Question for judges and experienced builder connected with welding

[Jonny Rochester]

OK, Z and I agree on something! I'll make a note.

[Alumni]

And we shall agree to disagree.

I was never certified or got to enjoy underwater welding but there was nothing like a bit of x-raying while standing inside of a 250,000 lb forge press! (Also a stick weld repair.)

[rwstevens59]

First things first. The weld as shown in the photo would never pass any technical inspection by anyone including any experienced welder. Not trying to belittle your work, just stating a fact. Being mild steel tubing grind and wire brush clean and fill the gap all the way around. Having done my fair share of tube frame race car and airframe welding (a lot with just an oxy-acetylene torch) I will have to say these narrow angle joints can be a real pain.

As stated above any manual welding process is as good as any other for this type of work and material. It is much more about the technique and experience of the welder than any particular process.

I am having trouble with the idea that a properly welded tubing cluster, with good weld bead reinforcement, introducing any major compliance issues that matter. I will concede that a welded joint is in fact a localized casting process and will have an annealing effect on the material some distance back from the bead. Even with this mild steel tubing you will loose some of the tubings strength that was provided by the work hardening that occurred during the manufacture of the tubing.

All of that said, tube frame compliance issues, at least in my experience, are more a function of the design of the structure itself i.e. poor load paths, bad geometry and with tube members loaded in bending rather than tension or compression. I see gross examples of the preceding almost daily.

I will however agree that if the structure is properly designed and IF a comparable structure could be made homogeneous somehow it would most likely be less compliant than the welded structure. But for the business at hand it is of theoretical interest only.

Just my two cents worth.

Ralph

[Tim.Wright]

Interesting discussion...

Even though I'm not a design judge, or a builder, or connected to welding I will add my 0.02c.

My opinion is that unless a weld has failed (i.e. gone past its elastic range) then it's effect on compliance for a fully welded joint should be practically negligible. Overall strength and durability might be questionable, but this is another discussion.

Reason being is that a weld is such a short part of the overall structure. Given that compliance of a member in most modes of deformation are proportional to the length of the member, then its practically impossible for a weldment of say 5mm of size to deflect by a magnitude similar to that of a decent length of tube.

For example:
Consider a typical FSAE spaceframe application of a 800mm length of tube (25mm x 1.6 wall) with a weldment 5mm long at each end. Something which could be a side intrusion bar. If we start by assuming the weldment has the same cross sectional area and youngs modulus of the tube then you have the following 3 stiffness' in series
k_weldment = EA/L = 4940kN/mm
k_tube = EA/L = 30.9kN/mm
k_other_weldment = EA/L = 4940kN/mm

So its clear that the weldments are pretty rigid compared to the tube. If we look at the parts in terms of compliance instead of stiffness its easier to visualise the contribution of each part:
p_weldment = 1/K = 0.20 um/kN
p_tube = 1/K = 32.4 um/kN
p_other_weldment = 1/K = 0.20 um/kN

p_total = 32.8 um/kN

So here, each weldment is contributing only 0.6% of the total compliance.

If we simulate a "bad weld" by making it really soft. I.e. lets half the youngs modulus from 210 to circa 100GPa we get the following compliances from each part:
p_weldment = 1/K = 0.20 um/kN
p_tube = 1/K = 32.4 um/kN
p_BAD_weldment = 1/K = 0.42 um/kN

p_total = 33.02 um/kN

So you have increased the compliance by 0.7%. Bugger all basically.

If you consider this tube in a loaded condition to say 100MPa, then the bad weld is going to add a grand total of 2.5microns of extra deflection in this tube which is nothing compared to the total deflection of the tube + weldments which is 388microns.

Sure, my example is very simplified in that its tensile forces only on a single member but given that a good spaceframe should load its members primarily in tension/compression and that compliances are additive throughout the structure then for a full spaceframe you should also find that, within an order of magnitude, the portion of your total compliance coming from your welds is equal to their portion of the total length of the structure which is pretty small. So something in the order of a few %.

T