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National Advisory Committee for Aeronautics, Technical Notes - Elastic and Plastic Buckling of Simply Supported Metalite Type Sandwich Plates in Compression

The necessary condition that the wing surfaces of modern high—speed
aircraft remain smooth under high loads has led to the use of the sand—
wich plate as a substitute for sheet—stringer construction. Sandwich
plates consist of two thin sheets of metal separated by a lovadensity,
low—stiffness core which, though contributing little to the strength of
the plate, serves to increase tremendously the flexural stiffness of the
load—carrying faces. The increase in flexural stiffness is somewhat
offset, however, by deflections due to shear which become appreciable
because of the low stiffness of the core.
Several papers which extend ordinary plate theory to take deflections
due to shear into account have appeared recently in this country. The
extension is made approxrmately in reference 1 by.means of the assumption
that any line in the core that is initially straight and normal to the
middle surface of the core will remain straight after deformation but will
deviate from the normal to the deformed.middle surface by an amount that
is proportional to the slope of the plate surface, the proportionality
being the same throughout the plate. The theory is used to obtain approxi—
mate criterions for the compressive buckling of plates with various
edge—support conditions. The criterions are corrected for the effects of
plasticity by replacing the Young's modulus of the face material every—
where it appears_in the buckling formulas by a reduced modulus.
This method of correction being partly-Justified by the consideration of
its theoretical effectiveness in connection with the plastic buckling of
simply supported sandwich columns. Reference 2 presents a smallrdeflection
theory for elastic bending and buckling of orthotropic sandwich plates which
considers shear deformations in a more refined manner. Reference 3 presents
a large—deflection analysis of elastic isotropic sandwich plates and
reduces the equations to smallrdeflection form to solve the problem of the
compressive buckling of simply supported sandwich plates.