by extracting and fitting the measured data in MATLAB. It was proved that the Fourier series model can perfectly describe the initial geometric imperfections of roll-welded cylindrical shells that manufactured in the same or similar process by comparing the fitting Fourier series with the measured data.In order to verify the validity of the fitting Fourier series model, an experimental device was designed and manufactured to make external pressure tests on eleven roll-welded cylindrical shells. Buckling pressure, buckling mode and strain of cylindrical shells were obtained. Meanwhile, finite element software ANSYS was adopted to simulate the cylindrical shells. Bilinear material model was adopted in the nonlinear instability analysis of three different initial geometric imperfection models(consistent imperfect buckling mode model, Fourier series model and scanning model). The results show that both the simulation results ps obtained by the scanning model, and the simulation results pfl obtained by the Fourier series model agree well with the experimental value pT. Meanwhile the critical buckling pressure pyz obtained by the consistent imperfect buckling mode model is generally smaller than the experimental value pT. Generally speaking, the buckling position predicted by Fourier series model and scanning model roughly agree with the experimental results.In conclusion, the Fourier series model is reliable in describing the initial geometric imperfections for the roll-welded cylindrical shells manufactured in the same or similar process, and it can predict the critical buckling pressure and show buckling position of cylindrical shells to some extent under external pressure.