The purpose of the composite laminate's core material, whether foam, Nomex honeycomb, or aluminum honeycomb, is to separate and stabilize in shear the composite inner/outer skins and thus increase the buckling margin of the structure and its bending and torsional properties.
Nomex honeycomb has the benefit of a CTE that closely matches the graphite composite, so there is no thermal strain produced during an elevated temperature cure cycle of the composite. Aluminum honeycomb has a thermal mismatch with the carbon fiber composite, but the aluminum honeycomb also performs much better in a crash condition due to its high elongation rate.
The optimum thickness of the core material varies throughout the chassis structure, depending upon the local stress conditions.
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