Equivalent Strength - FG and Aluminum

So I'm thinking of project where one might use a .063" piece of aluminum of about 5" x 5" as a door. There is no structural concerns, it's just an access door if you will. The .063 is typically used to protect against vibration and cracking from fatigue. If I were to do this same size "door" in fiberglass or carbon fiber, is there a way to approximate the tensile strength of the aluminum .063 and determine the best carbon or fiberglass thickness to achieve similar strength?

Stiffness is the thing I think

Hi Charlie,

I think that for a non-structural door, matching the stiffness/bending strength of the two materials is the way to go.

For the 5" wide .063 aluminum door the effective moment of intertia (I) is (width*thickness^3)/12. If my math is correct, that comes to 1.04e-4 in^4. The stiffness is modulus * I, so 10e6 * 1.04e-4 --> 1042.

Fiberglass laminate has a modulus of about 3e6 psi, so you'd need I to be about 1042/3e6 or 3.5e-4 in^4. That requires a thickness of about .094".

I don't have a good modulus value for carbon laminate, but hopefully someone here can help with that number.

Take care,
David

Carbon Fiber for door

You should not use carbon fiber for the door. Carbon and aluminum don't like each other and you will have aluminum corrosion where the carbon touches aluminum. Same as reason you don't use graphite pencil for marking aluminum. Also carbon fiber composites in thin sections have brittle fracture issues.

I assume you are looking at it for weight savings, but maybe not. If you want a good stiff lightweight door you could do a sheet of glass, then a thin sheet of balsa with the edges taperd. Then a back sheet of glass so that the edges of the door are only glass, but the bulk of the door is the glass/balsa sandwich.

It will make a very stiff, lightweight door panel.

Tim

You could separate the carbon from any aluminum with a thin fiberglass layer on the surface. Or even paint, if you were sure it wouldn't chip or rub off.

For bidirectional carbon cloth, you can probably use a modulus of 8e6 psi, which would suggest a thickness of about .070 inches. Any fiberglass would be on top of that. If you alternated layers oriented 0/90 degree and 45/-45 degrees, you'd be slightly better off, approaching quasi-isotropic material properties. Assuming, of course, that a specifically-oriented layup isn't necessary for your design concept.

If the door is going on aluminum structure, I'd tend to favor an aluminum door. If it's going on fiberglass, then I'd be tempted to build a carbon door.

If the door is in a location such that cracking is a real issue, then a door redesign or vibration alleviation are definitely approaches to consider. It might be worth comparing your door to similar doors on production aircraft.

Dave