Residual Stress After Bending Sheetmetal
Residual Stress After Bending Sheetmetal
(OP)
First post on Eng-Tips!
This is as much a question as it is something I'd like to get a discussion going on. I'll phrase it like a question:
When bending sheetmetal, the material at the bend has clearly been stressed past the yield stress but less than the ultimate/failure stress. So there are residual stresses at the bend and material has yielded before the part is ever loaded.
When analyzing a part for structural integrity, is it appropriate/prudent/good practice to consider this residual stress/yielding? Or is it ignored to make the problem more manageable? Does anyone have experience either way?
This is as much a question as it is something I'd like to get a discussion going on. I'll phrase it like a question:
When bending sheetmetal, the material at the bend has clearly been stressed past the yield stress but less than the ultimate/failure stress. So there are residual stresses at the bend and material has yielded before the part is ever loaded.
When analyzing a part for structural integrity, is it appropriate/prudent/good practice to consider this residual stress/yielding? Or is it ignored to make the problem more manageable? Does anyone have experience either way?





RE: Residual Stress After Bending Sheetmetal
1) plastically form the s/m (usually in O condition) and then heat treat to remove the strains and to increase the temper (= strength) of the finished part. Thus no residual stresses.
2) plastically form the s/m and live with the residual stresses. Forming to standard shop practices (eg 3t bend rad) has shown (over the 50+ years we've been doing this) that this ok.
Quando Omni Flunkus Moritati
RE: Residual Stress After Bending Sheetmetal
Option 1 would certainly be the best structural solution. Unfortunately, the specific design I'm working drives us to Option 2 for various reasons (cost, material availability...).
The reason I'm a bit concerned is that the bend radius is the tightest within the recommended forming range and the part will be loaded in the exact same direction as when it was bent. Thus it seems unsafe to neglect the residual stress.
I'm wondering if there is any standard/textbook way of analyzing this problem, short of some complicated non-linear FEA modeling the bending process and capturing whatever residual stresses develop during forming.
RE: Residual Stress After Bending Sheetmetal
Quando Omni Flunkus Moritati
RE: Residual Stress After Bending Sheetmetal
Teasing aside, the answer to your question is "yes" and you will find much detail in the textbooks under this forum subject's FAQ. here: http://www.eng-tips.com/faqs.cfm?fid=919
I believe some research under "tension clips", either with the forum search tool, or the old-fashioned google approach, may also reveal some design charts that you can use, but the source may be uncertain.
STF
RE: Residual Stress After Bending Sheetmetal
My Google efforts have turned up some interesting stuff on FEA modeling of springback after bending, but nothing from a stress analysis perspective.
For what it's worth, in my experience I've done as rb1957 suggested, either 1) heat treating the part to remove the residual stress or 2) bending per standard practice, leaving a solid design margin, and not worrying about it. I also spoke to a structural DER and he echoed the same sentiments that option 2 has been historically shown to work and be acceptable.
But it's my job to worry about this kind of thing, so I posed the question. I'm hoping to draw on some collective wisdom on the subject. SparWeb, if you know of any specific design charts or applicable references, I would be indebted to you if you could pass them along.
Perhaps this question would be more suited for a mechanical or materials forum?
RE: Residual Stress After Bending Sheetmetal
we did a bunch of FEA ... it showed the most unexpected results, i think it showed a stress peak on the free edge of the undeformed flange of the angle (the one to attach to the fuselage), i was expecting to see nasty stresses at the corner. it fell onto the back burner but it'll have to be looked at again (i thought we used plastic FEA (we should have), maybe we'll try photo-stress ?)
residual stresses are something to worry about; saying we've done it that way for 50+ years could easily be the beginning of a "mayday" episode.
Quando Omni Flunkus Moritati
RE: Residual Stress After Bending Sheetmetal
Link
I've seen statements that the hysteresis loop returns the stress to zero when the sheet is unloaded after being bent, just with permanent deformation, but I don't buy that.
It seems like there ought to be design tables for this somewhere. I've been in the industry a couple years and haven't seen any.
RE: Residual Stress After Bending Sheetmetal
Quando Omni Flunkus Moritati
RE: Residual Stress After Bending Sheetmetal
It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.
RE: Residual Stress After Bending Sheetmetal
Long story short, best to just heat treat it if it's a structural part and save yourself the headache and worry.