I made one more “confidence layup”. This one I did according to the instructions. When I made the first one, I had misread the instructions and used polyvinyl foam instead of polyurethane. The PV foam is a lot stronger and is used in structural applications on the airplane, so this should have made the piece stronger. What I didn’t notice until recently was that the instructions also say to use 0.5″ thick foam, and the PV foam is 0.75″. Since the strength of a beam depends strongly on its height (I seem to remember going as h^2 from the days of structural mechanics many years ago) that would also make the piece stronger. So no wonder it could take Greg’s weight…<\/p>\n
I figured I should try making one out of polyurethane just to try working with it. Wow. I knew polyurethane is a lot more brittle than the other foams (on the airplane it’s used in non-structural applications where ease of shaping is important, like the nose) but actually seeing it in action was an eye-opener. It basically turns to dust if you even look at it sharply. This made making the “beam shape” of the confidence layup a piece of cake. (The PV foam was surprisingly hard to cut and sand.) Here’s how it came out, in comparison with the carbon fiber piece:<\/p>\n
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<\/a>The polyurethane piece on the scale, weighing in at 103g. Note the as-asked for 16″ length compared to the oversized carbon fiber one.<\/p><\/div>\n
I was interested to see if this piece would crack right away when subjected to the strength test. Nope, it hold up to at least my 150lb without any problem.<\/p>\n