Prediction of tensile stress fracture in sheet metal bends

[SteveThePirate]

Hi, I'm trying to calculate whether a 2.5mm thick sheet of 5052-H32 Aluminum will fracture if bent at a 90deg angle with a 1.5mm inner radius. For the part in question, the manufacturer originally agreed to the 3mm bend radius in my CAD & drawing, but the parts they shipped us had no radius at all and I found tensile stress cracks on the outside of the radius. They're now saying they can't do better than 1.5-2mm due to the geometry (two consecutive 90deg bends forming a U ~9mm apart). I've googled around for it and can't find an example problem showing how to calculate this.

As a follow-up question, I'm curious if anyone knows whether anodizing the Al after bending will potentially also cause cracking across this bend?

 

[3DDave]

Typical limit is 1.5t; the original value was a little smaller than the recommended 3.75mm bend radius. Grain direction from rolling can help a little.

https://www.cumberlandmetals.com/aluminum/minimum-bend-radii/

 

[SWComposites]

Yes, grain direction will make a difference.
Also, can they bend in the O condition and then heat treat?

To estimate cracking, you could make some assumptions about the amount of elongation on the inner and outer arc length, and see if the outer surface strains enough to reach max elongation. Should be able to find stress-strain curve in MMPDS.

 

[rothers]

From the spec, "for information only", whatever that means.

 

[MintJulep]

Why not just ask for some samples?

 

[SteveThePirate]

Hi everyone. Thank you for your replies. Yes I know about the tables and rules of thumb. I did not know about grain direction, so thank you for that. I was hoping it was possible to calculate, but it appears not.

So for one of the parts I’ve replaced my two 90deg bends with a much larger 180deg bend and added a new alignment feature (was using the base of the U to align to mating part). For the other part I actually have two other bends where they tell me that can’t achieve the 3mm radius either - I believe due to the proximity of the two bends to each other again. So I plan to move them far enough from each other to allow for the full minimum radius.

I confirmed under the microscope with anodized vs bare parts that the anodizing process does indeed increase the depth of the “stretch marks” (for lack of a better term), so I’m pivoting to paint.

Unfortunately, in both parts these features are critical to safety. I have a FOS of 25 according to FEA, which is a great place to be, but of course a fracture will change that. Ultimately planning to load up the final design and test until failure to confirm. I’m on a (typical) short schedule so hopefully the design changes fix the issue and I don’t delay the project.

Thanks again!

 

[3DDave]

Anodize increases the thickness of the surface layer amplifying any defect. It's also not a good choice in fatigue sensitive applications even without pre-existing cracks.