ORIGINAL: bobsrc

If the bolt doesn't break, you are using a larger bolt than needed. This is a common misconception. If you want the wing to shear off in a crash, like on a trainer, the use of 2 one quarter inch bolts are as efective as steel bolts. Most .40 sized trainers will do just fine with a single 10-23 nylon bolt. You will want to check it after every rough landing, but it will shear if you use a wing tip as part of your landing gear. This size bolt has plenty of tensile strength to keep the wing on while in flight. I flew shoulder wing quickie 500s with Fox .40s with one 10-32 and never lost a wing in flight. One 1/4-20 nylon bolt holds the wing on my .60 size sport airplanes.
Good luck