Allowable loads in aluminium welds

Hello all

My little folding cat is having a slight redesign and I would like to change the beams from composite to welded aluminium trusses. The composite ones are fine but they are hard to build, heavier than alloy ones would be and tricky to align hinges on.

So I have been looking into welded trusses and come across some issues.

Of course welding is fine for many structures in alloy but what is a good stress limit for alloy at a welded joint. As these are cat beams they may cycle and fatigue (although most of the structure is pre-stressed I can't guarantee that the fore and rear beam trusses won't come out of compression and into tension)

I remember stories of Jim Brown's A frame Searunners having some problems but those boats are much heavier than mine. I would like to use RHS about 30 x 60 mm for most of the truss.

Do I use 6061 T6 or a more weldable alloy? In most of the structure it is not strength I need but stiffness - E. (Buckling is an issue) So I could use a better welding alloy and accept a lower strength one if that is appropriate.
What stresses are acceptable for a reasonable weld when fatigue is an issue?

Thanks in advance

Phil Thompson

 

6061 happens to be the strongest weldable alloy, but the welds will not be nearly as strong as un-welded portions, especially so for high 'T' hardness, eg 'T-6', therefore the allowable loads are rather disappointing for welds on
6061 T-6. Someone can chime in with the numbers, but what I'm saying is generally true. The strongest and stiffest alloys of aluminum are NOT weldable. This is the chief reason that aircraft are assembled with rivets and other fasteners instead of welds; they use 2024 or 7075 alloy. Many crossbeams, masts and etc on catamarans are assembled with rivets as well. Could the beam be so re-engineered as to avoid welding altogether?

Jimbo

 

Phil

You need to address this in stages

1) Design...what loads will be expected? This shall determine the scantling (or section) you will select. This is influenced by #2

2) Alloy. For the marine environment, your truss will be subjected to spray as well as 'normal' seawater. And as you noted, it shall be welded. The preferred choice is 6082-T6. The UTS is greater than 6061, but not much. However, the principal reason for selecting 6082, is the low copper content and as such, is much much less susceptible to corrosion, compared to 6061.

3) The design is also influenced by #2. The strength of weld value to use, shall be the as-welded strength. As a rough guide, the as-welded strength of 6082/6061 is roughly the same at around 110~120MPa. So this is your design limit.

4) However, being subjected to fatigue and being in a seawater environment, this value reduces from 120MPa down to 10~20MPa.

Hence your design limiting stress for a "safe fatigue" seawater environment will be to use 10~20MPa, as your design allowable.

The temper, T4, T5 or T6, is not really worth worrying about. Best to select one that is readily available. Since using the design allowable, the reasons for selecting a T4 over that of a T6 is negated by the "expected service" which has driven the design!

Trust this helps.

PS..i should add, if fatigue is a concern, then the detailing is much much more important. These details are usually the causes for structural failure, not the design allowables. If your detailing and manufacturing methods are not sufficient, it will failing owing to this. Correct detailing is very very important when designing for fatigue. As I've noted here:
http://www.boatdesign.net/forums/metal-boat-building/metal-frame-detail-28371.html