Craig,

Your project is a nice one. Dave and Paul have already given a few good suggestions and I’m sure there are more to come. The subject of materials testing that can be done at home is not easy (perhaps that is my own biased opinion because I work with mechanical testing equipment). The reason, of course is that to make good comparisons we need good measurements, and that is not always easy to do at home. But I think there are some things that you can do which will contribute to the subject.

Be aware that there are two constant problems with materials testing data that is intended to be used for design. One is, did the test accurately reflect the service conditions? And the other is, how strong (or rigid or tough or impact resistant) is strong enough? For the purpose of comparing different material combinations, the second problem is not so great.

Rigidity is pretty simple. The Gougeon brothers did use single cantilevers to demonstrate the stiffness of wood. But do we want to maximize stiffness, or is there an optimum stiffness? If there is an optimum stiffness, what is it? Regardless of the answer, you could do a good demonstration and learn quite a bit with beam stiffness experiments.

We all know that impact resistance is a good thing. I think this is a good area for you to look at. The basis for all impact testing is kinetic energy. The testing is done either with a weighted pendulum (big swinging hammer), a dropped weight, or a projectile (not a good choice). You can simply increase the force of the impact until failure occurs and then compare the forces for failure. Force is equal to one half of the mass times the square of the velocity. For our purposes, velocities are pretty low and masses are fairly large. For example, picture 200 lbs of paddler and kayak falling from the face of a two-foot wave onto a rock. In terms of testing, this is a semi-quantitative way to go and requires a lot of samples. In terms of a project, you can demonstrate some basic physics and contribute some real data. You would also have to establish some sort of failure criterion (complete puncture? delamination? delamination of both skins? first sign of damage?). The other thing to consider is sample support. If the samples are supported at their edges, larger samples will absorb more impact energy through elastic deformation than will smaller samples.

Another approach that you might consider is to not focus on mechanical testing, but on how wood epoxy and glass are used together in the shop. Show the cantilever beam response of wood strip, wood strip sealed with epoxy, glass/epoxy skins alone, and finally the complete composite. Vary the grain orientation in the beam and see what happens. What does this tell you about how composites work? Measure the weight of wood epoxy and glass that you put into a panel. Does your glassing technique produce a good ratio of epoxy and glass? How much epoxy do different wood species absorb? Do you get good bonding between strips with your techniques? These are simple yet important things to know about.

You might like to check out the testing that Nick and I did not too long ago. We learned a few things that not everyone expected. Most of all, have fun, be creative, and keep us posted.

Sam