Trying to understand welding recommendations

[Juanda]

TL;DR Summary

Shown welding recommendations seem to contradict Von Mises' failure theory and I don't understand it.

Recently I found these notes with recommendations for welded joints and something is not sitting right with me. It's in Spanish but I'll translate it as close to technical English as I can.

Original source

http://educacion.sanjuan.edu.ar/mesj/LinkClick.aspx?fileticket=dDgGbMeIr3A=&tabid=678&mid=1743

Translation to English

Angled joints with crossed cords
When only 2 cords are crossing (Figure 16) the welds must be done following a), because even if b) avoids biaxial traction, the effect from the hole (entalla) is more negative than the biaxial traction itself.

When the joint has 3 cords (Figure 17), the effect from the from the triaxial traction and its consequent danger of fragile fracture implies that the recommended configuration is a) instead of b) although the best possible solution would be to avoid having 3 cords intersecting in a point.

I have checked a few sources on the internet and they all say basically the same thing. I couldn't find English sources. Maybe I'm missing the keywords in English. I know these recommendations are used, at least in Spain, because my home university has that kind of joint at the base of the beam pillars. It's a shame I don't have a picture because I've been wondering about this since I was studying there.I'm trying to understand what's the base for these recommendations. I believe the recommendations are related to the thermal expansion of the cords and the residual stress once it cools down. I'm especially troubled with this part of the text "The triaxial traction and its consequent danger of fragile fracture".
Why is it that triaxial traction causes fragile behavior? I could not find other sources confirming this and explaining the reasons for it.
According to Von Mises's failure theory, triaxial traction is easier to withstand than uniaxial traction since its deviatoric stress should be lower even if its hydrostatic component is greater because the last ones do not contribute to failure so I don't understand why the emphasis on this triaxial tension state.

$\sigma_{VM}= \sqrt{\frac{(\sigma_1-\sigma_2)^2+(\sigma_2-\sigma_3)^2+(\sigma_1-\sigma_3)^2}{2}}$

Source:

Does the fabrication process of the 3 cords contacting each other change the chemical properties of the metal so it's no longer ductile? Is it that the high temperatures involved and rapid cooling make it fragile?

 

[anuttarasammyak]

Reheat cracking might be a concern.

 

[Juanda]

Reheat cracking might be a concern.

Could you provide more details about that?

 

[anuttarasammyak]

I have read https://www.twi-global.com/technical-knowledge/job-knowledge/defects-imperfections-in-welds-reheat-cracking-048#:~:text=Reheat cracking may occur in,temperature service (typically in the .　I hope it informative to you.

 

[Juanda]

I have read the article and it's related and definitely interesting. However, it doesn't answer the initial concern. Why is the triaxial traction related to fragile behavior?

I'm especially troubled with this part of the text "The triaxial traction and its consequent danger of fragile fracture".

 

[Lnewqban]

I have read the article and it's related and definitely interesting. However, it doesn't answer the initial concern. Why is the triaxial traction related to fragile behavior?

I believe that the article of post #4 shows an example which makes those recommendations of your book more relevant.
That is not the case for the mentioned base of the beam pillars.

The difference is determined by the types of steel used for a nuclear pressure vessel (heat treated alloy steel working under high and variable temperatures and pressures) and for a metal structure (hot rolled low carbon steel).

 

[Juanda]

This thing is still in the back of my mind. I recently came across some additional information that might be illuminating for someone else to fully unravel the mystery.

Again, the documentation I could find this time is only available in Spanish. It's from Valladolid University (Universidad de Valladolid). The relevant part is chapter 3 (Elastic Solid) which talks about historical tests (traction and torsion), the generalized Hooke's Law, failure theories, etc. I'll try to translate and process the key bits most relevant to this conversation.

From page 35

Finally, notice that the experiences from Bridgman cannot be extrapolated to the triaxial traction. Although for triaxial compression the component could survive pressures as high as the equipment would allow, when the experiment is done with triaxial traction on "sweet steel" (acero dulce, carbon below 2%) a fragile fracture may happen at tensions higher than the elastic limit.

So the behavior predicted by Von Misses, an accurate and well-accepted theory for these materials, in the triaxial traction case differs from the experimental results. It makes sense we can't infinitely traction the part when pulling from all sides simultaneously but the change in behavior (fragile fracture in ductile material) baffles me.

From page 37

In the quadrant where all tensions are traction, it's not valid to think the plastification surface can be extended indefinitely. As it was previously pointed out, a fragile fracture will happen.

So the plastification surface predicted by the theory (what's shown is for Tresca, Von Misses would be a cylinder) must be capped somehow to agree with experimental results. I don't know if that cap can be derived mathematically or if it's obtained experimentally. The document shows the shape later.

From pages 40 and 41

To finish, let's revisit the possiblity of a fragile fractule of a normally ductile material when working in a triaxial traction state. Let's define a fracture surface as the geometrical place where the main tensions $\sigma_I$, $\sigma_{II}$ and $\sigma_{III}$ would cause the fracture of the material. If the fracture surface were reprensented overlapped with the plastification surface the result would be similar to what's shown in Figure 3.15a in which can be seen how fracture can happen inside the plastification surface. It is then possible to load the element is such way that it breaks before crossing the plastification surface. In other words, a fragile fracture.

The current version of the Techical Code for Edification (CTE: Código Ténico de la Edificación) does not mention this phenomemon. Perhaps because how unlikely it is to find building structures where triaxial traction occurs. However, such states can happen for example in welded joints due to residual stress depending on their geometrical disposition (as shown in this thread in the original post). Previous versions of the technical codes (EA95) indicated that even if Von Misses is satisfied it's also necessary to check $\sigma_I<2\sigma_e$.

I believe the red surface on the left picture is from experimental results. Then, on the right side, there's the engineering simplification to cap the predictions done by the Von Misses failure theory.

In conclusion, welding cords are just a scenario that makes triaxial traction more likely depending on how they are positioned. But it's not the reason behind the fragile fracture because such a phenomenon has been observed in cases where there are no welding cords involved.

It feels like there are two mechanisms at play to describe the behavior of a chunk of metal under load. Theories like linear elasticity + Von Misses in one of them and it predicts the results pretty well within certain limits ($\sigma_I<2\sigma_e$). But there is a point where the similarities with reality break apart and not by a little bit. According to Von Misses the chunk of metal can be loaded indefinitely and in reality, we see a fragile fracture. (We could say the theory breaks in both directions since if we keep compressing the component then nuclear things could start happening at some point but that's beyond practical applications and the intended purpose of this post)

This post kind of answers the original question.

Does the fabrication process of the 3 cords contacting each other change the chemical properties of the metal so it's no longer ductile? Is it that the high temperatures involved and rapid cooling make it fragile?

We now know that welding cords are not the origin of the phenomenon. However, it opens even more concerning/interesting questions. What's that second mechanism at play? Why does the change in behavior (ductile → fragile) happen in the shown scenario?

Anyway, that's all I could find for the moment. Just like with anything else, this is probably a well-studied and documented phenomenon with math already developed around it. If you know of it let me know, please.