A Nondestructive Technique for the Evaluation of Thin Cylindrical Shells' Axial Buckling Capacity

The axial buckling capacity of a thin cylindrical shell depends on the shape and the size of the imperfections that are present in it. Therefore, the prediction of the shells buckling capacity is difficult, expensive, and time consuming, if not impossible, because the prediction requires a priori knowledge about the imperfections. As a result, thin cylindrical shells are designed conservatively using the knockdown factor approach that accommodates the uncertainties associated with the imperfections that are present in the shells; almost all the design codes follow this approach explicitly or implicitly. A novel procedure is proposed for the accurate prediction of the axial buckling capacity of thin cylindrical shells without measuring the imperfections and is based on the probing of the axially loaded shells. Computational and experimental implementation of the procedure yields accurate results when the probing is done in location of highest imperfection amplitude. However, the procedure overpredicts the capacity when the probing is done away from that point. This study demonstrates the crucial role played by the probing location and shows that the prediction of imperfect cylinders is possible if the probing is done at the proper location.

Analysis of Mechanical Behaviors of Waterbomb Thin-Shell Structures Under Quasi-Static Load

Waterbomb structures are origami-inspired deformable structural components used in new types of robots. They have a unique radially deployable ability that enables robots to better adapt to their environment. In this paper, we propose a series of new waterbomb structures with square, rectangle, and parallelogram base units. Through quasi-static axial and radial compression experiments and numerical simulations, we prove that the parallelogram waterbomb structure has a twist displacement mode along the axial direction. Compared with the square waterbomb structure, the proposed optimal design of the parallelogram waterbomb structure reduces the critical axial buckling load-to-weight ratio by 55.4% and increases the radial stiffness-to-weight ratio by 67.6%. The significant increase in the radial stiffness-to-weight ratio of the waterbomb structure and decrease in the critical axial buckling load-to-weight ratio make the proposed origami pattern attractive for practical robotics applications.

Bi-Axial Buckling of Laminated Composite Plates Including Cutout and Additional Mass

In the presented paper, a study of bi-axial buckling of the laminated composite plate with mass variation through the cutout and additional mass is carried out using the improved shear deformation theory (ISDT). The ISDT mathematical model employs a cubic variation of thickness co-ordinates in the displacement field. A realistic parabolic distribution of transverse shear strains through the plate thickness is assumed and the use of shear correction factor is avoided. A C° finite element formulation of the mathematical model is developed to analyze the buckling behavior of laminated composite plate with cutout and additional mass. As no results based on ISDT for the considered problem of bi-axial buckling of the laminated composite plate with mass variation are available in the literature, the obtained results are validated with the data available for a laminated composite plate without cutout and additional mass. Novel results are obtained by varying geometry, boundary conditions and ply orientations.

Comparative Study Concerning the Methods of Calculation of the Critical Axial Buckling Load for Stiffened Cylindrical Shells

As demonstrated in numerous theoretical and experimental studies [1], the buckling behaviour of stiffened cylindrical shells (SCS) is strongly influenced by the presence of geometric imperfections caused by the manufacturing process and/or exploitation. Therefore, the design norms recommend the use of reduction coefficients with very low values, resulting in a significant reduction of the maximum load applied. In order to calculate the critical buckling load as accurately as possible it is necessary to know the real geometry of SCS. In case of SCS, the structural analysis based on the use of the finite element method (FEM), using models that reflect the real geometry of the shell determined from measurements, lead to a better evaluation of the critical buckling load. The structural analysis with FEM is accepted more and more by standards, EN 1993-1-6:2007 [2] specifying the types of numerical analysis accepted for cylindrical shells. The aim of this study is to compare the results concerning the critical buckling load for SCS under axial compression, obtained with both the analytical and FEM methods for real geometries obtained from measurements. For this purpose, scale models of SCS were used, for which were determined, by measuring, the values of the deviations from the median radius at several points on the shells surface. These deviations were then incorporated in the numerical analysis with FEM and it was determined, for each cylindrical shell, the value of the critical axial buckling load, by using geometrically nonlinear analysis. In order to validate the results of the numerical analysis, the analysed SCS were subjected to axial compression within an experimental program and the experimental data were compared with the results established on the basis of analytical and numerical calculation.