Residual Stress in a Bent U-Bolt

[jorton]

I'm looking at a U-bolt that has been bent from A36 rod. It will be loaded in such a way as to produce both tension in the legs and moment in the bend. Does anyone know of a reference that would help me estimate the residual stress in the bend? I plan to stay in the elastic range with a healthy factor of safety, but I would still like some idea of the stesses already in the bolt. I know I could heat each piece to relieve the stress, but I would like to avoid this if possible. Thanks for your help.

 

[GregLocock]

I'd have thought some carefully sketched plots of stress and strain vs distance from the neutral axis would answer that.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[kelowna]

Residual stresses on the bolt? If it was bent, obviously the stresses go beyond the elastic limit. Heat treat it? I would not go there, you might end up affecting the strength and it will distort for sure.

In my mind this is no different from a welded I beam or a rolled section, which everybody knows have residual stresses, sometimes beyond elastic limit and everybody just choose to ignore.
We do de same with A320 bolts that can be re-used after tensioning, even if that tensioning took them over the elastic limit, which it does.
I do not see why the approach would not work with a U-bolt although I would like to hear what other engineers have to say.
Incidentally, I would assume that all U-bolts commercially available have been formed by bending straight rods, I doubt they are cast that way.

 

[40818]

I agree with kelowna,
To achieve the desires shape it has to go plastic, and apart from going through the heat treatment to relieve the stresses you will have residual effects. But if you get some bloke on the shop floor bending a but of plate sheet in to an angle the same thing occurs. If memory serves correct then wouldn't the stress/strain will follow a hysteresis loop back down to zero stress with an associated permanent strain? Havnt had a coffee yet so I might be confused.

 

[jorton]

It definitely goes plastic across the entire cross section (finished bolt is longer than original bar, but I don't know by how much). There is also a somewhat uniform change in cross-sectional area through the bend. I feel comfortable ignoring the residual stress in a beam as it is localized at the flange tips and flange/web connection rather than across the majority of the section. I can't really get my head around how it will be distributed here. I have thought some about the hysteresis loop also. I agree it will go to zero stress with an associated permanent strain, but won't this be an average stress? Could there still be significant stress at the extremes which would greatly affect the bending strength?
Thanks all for your help.

 

[kelowna]

http://www.boltcouncil.org/files/2ndEditionGuide.pdf

Page 62 talks about reuse of bolts. It might help.

I still think that the bolt goes plastic, deforms, and when tension drops to zero, the deformation stays, but when loading the bolt again it loads elastically. 40818 put it quite nicely.

 

[PersonalProfile]

My statics book (at work, not with me at home) has the basics (think of cambered beams as a structural analogy).

AS4100 is based on plastic analysis for compact sections.
AS4100 also, if I recall correctly, stretching the memory a bit, states that a bolt can be reused / retightened, once only, in its original location, then must be replaced with a new version.

Would suggest you investigate specify a "u bolt" "off the shelf" - less risk, hopefully the OEM will specify a rated capacity and should be cheaper (and quicker) then having a contractor complete the deed, in unknown circumstances (doesn’t matter what you tell them, unless you watch and enforce the required procedure).

As stated earlier, suspect some form of stress relieving process may retrieve some of the original properties, subject to details by others. Maybe you could do the modifications with the material at an appropriate temperature (hot work, rather than cold) again, details by others.

Regards,
Lyle