Testing old boomis to check glue integrity

[michiganmarc]

After reading Dan Holdgreve's post about not wanting to use boomis older than 90 days due to CA glue losing integrity, I was curious to test that theory a bit myself.

Last year, I helped with a Division B team late in the season. We wound up with 7 unused boomis, but all of them were pre-tested to a reasonably high load. I just tested them all to 15kg to see what would happen. They were mostly over-designed builds (heavy), but that is still a valid glue test.

These boomis were stored in my basement on a shelf, so they would have been subject to the full temp and humidity swing of a summer/winter.

Mass; Pre-test in Spring 2019; Test on 2/21/2020

12.29 g; 10 kg x1; 15 kg
12.22 g; 10 kg x1; 15 kg
10.06 g; 10 kg x3; 15 kg
9.93 g; 10 kg x3; 15 kg
8.60 g; 10 kg x3; 13.41 kg
8.47 g; 10 kg x3; 13.02 kg
7.29 g; 10 kg x2; 11.25 kg x1; 15 kg

From this data, it looks like the glue (and wood) held up fine. I'm not saying it's not better to have a "fresh" build, but from this testing, it doesn't look like it's an automatic fail if you have to use an older device.

[dholdgreve]

LOL... Well done Marc,
It seems like my theory may be all wet, but I have seen it time and again where we have had a boom carry consistently 4,5, even 6 times, then break on the 7th... Sure, maybe it is damaged wood from the previous tests, but in nearly every case it was at a joint, not in the middle of a span. To me this indicates glue failure, not wood failure.

Thanks for the update!

[michiganmarc]

My simple test is certainly not a definitive study, but it's some data to consider.

I have definitely seen where testing near the limit damages the structure without anything visible. This may be the wood, which I think can often be heard with "cracking", or perhaps the glue joints themselves get internally stressed and get weaker to the point of failure on the next test. I'm not certain the glue fatigue has an age component or not. It would take dozens of controlled builds and testing to really have confidence one way or another.

I remember doing some loading tests 4 or 5 years ago with bridges when I was first playing around with pre-testing devices. I found that if we were relatively conservative 70-75% of eventual max load, we could test many times. I think I even pre-tested a bridge 15+ times once. It's when you get closer and closer to the max eventual load that all bets are off.

Of course, this year's rules makes it almost necessary to test to 15kg for an important competition, so unless you are going to design to close to 20kg (which I don't advise), it's going to be a little more risky.

Last year, our #3 National boomi held 14.95kg, which was almost a perfect result. This year, that would be a disaster!

[dholdgreve]

well... if things stay on track as currently planned, looks like we will be testing both theories in earnest next season! LOL

[michiganmarc]

Very true! I feel really bad for the seniors on the team. For next season (assuming it happens as scheduled), everyone should have a nice head start on their boomi design!

Some of our best builds this year were fairly recently re-tested to 15kg in preparation for Regionals that never happened, so we'll definitely try those again next season at some point.