Stability of laminated composites with delaminations

Abstract

In this thesis, investigation on buckling problem of composite beam is carried out. A new formula which is denoted as CLT method 3 is derived to calculate the effective flexural stiffness of the thin composite beam. Two other existing equations indicated as CLT methods 1 and 2 that can also be used to evaluate the effective bending rigidity are presented. Furthermore, in order to examine the buckling phenomena of moderately thick composite beam, three different collections of expressions designated as FSDT methods 1, 2 and 3 are presented to compute three separate sets of values for the effective flexural stiffness, effective shear modulus and shear correction factor in the first-order shear deformation beam theory.

In conjunction with the above-mentioned equations, various analytical models are developed to analyze the thin and moderately thick composite beams with a single delamination, thin composite beam having semi-embedded delaminations with and without contact and thin composite beam with non-coinciding delaminations under combined axial and transverse loads including contact effect. These mathematical models are formulated via the suitable boundary conditions, kinematical compatibility equations and equilibrium of forces and moments. The contact effect is implemented appropriately into some analytical models to prevent the physically impermissible interpenetration of sub-laminates.

Various parametric studies are then conducted and they include boundary condition, type of composite materials, number of layers, ply orientation, stacking sequence, through-the-thickness location of the delamination, delamination length and along-the-span position of the delamination. For all the single-delaminated and multi-delaminated thin composite beams cases that are examined, the CLT methods 2 and 3 are able to form the desired upper and lower bounds, respectively, for the actual buckling loads. Analogously, for all the single-delaminated moderately thick composite beam cases, the FSDT methods 2 and 3 are competent of generating the required upper and lower limits, respectively.