Methods to reduce Residual Stresses from Forming 1

[ORnerd]

I have a formed channel 1.25 legs x 2.2 height 19" long, 0.25" thickness, A36 material, with an inside bend radius of 0.18". The channel is loaded with a 1.5" dia roller that rides against the legs of the section. The loading is not extreme, usually around 500 lbs. We see the channel opening up, on the loaded side, by an average of 0.030" after 20,000 cycles. We would like there to be zero deformation and are considering a bend radius of 0.25 to help the situation.

I am concerned that the tight bend radius is causing this issue. Will increasing the bend radius provide a solution entirely or will thermal stress relief have to be added as a post op.?

Allied Systems Company
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[desertfox]

hi ORnerd

Can you post a sketch of how the roller loads the legs?
An increased bend radius will certainly reduce the stresses at the bend, in fact in practice you should not use a bend radius less then the thickness of the material.

desertfox

 

[ORnerd]

Here's a cutaway of the channel to show the loading direction.

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[desertfox]

Hi ORnerd

Thanks for the pic, well the channels loaded in torsion as well as bending as anybody done an analysis on it, are there any cracks in the inside of the channel? I am thinking of maybe a fatigue issue.
I would still change the bend radius.

desertfox

 

[seymours2571]

ORNerd,

Is there any possibility to create the formed channels out of a steel with a higher yield strength than A36? For instance, A572 Grade 50 might be an option. Reason being, the stress levels at the corner of the bend might be near or just over the yield strength of the material allowing for very small plastic deformations that culminate into a 0.030" deflection over time.

It also might be a function of material creep, but others in the forum would be more knowledgeable on that subject.

Increasing the bend radii might help some. You could perform an FEA to optimize the radius of the formed angle.

Steve

 

[ORnerd]

desertfox,

FEA shows 20ksi at the inner radius(0.18), SF of 1.8. There are no cracks on the inside or the outside of the channel from forming or otherwise.

Changing the inner radius to 0.25" lowers the stress to 13.2ksi, SF of 2.7.

seymours2571,

Higher strength material may very well be the final solution. A variant of A572 could be used, my records indicate that what we stock is not readily formable, but that there are some grades out there. I think the spec would be A715 or A656?

Thank you!

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[desertfox]

Hi ORnerd

Well maybe the cause is creep as suggested by seymours2571.
Well whats the cheapest option a stronger grade of material or changing the radius on the existing one.
I think you should try the latter first and I still think its not good practice to bend a piece of material with a radius less then its thickness.

desertfox