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Old 04-03-2013, 02:32 PM   #328 (permalink)
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Originally Posted by jjackstone View Post
IIRC Neil,

Each time the core thickness between the layers of composite doubles, the strength cubes. Just for an example, if the 10mm layup could handle 10 pounds of stress, then a 20mm thick layup should be able to handle a 1000 pounds(10 to the 3rd power) of stress. These are just made up numbers. No idea what these thicknesses can really handle. I do have a two foot long, twelve inch wide, 3/4" honeycomb core laminated with just glass that I can't even begin to bend by placing it over my knee.
Hi Jack,

Actually for a "rectangle" shape in bending, when the thickness doubles, the strength goes up by four (T^2). This is for a material of solid section, i.e. one material, not a composite.

In the Machinery Handbook this is known as Z Section Modulus.

The theory is something like this:

1) When the section thickness doubles, there is twice as much material to take the load.
2) Since the thickness is now double, there is fiber strength that is twice as far from the neutral axis, and therefore twice as much strength based on that alone.
3) When you add 1) and 2) together, one ends up with four times the strength.

Now when talking composites, all this assumes that the "core" (foam) can handle the shear loads and/or compressive loads from the "skin" in the composite, without delaminating and/or buckling.

Composites modify the above listed Z-Section due to the single layer of glass on the thinner and thicker sections of foam, so the strength increase is not strictly defined by T^2 anymore. You can see that modeling composite structures is not an easy undertaking.

If one were talking about the torsional stiffness of a drive-shaft, then you are correct, that it goes up by the cube of the shaft diameter (D^3).

Hope this helps, Jim.

Last edited by 3-Wheeler; 04-03-2013 at 02:45 PM..
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