Fatigue cracking in bending zone - photos and case study

[MateuszM]

Hello,
Edit: I know thesis posts are forbidden, but I think it is not a typical "help me with my thesis" post. Please consider it as an experience exchange between engineers in mechanical field.
I am currently working on my PhD thesis which is concerned about the influence of the bending process parameters and sheet metal anisotropy on the fatigue strenght of metals.
I have encountered this problem during my work in the special vehicles construction company (cracking heat shield holders, mudguards holders etc.). Unfortunately I do not have a photographic documentation.
So I would like ask here for a help - does anyone encountered such a problem and could support me with a photo of fatigue crack in the bending zone with a permission for publication?
Also I would like to herar your thoughts on this problem - have you ever faced it and what was your solution.
Thank you in advance.
Mateusz.

 

[SWComposites]

Are you refering to "bending" during forming the part, or "bending" under load in service? For the latter, solution is redesign the part.

 

[mfgenggear]

MayeuzM
There is a wealth of information on the web about failure due to cracking. As SW said forming or Service

 

[rb1957]

"the bending process" = sheet metal forming ?
In our business, maybe not your construction vehicles), we normally stress relief after forming, to remove the adverse effects (of plastic straining the material).
But on elements which are not flight critical, or which don't carry flight loads, we can leave in the formed condition. I've never seen a fatigue failure with these parts.

"cracked heat shields" are probably steel (stainless steel) under thermal loads and or thermal fatigue ... probably vibration (VHC fatigue) at elevated temperature. Not a classical fatigue.



"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[MateuszM]

@SWComposites - I mean cracking in bending zone during operational loads under service, but this bending zone is previously shaped using sheet metal forming, air bending in this case. Redesigning the part is always a solution. The object of my work is to find the guidelines how to redesign it in terms of process parameters.

@mfgenggear - yes I know there is plenty of information, I have done the literature review. I just wanted to ask for a specific cases encountered in various industries.

@rb1957 - exactly, I meant sheet metal forming, specifically air bending. It is a valuable point with stress relieving after forming - we do not use that. Thank you.

 

[rb1957]

"air bending" ? If you form metal (plastically) then you build in stresses and change the material (work hardening).
It might be interesting to compare fatigue life of a formed sheet compared to an unformed sheet.
You might be able to FEA the forming to see the internal stresses.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[MateuszM]

Air bending is shaping metal using punch and V-shaped die, but when the shaped material is not pressed against the die walls. It allows for achieving various angles without change of tooling (angle depends from the punch stroke):

https://www.komaspec.com/about-us/b...r-bending-and-bottom-bending-for-sheet-metal/

I have already done FEA simulation of bending process using Barlat's plastic anisotropy model and I have some hints what influences the fatigue strenght.

I just wanted to support my research with some real industrial cases. Unfortunately we solved our problems already and I do not have a documentation I could use.

 

[dvd]

This article has some interesting discussion of charpy notch toughness and bend zone effects on ductile-to-brittle transition temperature - Charpy v-notch

Don't bend along grain direction, either bend perpendicular to grain direction, or obliquely - grain direction and bending

 

[malzeri83]

@MateuszM we supply press brake tools but mainly there is no any detailed feedback if everything is working properly.
What I can tell:
1. Nobody buys tool for bottom bending / coining. Maybe only for somewhere in very precise industry but we encountered no any requests.
2. If you have correct die opening according to your thickness so you are not looking for ways to make radius 0,5 on 2 mm metal and have it R > 1*S=thickness if you will not have any damages. Maybe only with some AL alloys, it will be a reason to increase the radius for bending.
Sorry if I understood something wrong. Meanwhile we are making big article about tooling, maybe after couple of weeks you can take something for your work if you want.
3. If you are talking about cracking of part after couple of years it is quite hard to trace and moreover to understand the reasons you should know perfectly all the conditions of use during this time.
@ dvd Second article is interesting but not rich with details. For example minimal flange as 1R + 3*S for 30 degrees of bending probably will not work)

 

[IFRs]

For cracking during manufacturing, after the thickness and grade of material, the two most important questions are what is the bend radius and what is the grain direction. You can generally solve this problem with a large enough bend radius or if that is not practical then bending while hot or stress relieving after bending, or both. You can try a few variations, and then measure the hardness of the metal in the bend area compared to virgin area and look at the grain structure under a scanning electron microscope.

 

[malzeri83]

@IFRs
Agree that during manufacture bending radius is factor N1 and the most important. About grain direction - I know that it values but mainly nobody cared seriously to take it in consideration in production, at least we never heard. Moreover the same material from other batch can bring other results.