: Any other ideas?

Hi there.

I've been toying with the idea of nondestructive material testing for a while, and in the photo you can see the prototype of what I've come up with so far. Sam McFadden's recent posting on test results of Nick Schade's cedar strips was the inspiration, but I wanted some tests done on plywood as I pretty much just do Stitch & Glue kayaks.

While I don't think I'll be getting absolute numbers with this setup, I'm hoping to use it to compare various fiberglass layups that I have made up. Taking a full sheet of 4mm BS 1088 plywood, I cut it into 64- 6" x 12" panels, with the face grain running parallel to the short sides, i.e., the 'weaker' way. So far I've made up about a dozen test panels using two kinds of 6 oz.(plain and tight weaves) and 3 kinds of 4 oz. (plain, tight, and satin weaves), and I'm sure I'll be able to come up with at least 50 different layups to compare.

My ultimate objective is to be able to make up some simple charts or graphs comparing stiffness (as measured below) to weight/area of each layup configuration. If I can get them to make any sense, I'll post them on my website.

I've also been getting some advice from a retired UW engineering professor, Paul Ford. Like you, I had at first wanted to test panels by clamping them at one end and measuring the force required to move the other end "X" inches. He advised me that this cantilever method wouldn't give me the kinds of results I sought because most all the force becomes concentrated at the clamped end of the panel. Instead, he recommends the traditional (?) 3-point test setup, as it distributes the force more evenly over the entire panel. I guess that makes sense; anyway, it was easy to change my setup to accomodate that.

Taking some of the details from Sam McFadden's postings, here's a brief description of, are you ready for this, the Cat's Paw Nondestructive Materials Testing Apparatus, or CPNMTA for you acronym lovers.

On a sturdy, non-flexing base, I've mounted two 1/2" diameter steel rods 6" long, parallel 12" apart, with the tops 1" off the base. A 1" diameter by 6" long alloy tube, also parallel to them, sits in the middle. It is connected to a short piece of 3/4" plywood which hangs from a bolt which runs through a roughly horizontal piece of plywood about 3' long. 12" out from there, I have a 0-75 lb. range spring scale (which I picked up at a garage sale some time ago and which I think is accurate and reliable enough for my purposes) hanging from another bolt. Well, right now it's actually a nail, but there WILL be a bolt in the real setup. 12" back the other way from the tube pivot point is another bolt through a couple of plywood 'posts' securely mounted to the base. Farther back from there is enough length of plywood to mount a counterweight which I place so that the entire setup balances nicely when the tube rests about 1" above the base.

On the base, directly under the alloy tube, I'll be glueing two small copper strips that, when the tested panel is depressed the 1", will close an electrical circuit, and a flashlight will come on. This should make each test run as identical as I can think of right now.

So, when I put a panel across the two steel rods and start pulling down on the spring scale's bottom hook, when the panel has been deflected 1", the light will come on, and I (just) need to note the scales' indication and multiply it by 2 to get the actual value.

It took about 2 hours to create this CPNMTA prototype setup with scrap material I have around the shop. I figure twice that to build something more 'tight' and reliable, but it will still be a simple and compact setup which I think folks will be able to duplicate if they care to.

One of the advantages I hope to gain from this nondestructive method is that I can flip each panel over to switch compression and tension sides. Also, if I keep the data from the panels I have made up already, I'll still have those panels around to compare for variation when I have others made up.

Does this make any sense, or do I just have 'way too much free time on my hands?

Suggestions and criticisms are welcome, here or direct via e-mail. Be gentle, please.

Pete Roszyk, in Snohomish, WA