Welding Direction: Understanding \tau_\| and \sigma_\bot

In summary, an expert summarizer of content observed that an L-shaped structure with a downward force on the horizontal arm induces a shear in the weld, which causes a moment at the vertex of the L. The question is why the equivalence \sigma_\bot = \tau_\bot exists. The solution may be found by thinking of the angle of the 'L' as a hinge attached to the pipe, and applying leverage formulae to figure out the pull-away force at the top.
  • #1
TSN79
424
0
This is kinda hard to explain without a drawing but I'll try. Imagine an L-shaped structure with a weld going down the vertical part connecting the L to a vertical pipe on the left side of the L. The curvature of the pipe is insignificant. Then on the L's right end is a vertical downward load. I find that I have a [tex]\tau_\|[/tex]. This I can see, since the weld is being "streched" in a parallel direction, but my book also tells me that [tex]\sigma_\bot = \tau_\bot[/tex] is also part of the problem, and I am not able to see why this is...can someone please help me?
 
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  • #2
I think I'd like a drawing to think about this one.
 
  • #3
Ok, drawing is on its way, I'm just relieved that someone is actually trying to help me here, didn't have too much hope...
 
  • #4
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with a vertical force on the - of the L.

Well the L has a downward force on the horizontal arm, which induces a shear in the weld. But the downward force at the end of the arm produces a moment at the vertex of the L, i.e. where the vertical and horizontal parts meet.

The question is why the equivalence [tex]\sigma_\bot = \tau_\bot[/tex], I imagine. I'll have to think about it.

I suppose PerennialII will have some insight.
 
  • #5
I think Astronuc has this one in the bag.

The L part will be wanting to rotate about it's 'elbow', causing the welded seam to pull radially outwards on the pipe.
 
  • #6
Yes, that might be it. The question Astronuc is not why [tex]\sigma_\bot = \tau_\bot[/tex], but why they are there at all. As far as I can see the weld isn't exerted by any perpendicular force, only paralell, as noted by [tex]\tau_|[/tex]. But since the force will try to turn the whole thing around clockwise, that might cause some "perpendicular-like" force...or?
 
  • #7
Yes TSN.

Imagine the L is just attached at its base. If you apply a force downwards on the _ of the L, the member will rotate clockwise about the left hand side of the _.

As a result, the weld provides a force on the L, acting towards the centre of the pipe.

If you're still stuck, read up on couples and moments.
 
  • #8
Yes, that might be it. The question Astronuc is not why [tex]\sigma_\bot = \tau_\bot[/tex], but why they are there at all. As far as I can see the weld isn't exerted by any perpendicular force, only paralell, as noted by [tex]\tau_|[/tex]. But since the force will try to turn the whole thing around clockwise, that might cause some "perpendicular-like" force...or?

Here is a link that explains the notation by the way:

http://www.gowelding.com/calcs/c2.html
 

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  • #9
Something like this -

|->
|
| . . . . .|
L______V

I believe the bending moment is greatest near the right angle in the L, and it should fall off.

Basically the weld provides a distributed load on the vertical branch of the L, and the vertical load at the end of the horizontal arm is a pointwise load.

Distributed load on vertical arm
|<-
|<-
|<-
|<-

and a moment at the right angle.
 
  • #10
... I'm getting this the same way ... applied load is shear with respect to the "L's tip", the shear causes a moment to the pipe, the moment in the pipe causes a support reaction, and the support reaction is a shear load as far as the pipe is concerned. I think the notion about the equality in | and -- can make sense if the "support span" in the pipe is equal to the "length" the L ... this way I think the shear forces will be equal in both directions (but stress wise needs some further thought if we're using e.g. beam theory ... shouldn't result generally but rather a specific result...).
 
  • #11
So to wrap up, would it be correct to say that we have a component [tex]\tau_\|[/tex] initially, but because the downward force tries to rotate the L-shape clockwise, this results in a perpendicular force on the weld, namely [tex]\sigma_\bot[/tex] or [tex]\tau_\bot[/tex] if you will. If someone confirms my thoughts on this I'll be so happy! :rofl:
 
  • #12
Pretty much, yes.
 
  • #13
I don't know any of the math, but my approach if I had to build the thing would be to think of the angle of the 'L' as a hinge attached to the pipe. Then you can do whatever number stuff you have to do in order to figure out the 'pull-away' force at the top by leverage formulae. Then factor in that the weight would also be trying to slide the whole thing down the pipe, which I guess is the 'sheer'. When in doubt, use bolts instead.
 

1. What is the significance of \tau_\| and \sigma_\bot in welding direction?

The symbols \tau_\| and \sigma_\bot represent the shear stress and normal stress, respectively, that occur during welding. These stresses play a crucial role in determining the strength and quality of the welded joint.

2. How do these stresses affect the welding process?

\tau_\| and \sigma_\bot can affect the weld in different ways. \tau_\| can cause the weld to deform or distort, while \sigma_\bot can cause the weld to crack or fail. Understanding and controlling these stresses is essential for creating a strong and durable weld.

3. How does welding direction impact \tau_\| and \sigma_\bot?

The direction of welding can greatly influence the magnitude and distribution of \tau_\| and \sigma_\bot. For example, welding in the direction of \tau_\| can result in higher shear stresses, while welding perpendicular to \sigma_\bot can lead to higher normal stresses.

4. Can \tau_\| and \sigma_\bot be controlled during welding?

Yes, \tau_\| and \sigma_\bot can be controlled through various techniques such as proper joint design, material selection, and welding parameters. By understanding the direction of these stresses and implementing appropriate measures, welders can minimize their impact on the weld.

5. What are some common challenges related to \tau_\| and \sigma_\bot in welding?

Some common challenges include distortion and cracking of the weld, lack of fusion or penetration, and poor joint strength. These issues can arise if the welding direction is not considered or if the stress levels are not properly managed during the welding process.

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