: Sam,

: As a structural engineer and hobbyist this is very exciting information for
: me. I once developed a computer program to calculate the strength of model
: sailplane wing spars using different materials, say balsa for the webs,and
: a combination of carbon fiber and hardwood for the flanges.

: What I was trying to do computationally, perhaps you could do through
: testing. I am trying to determine how much stronger a composite section is
: of say 4 mm Okume ply with 6 oz e glass on both sides versus a section of
: 4 mm Okume ply with just 6 oz e glass on the outside. I believe I came up
: with a rough estimate that the section with glass on both sides was
: roughly 32 times stronger than the other section, but this was only
: considering the position of the fiberglass and not considering the
: strength of the wood. Considering the wood, the strength ratio should be a
: lot less.

: I know that the most efficient section would be if the glass was symmetrical.
: I have read of so many people just glassing only the outside of their
: boats that I think a test of this info might be enough to discourage them
: of this practice, or at least make them aware of the advantage of glassing
: the wood symmetrically. I think it makes a big difference in say the
: strength of a deck to flexure when leaning on it, when you glass both
: sides.

: I have a wood handbook from the 1930s giving info on hardwood strengths, back
: when wood was considered a valuable material for wing construction. I
: don't have any information on the strengths of Okume ply, or of glass. I
: would also need how much glass per inch is actually in a certain weave in
: order to estimate its strength. If we could get this info together perhaps
: we could compare theoretical bending strength results to your impact
: testing results. Just a thought.

: If I am correct in reading your last graph it did not show a section with
: glass just on the outside.

: Keep up the good work.

: Regards,

: Steve Pituch

Steve,

Computation is a great way to develop possible solutions and to start off in the right direction. It also helps us think about the way structures actually behave. One big problem with modeling, as you obviously know, is having good properties data. For composites that means a minimum of 4 elastic constants, and a reliable failure criteria - which adds on at least another 2 to 3 failure strengths. That’s for each lamina! If we change from plain weave E-glass to satin S-glass, we need to go measure those properties again, and do it for the wood too. So this kind of testing goes straight to the experimental solution - but it is a particular solution for local loading normal to panels (thin shells). We would have to replicate (approximate) the correct loading condition for airplane wings to take the same approach.

There is more and more properties data available for composites, but the vast majority is for vacuum bagged samples made with elevated cure resins.

You are correct: there are no results shown for glass on only one side of the samples.

Thanks,
Sam