After digging a bit more into bolts and nuts a learned that torque isn’t tension.
Goal for a bolted joint is sufficient clamping force / tension. Because this is difficult to measure we are used to use torque because this is easy to measure with a torque wrench.
After reading Tricks Engineers Need to Know About Fasteners and seeing these YouTube videos Torque and Tension and Applied Bolting – Torque isn’t Tension I was surprised that that torquing a bolt is not as accurate as I thought.
On an Ebay auction I purchased a Skidmore-Wilhelm meter for not too much money and could do some tests myself.
My setup: AN3-17A bolt + washer + Skidmore-Wilhelm + 6 washers + AN365 nut (I used a longer bolt and that many washers to simulate the multiple layers that the wingspar has at the root).
I used my Garmin action camera to record the tests:
I first torqued the bolt and nut to 29 inch/lbs that resulted in 600-650 lbs clamping force.
Then I torqued the bolt and nut to 37 inch/lbs and that resulted in 800-850 lbs clamping force.
(37 inch/lbs is the latest total torque value I received from TAF for AN3 bolts)
I also tried another setup: AN3-13A bolt + Skidmore-Wilhelm + 1 washers + AN365 nut and at 29 inch/lbs this gave almost 1000 lbs clamping force.
Conclusion: less layers in a joint results in more clamping force.
The difference in torque before the bolt or nut fails is quite huge: between 75 and 137 inch/lbs. The clamping force is between 2000 and 2500 lbs before the joint fails which is less than the 2640 lbs (see previous post) due to the rotational force.