Most common fin materials are strong enough to withstand the static loads associated with hobby rocket flights up to M~2. What casues fins to fail are the unstable transient aerodynamic loads encounter in the transonic flight region at M>0.8. The cause of the failure in this region is a lack of stiffness which allows the fins to flutter to the extent where an inelastic failure occurs.
The standard fin flutter spreadsheet uses some simple relationship to estimate the "flutter velocity" of a fin. For a fin of a give size and dimension, the increase in the flutter velocity varies as the thickness to the 1.5 power and as the square root of the Shear Modulus.
Based on the model, for a fixed fin size and composition, the flutter velocity increases by the following factor if you were to start with a 1/16" thick fin.
Thickness factor
1/16" 1.00
3/32" 1.82
1/8" 2.83
3/16" 5.20
1/4" 8.00
At the extreme ends, increasing the thickness by a factor of 4 raised the flutter velocity by a factor of 8! Thickness is good.
The resistance of flutter is also a function of the fin material. It's really easy to predict what happens with a metal, but it becomes a bit more difficult to predict what happens to a composite. Based on the numbers in the fin flutter spreadsheet, the table below shows how the flutter velocity of identical fins change with materials.
Material Factor
Balsa 0.05 this value seems low
Balsa (end grain) 0.49
Plywood 1.00
Carbonfiber Sandwich 1.77 Specific CF/epoxy layup over endgrain balsa
G10 4.31
FR4 4.47 not a significant difference
7075-T6 Aluminum 6.20
6061-T6 Aluminum 9.93
Carbonfiber Composite 11.40 Commercial product
It's also not clear that the brittleness of materials is considered here. For example, a pane of window glass is very stiff, but if you try to bending it you will crack it after being bent very slightly.
Finally the density determines the relative weights of the materials. A strong material that is very stiff may be very heavy and a weaker but thicker material may be a lighter weigh solution. A table of densities is shown below.
The densities of common materials
Balsa: 0.1 - 0.2 g/cc
Foam: same as balsa
Plywood: 0.5 - 0.65 g/cc
Water: 1.00 g/cc
CF composite: 1.4 - 1.6 g/cc
FG composite: 1.6 - 1.8 g/cc
For example a plywood fin is about 1/3 the weight of a fiberglass fin of the same thickness, and the fiberglass fin has a 4.4x higher flutter velocity. If we triple the plywood fin thickness to obtain the same weigh as the fiberglass fin, the flutter velocity increases by 5.2x, so in this case the thicker plywood fin is actually better than the fiberglass fin on a weight basis.
On the other hand, a solid carbon fiber composite fin made from commercial plate is 11.4x stronger than plywood, and even 2.6x stronger than fiberglass composite. For equivalent strength it can have 40% less thickness than a fiberglass fin. It wins in every way, but it is by far the most expensive solution.
The strength of hand laidup sandwich composites can not be readily predicted by a simple model. It should be noted that the carbon fiber balsa sandwich in the fin flutter spreadsheet has only a 1.7X higher flutter velocity than a plywood fin of the same thickness. A 50% thicker plywood fin will have the same flutter velocity as the example. Which will be lighter. It will depend on your construction techniques. At a minimum, if more than 1/3 of the weight is CF composite, the the plywood fins is actually lighter for the same strength!
There are really no simple answers when you use sandwich materials unless you actually do some testing.
Bob
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