Hi Guys,
I've just begun building some wing ribs for my Pietenpol Air Camper. I have made a few already but was made aware that I have made a misstep in that I have not done the following:
1) Make a log for the build (every piece that was ever glued).
2) Make a set of "test" strips for every batch made (I am guessing it's per rib, etc..)
Can anyone clarify what has to be done so the AME can approve all of this when i am done?
PS. I am in Canada.. lol
Can you provide the source for the test strip tests so I can see what they want?
Generally, adhesives are tested with adherencds of the same materials that are being bonded, then those "lap shears" are pulled in a tensile testing machine to measure the force required to break the bond. That force divided by the bond area gives the shear strength of the coupon.
Adhesive failure modes are "adhesive" and "cohesive". Adhesive failures are between the base material and the glue; cohesive failures are within the glue. Ideally, you want failures to be cohesive because that indicates that the glue has reached all of it's capability. But most failures are a combination of the two modes and are reported as percentages of each mode.
When an epoxy adhesive vendor reoprts his adhesive's strength, it's usually done on aluminum adherends 1" wide and 6" long and .025" thick. This gives high numbers, like 3500 psi. A wood joint would probably never come close to such values and the failure would be in the wood peeling apart. I would consider that a successful test of the adhesive.
Actually I believe you are looking for a percentage of wood failure when doing the common adhesive tests for wooden construction. Basicly testing both the physical material, and the method of use.Originally Posted by Bob H
Why are you using epoxy with wood? Epoxies, especially T-88, gets soft and weak when it gets warm, like it does inside a wing on a bright summer day, tied down at a fly-in. I performed a controlled comparison test of several different wood glues a few years ago, and my results pretty well duplicated the results you can find online. The two best glues for building wood airplanes are TiteBond III and the only FAA approved glue: Resorcinol. Maybe ol' Norm Abrams swore by one glue or another when he had his New Yankee Workshop show, but he wasn't building life support equipment like we are, was he?
In answer to your question, should you decide to use epoxy to make something out of composite, mix more epoxy than you'll need and put it in a small Dixie cup and keep it with the item you laid up with it, so you can check its hardness and consistency after the batch goes bang and all the molecules have linked up. Label the cup with a number and label the layup with the same number. Whenever I built wood ribs, I always dated the rib as it came out of the jig. If you wish to "log" each rib build in a journal with environmental info and maybe the batch number of the glue you used, it may or may not impress your AME and Tech Counselors. And yes, you can make sample pieces to later test your glue batches, but the samples must be identical, so only the glue batch is the variable, for the samples to mean anything. For this you'll have to create a jig just for holding the two sticks in place while the glue dries/cures, and another fixture for making all the sticks from the same board, so they're all identical. Then you'll have to acquire a calibrated test rig with a load cell and data acquisition software to record the tests when you pull the glue samples. E-gads!!!
My advice is to just focus on your craftmanship so your joinery is accurate, with consistent gaps, and you wipe away as much of the excess squeeze out as you can without drawing any glue out from the joints. If your AME sees nice, clean ribs with just enough squeeze out to confirm the glue joints aren't too "dry" and all the ribs are identical, he's going to be more impressed than if you bombard him with a bunch of data and results from your pull tests. Just build it and if you muck up a part, learn from it, destroy it, but DO NOT take a photo of it!
Piets are a LOT of work, so don't unnecessarily add to your workload by overdoing the documentation. Take lots of photos and store them on a flashdrive or DVD, far from viruses and ransomware.
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