residual stress in bent steel
residual stress in bent steel
(OP)
I am in process of designing a steel truss that utilizes bent steel shapes as web members. I would anticipate that bending steel would result in residual stresses that I would need to include in my analysis.
DOES ANYONE KNOW HOW TO FIGURE THE MAGNITUDE OF THE RESIDUAL STRESS?
DO DIFFERENT BENDING PROCEDURES RESULT IN LESS RESIDUAL STRESSES?
DOES ANYONE KNOW HOW TO FIGURE THE MAGNITUDE OF THE RESIDUAL STRESS?
DO DIFFERENT BENDING PROCEDURES RESULT IN LESS RESIDUAL STRESSES?






RE: residual stress in bent steel
Residual stress can be tested. Using a special strain gage, you can determine the residual stress in a member after bending. It is almost non-destructive in that it only requires drilling a small hole. Not sure where you are located, but some testing labs have that capability. Otherwise call MicroMeasurements in Raleigh, NC.
RE: residual stress in bent steel
As already mentioned by Ron, such a calculation can't give sufficiently realistic figures, simply because the stress patterns will be very different depending on the bending (or forming?) procedure.
Moreover you will inevitably find stresses that are close to yield, if, as I understand, your process is a cold forming process: what usage do you intend to do with them?
If you are assessing the strength of a structure, then you shouldn't worry about residual stress, unless you are in a high fatigue environment.
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RE: residual stress in bent steel
The bent truss member cannot be tested until it is bent, which is hopefully going to happen after the design. I was hoping I could somehow estimate the residual stress level and take that in consideration when evaluating the stress level under gravity and wind loads. The stress level of the bent HSS8x6x3/8 is at 76 % of its allowable stress level under gravity and wind loading, but not considering the impact of bending.
The stresses being close to the yield makes sense in order to have plastic deformation. Wouldn't that mean that the remaining capacity for additional loads is essentially zero when bent by cold forming process?
The truss will support a roof for a new church, has a 60'-0" span and a 22'6" tribuarty width. The top chord on each end of the truss pitches at 6:12 over a 15'-0" length. The center 30'-0" section of the truss has a flat topchord that is approzimately 6'-0" from the bottom chord. The webing of the center 30'-0" section consists of a single arch member with the crown of the arch pointing upward. The project, has am I, are located in Asheville/ North Carolina.
RE: residual stress in bent steel
You can assume a simplified stress-strain curve (IE, linear up to yield point, constant stress after that) and calculate the residual stresses. It gets messy pretty quick. In a bent plate, the residual stress level varies depending on the radius. As the radius becomes smaller, the plate acts more like a plastic hinge, with yielding across the full face.
Don't give this too much thought or you'll start wondering if beams come out of the mill with residual stresses, and what should be done with them.
RE: residual stress in bent steel
If you remove the load (stress) you have material which is plastically deformed (strain) but has the same material properties as the original material.
Put simply, residual stresses do not exist within the theoretical model. Member design is conducted on the portion of the stress/strain curve which is elastic.
NOW - bending a section does mean that the internal forces have to act around the curve. This generates forces at 90 degrees to the section and these do have to be considered. Their magnitude depends on the survature of the section and on the applied forces
RE: residual stress in bent steel
RE: residual stress in bent steel
JRC
RE: residual stress in bent steel
I work in the UK and our book for this is produced by the Steel Construction Institute and is Publication P281 ' Design of Curved Steel'.
The UK company which specialises in curved steel is 'Angle Ring' they have been a great help in the past with the technical aspects of the design. Is there a similar company in the USA? (I'm assuming you work in the USA).
RE: residual stress in bent steel
RE: residual stress in bent steel
The material at the surface is stressed to the yield point, and at that point, continues to be stressed to the yield point as the material is bent more. (Based on the simplified stress-strain curve mentioned above).
When you stop bending the material, it moves back the other way ("springback"). It has a permanent strain near the surfaces. As it comes to equilibrium, the surface that was stretched in tension will have compressive elastic stresses in it, and vice versa at the far side. The interior of the piece was stretched in tension will now be balanced by the outer surface in compression (and vice versa at the other face). So yes, you do have residual stresses. But as I mentioned above, these are not normally taken into account in the design.
Of course, any type of welding or cutting on the member will induce internal stresses in it, and these are not normally considered, either.
RE: residual stress in bent steel
Within the elastic range the load-bearing properties of the material are effectivelty unchanged, although there will be some loss in ductility.
For normal strength steels the changes are not sufficient to require stress relieving.