Composite Sandwich Stiffness Equation and Calculator

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Composite Sandwich Stiffness Equation and Calculator

Structural Composite Sandwich Stiffness Equation and Calculator

The analysis procedures described on this webpage apply to composite sandwich structures having isotropic facings and either orthotropic or isotropic cores. The isotropic materials are those having essentially constant properties in all directions. The orthotropic materials are those whose strength properties are not constant in all directions, such as honeycomb cores.

The assumption is made that adhesive failure does not occur, a reasonable assumption if proper care is taken in the selection of the adhesive system. This requires that the adhesive shear and flatwise tensile strength be greater than the respective core strength.

If the sandwich has thin facings on a core of negligible bending stiffness, as is usually the case, and after assuming λ₁ = λ₂ = λ₃, the bending stiffness is given by the formula:

Eq. 1, for unequal facings

D = \frac{E_{1}^{'} t_{1} E_{2}^{'} t_{2} h^{2}}{(E_{1}^{'} t_{1} + E_{2}^{'} t_{2}) λ}

Eq. 2, for equal facings

D = \frac{E_{1}^{'} t h^{2}}{2 λ}

Eq. 3
λ = ( 1 - µ² )

Where

E' = effective Modulus of elasticity, lbs/in²
t = thickness, in
µ = Poisson's ratio
h = distance between facings centroids, in
D = Bending stiffness, lbs·in²

The notation used

1- subscript denoting facing 1
2 - subscript denoting facing 2

Effective elastic moduli of a heterogeneous body would be the ratios of the average stresses to the average strains that result in the body when it is subject to pure shear or pure compression on its outer boundary.

Figure 1, show the notation used for the analysis of sandwich panels in this webpage.

Source

Bell Helicopter Structural Design Manual, 1977