Have you ever wondered how aircraft interiors and Formula 1 cars are so stiff and light? The secret is sandwich panels. Sandwich panels are used whenever lightness and stiffness are required for a flat or irregular shape. The main theory behind sandwich panels is the concept of mass moment of inertia. Increasing the mass moment of inertia directly increases the stiffness. So how do you increase the mass moment of inertia while keeping weight down?
First, let's consider a solid carbon fiber plate. Thicker plates will be stiffer, but not just because there is more material. When layers are further from the center (commonly called the neutral axis) they contribute more stiffness. Consider two tubes of different diameter. The large diameter tube will be stiffer than the small diameter tube, even if the amount of materials in them is equal! This is because the larger diameter tube has more material further from the center.
An I-beam works using the same principal. The web of the I-beam (the skinny center section) holds the flanges (the end pieces) far from the center, making the beam much stiffer than a solid plate using the amount of material. (See Figure 1.)
So, we know a thick plate will be stiffer but we need something lightweight. That's when the sandwich comes in. When a plate bends, the outermost layers don't just add the most stiffness, they also take a majority of the load. Both faces will experience stress with one in compression and one in tension. Strangely enough, the center of a bending plate sees no stress at all! Therefore, if we know the center layers of a plate don't contribute much we can eliminate them and replace them with foam or honeycomb.
Replacing the center layers reduces overall stiffness a little bit, bringing us right back to the mass moment of inertia by effectively moving the load carrying materials (also referred t as the panel "skins") further from the neutral axis. If we make the core thicker we've increased the mass moment of inertia and increased the bending stiffness and strength. If we add more layers we can also increase the strength and stiffness since we add more materials far from the center, and we can distribute the load over more cross-section. Getting the thickness right results in a panel that is considerably lighter than an equivalent solid panel. Rock West Composites offers sandwich panels in a multitude of thicknesses and constructions to suit your needs. And as always, if we don't have it shoot us an email or give us a call and we CAN build it for you!
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