Buckling Characteristics And Engineering Applications Of Axially Compressed Composite Cylindrical Shells

Composite has many good characteristics like high specific strength,large specific modulus,outstanding designability,excellent corrosion resistance,etc.Cylindrical shells have high load capacity and space utilization.They are usually used in manufacture of structures like ocean platform spud legs and submersible pressure hulls.This paper investigates on buckling characteristics and optimal design methods of composite cylindrical shells under axial compression in order to improve their axial load capacity.According to actual manufacture and engineering condition,design calculationprocess and buckling analysis method of composite cylindrical shells are established for guiding design,strength check and optimization of composite cylindrical shells.Firstly,mechanical properties of CFRP(Carbon Fiber Reinforced Plastics)cylindrical shells aregot from uniaxial tensile tests.Linear buckling analysis is performed for cylindrical shells under axial compression condition to obtain the first fifty buckling modes,defined as mode imperfections.Analytical and practical buckling critical loads of axially compressed composite cylindrical shells are obtained from composite thin-walled theory and NASA SP-8007 standard so as to verify correctness of linear numerical models.Nonlinear buckling analysis of the cylindrical shell under different mode imperfection is proposed based on the Riks method.Initial geometric imperfections of composite cylindrical shells are got from the imperfection testing experiment and axial compression buckling characteristics areobtained from the axial compression experiment.Secondly,Buckling loads respectively calculated using numerical analysis results and analytical theory results are compared with axial compression test results.Results show that the first mode imperfection is not always the worst imperfection for composite cylindrical shells under axial compression condition,and buckling load obtained from higher mode imperfections agrees well with experimental results.The decrease of buckling load is influenced by shape and amplitude of imperfection synchronously.Multi-mode effects should be considered on buckling analysis of composite cylindrical shells.Buckling load obtained from NASA is much lower than the test result,which always results in heavy structure,material redundancy and poor performance.Thirdly,both linear and nonlinear buckling analysis methods are used to research on the influence of stacking sequences and shell shapes on buckling characteristics of composite cylindrical shells under axial compression.Reasonable stacking sequences and shell shapes are obtained.Bionics principle is used to conduct shape optimization on cylindrical shells.Two methods of equal mass and equal volume are used to design egg-shaped shells which have same aperture diameter with cylindrical shells;linear and nonlinear buckling analysis are conducted.Results show that composite egg-shaped shells have higher axial load capacity and lower imperfection sensitivity than cylindrical shells.Fourthly,according to actual engineering projects,spud legs of single column fixed ocean platforms are set as background and sectional modularized design method is adopted.Axial buckling characteristics ofnormal steel cylindrical spud legs,CFRP cylindrical spud legs and CFRP multiple intersecting egg-shaped spud legs are investigated under five per cent multi-mode imperfections.Density of composite is obtained from DH-300 electronic hydrometer.Results show that CFRP multiple intersecting egg-shaped spud legshave lighter mass,lower imperfection sensitivity and better stabilitywith the same load capacity.