U bolted 1x1 6061 T6 tubing 1

[amorrison]

I am designing a homebuilt using 6061 T6 and don't like the common joining methods available.

If you are stressed near maximum (considering fatique(0.3of yield) and general safety factor (3)) there isn't much strength left (39000x0.3x0.3=4000psi) BEFORE welding strength and SCF reductions - -
Additionally
1. Welding without post heat treating looses half the strength of the base metal.
2. Joining with bolts/rivets locally looses 1/2 to 2/3 the strength because of stress concentration.
If you have to rivit or bolt near a untreated weld strength is down to 4000psi x 0.5(weld reduction) x 0.33(scf reduction)= 666psi. NOT GOOD !

I am considering U bolting the 1x1 aluminum at high stress connections - using some SS steel "pads" under the bolts to distribute their clamping stress. Additionally a neoprene pad between the 1x1's to increase friction and absorb some vibration. This way I have at least say 3,000psi to work with.
Comments please.

 

[SuperStress]

If you are talking about having primary structure in your homebuilt fastened with friction joints, I think that's a bad idea. I wouldn't fly in it that way.

Is it that hard to find someone to post-weld heat treat your tubing? You can restore your strength significantly with this step and end up with a much more sound joint.

Niu gives a weld efficiency factor of 0.6 for "as welded" aluminum and 0.85 for post-weld heat-treated alloys (to T4 or T6). (Ref. Airframe Stress Analysis and Sizing, 2nd edition, Figure 9.6.5, p. 309).

You should always keep stress concentrations away from welded joints anyway because of the heat affected zone.

Consider also that adding neoprene between your tubes will add eccentricity to your joints, besides the inherent misalignment you get from having your tubes meet off-center.

It's my personal opinion that you should weld all joints in tubing since you can't get good clamp-up on them with bolts without crushing them. You can add bearing blocks to prevent the crushing, but then you're adding a lot of weight.

If you want to stick with bolted construction, stick with open extruded sections and design your joint using standard techniques for fatigue.