Wiseacre Dave
Well-Known Member
Greetings,
I thought I'd share the results of my recent epoxy tests and solicit some advice from the group.
Background: I've just started building a TEAM Mini-Max 1500V. I purchased the wing rib kit and have built two ribs. I've struggled a bit with mixing the epoxy by volume. It takes only a few millileters to build each rib, and I'm suspicious of the accuracy of measuring these small volumes with a syringe. In future, I'm switching to using the weight ratio for the mix. For the first two ribs I used a 1:1 volume ratio, Part A : Part B.
Testing Prep and Method: For rib #1 I prepared two test coupons corresponding to the two batches of epoxy I used. Only the second batch was used in the rib construction. For rib #2 I again used two batches, and both batches were used in construction. Test coupons were glued up just after the batches were mixed. There is sufficient bondline for the rib #1 coupons. For rib #2 I made the test coupons differently, as per Fisher Flying Products/Aircraft Spruce recommendations, and I presume this method is standard practice. For those coupons, in the shape of a tuning fork (see photos), the bondline width is between 2 to 3 time the part width (1/4 inch cap strip material). The coupons were virgin wood, not prepared or sanded and represent the same condition of wood used in the build. The wood is Clear White Pine (Nothern White Pine species). The coupons exhibited adequate squeeze-out and were clamped with small spring clamps. The coupons were tested on a bench vice, with compression force applied until failure. I don't have a photo of the coupon in the vice, but it was oriented as you'd expect---with one end of the "tuning fork" contacting one of the vice jaws, and the other two ends of the fork contacting the other jaw. This tests the shear strength of the bond joint.
Results: As is evident in the photos, both coupons corresponding to rib #1 passed. For rib #1, the coupon for the second epoxy mix (batch "b") passed, but the coupon for the 1st expoxy mix (batch "a") failed. The failure is clearly along both bondline surfaces, and there is no evident distruption or failure in the wood.
Conclusions: I am now suspicious of both ribs and the epoxy mixes used. I suspect that there wasn't sufficience accuracy in the mix ratios by volume.
Questions: 1) Are there any further stress tests I could perform on the completed ribs? 2) Do you have any advice on how to proceed, e.g. rebuild the ribs using an accurate weight ratio mix? By the way, I've purchased a scale that can measure the weight to +/- 0.05 grams. I think this would be very accurate for this purpose and would certainly give me more confidence in the epoxy. Before I build another rib, I think I'll do a few test coupons using the weight ratio method.
Thank you for any advice you can give. I've been following this board for some time now, have read a lot of informative things, and respect members' knowledge and expertise.
Dave in Salt Lake.
I thought I'd share the results of my recent epoxy tests and solicit some advice from the group.
Background: I've just started building a TEAM Mini-Max 1500V. I purchased the wing rib kit and have built two ribs. I've struggled a bit with mixing the epoxy by volume. It takes only a few millileters to build each rib, and I'm suspicious of the accuracy of measuring these small volumes with a syringe. In future, I'm switching to using the weight ratio for the mix. For the first two ribs I used a 1:1 volume ratio, Part A : Part B.
Testing Prep and Method: For rib #1 I prepared two test coupons corresponding to the two batches of epoxy I used. Only the second batch was used in the rib construction. For rib #2 I again used two batches, and both batches were used in construction. Test coupons were glued up just after the batches were mixed. There is sufficient bondline for the rib #1 coupons. For rib #2 I made the test coupons differently, as per Fisher Flying Products/Aircraft Spruce recommendations, and I presume this method is standard practice. For those coupons, in the shape of a tuning fork (see photos), the bondline width is between 2 to 3 time the part width (1/4 inch cap strip material). The coupons were virgin wood, not prepared or sanded and represent the same condition of wood used in the build. The wood is Clear White Pine (Nothern White Pine species). The coupons exhibited adequate squeeze-out and were clamped with small spring clamps. The coupons were tested on a bench vice, with compression force applied until failure. I don't have a photo of the coupon in the vice, but it was oriented as you'd expect---with one end of the "tuning fork" contacting one of the vice jaws, and the other two ends of the fork contacting the other jaw. This tests the shear strength of the bond joint.
Results: As is evident in the photos, both coupons corresponding to rib #1 passed. For rib #1, the coupon for the second epoxy mix (batch "b") passed, but the coupon for the 1st expoxy mix (batch "a") failed. The failure is clearly along both bondline surfaces, and there is no evident distruption or failure in the wood.
Conclusions: I am now suspicious of both ribs and the epoxy mixes used. I suspect that there wasn't sufficience accuracy in the mix ratios by volume.
Questions: 1) Are there any further stress tests I could perform on the completed ribs? 2) Do you have any advice on how to proceed, e.g. rebuild the ribs using an accurate weight ratio mix? By the way, I've purchased a scale that can measure the weight to +/- 0.05 grams. I think this would be very accurate for this purpose and would certainly give me more confidence in the epoxy. Before I build another rib, I think I'll do a few test coupons using the weight ratio method.
Thank you for any advice you can give. I've been following this board for some time now, have read a lot of informative things, and respect members' knowledge and expertise.
Dave in Salt Lake.
