Cold working of holes 1

[anurag2801]

Hi,

I have a query related to cold working of the holes. Would like to know up to what extent the benefits of cold working can be assumed. That is to say what is the minimum distance from the edge of the hole unto which the cold working can be considered. Will there be any change in the benefits of a cold worked hole on which cold working is performed again?

 

[rb1957]

why would you be interested in the minimum distance from the hole ?

yes, there are "issues" with redoing cold working on a hole. You are yielding and repeatedly yielding the material at the edge of the hole.
The process you're using should give guidance on this.

research "Fatigue Technologies Inc", or "Forcemate".

another day in paradise, or is paradise one day closer ?

 

[anurag2801]

@rb1957, thank you so much for the response.
Actually I am concerned about the over sizing of the cold worked hole. The oversize could be beyond first nominal. Hence I was wondering if the benefit of cold expansion still effective in this case. The is no plan to cold work after oversizing.

 

[rb1957]

ok, that's a very different question to what was first asked (at least the way I read your post). Oversizing a cold worked hole is very problematic. By oversizing you are removing most of the cold working.

There is some room to manoeuvre, maybe 0.03", but not much; and substantiating it is difficult.

Better, off the top of my head is a freeze fit bush. Open up the hole (removing most of the cold working you've done. Install a "proper" freeze fit bush. There's still some issues with residual cold working.

Why do you need to oversize ? mis-matched hole centers ? The bushing would allow you to "drag" the centers back to alignment.

Edge distance is a concern (with opening holes)

another day in paradise, or is paradise one day closer ?

 

[LiftDivergence]

When a hole is cold-worked, there is a residual compressive stress field established around the hole. This benefits the fatigue capability of the hole greatly since local tensile stresses need to be in exceess of the residual compression to create an opening displacement for a flaw. In a sense, the highly localized stress at the hole has a -R. The hole severity factor is greatly reduced as can be seen in the hole condition factor for SF computation.

This residual stress field is not huge but obviously has some radius. There are several resources I can think of that discuss this:

Exploratory Investigation into the Durability of Beneficial Cold Worked Fastener Holes in Aluminum

https://ntrs.nasa.gov/api/citations/19990063973/downloads/19990063973.pdf

There is a discussion of strain and stress measurements around cold worked holes in test specimen, plotted against the radial distance from the hole.

Also useful might be:
NASA TN D-6955, An Elastic Analysis of Stresses in a Uniaxially Loaded Sheet Containing an Interference-Fit Bolt

https://ntrs.nasa.gov/api/citations/19720026264/downloads/19720026264.pdf

This provides formulae for determining the stress around a hole which has interference. You can plot the stress versus radial distance.

This shows the effect dies off pretty quickly, but not immediately. Most of the beneficial stress field is contained within a distance of 0.75R from the hole edge. However, oversizing a fastener will not remove that much material.

I think the answer is that you can enlarge a hole which has been cold worked without losing all of the advantage. But the beneficial stress field will be diminished.



Keep em' Flying
//Fight Corrosion!

 

[anurag2801]

Thank you so much...!

 

[WKTaylor]

This USAF T.O. may provide useful info on the FTI split sleeve hole-expansion process. This reference includes a discussion of residual strain and potential for over-over-sizing the hole for a corresponding larger fastener than originally intended

1-1A-1 General Manual for Structural Repair
Appendix C Special Considerations Regarding Fastener Holes

https://www.tinker.af.mil/Home/Technical-Orders/

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[rb1957]

man ! the amount of material that's out there, thx for the link Will.

another day in paradise, or is paradise one day closer ?

 

[WKTaylor]

Rb…

Yesterday one of my guys asked...

You aware of a MIL-STD for cold forming [sheet metal cold forming]?

Here is what I sent to him...

This is a difficult question since metal forming is typically taught... and the texts provides long-term references.

1. There are many detailed BOOKS on aircraft sheet metal forming... many are old but have amazing info. I have a few in my bookcase at work. Typical examples of currently available books, this subject...

How to Do Aircraft Sheet Metal Work

https://www.abebooks.com/servlet/Se...metal work&n=100121503&cm_sp=mbc-_-ats-_-used

Aircraft Sheet Metal (JS312633)

https://www.amazon.com/Aircraft-Sheet-Metal-JS312633-Bonacci/dp/0891002960

Aerospace Manufacturing Processes

https://www.taylorfrancis.com/books/9781315367965

2. Here are ‘authoritative’ documents that I normally refer to for ‘aluminum forming for repairs’... not per BAC5300...

TO 1-1A-1 General Manual for Structural Repair
Chapter 8 Metal Working

TO 1-1A-9 AEROSPACE METALS - GENERAL DATA AND USAGE FACTORS
[each chapter has information, various metals]

1C-135-3-1 Structural Repair - General
6.3 SHEET METAL FORMING.

Army TM 1-1500-204-23-10 AVIATION UNIT MAINTENANCE (AVUM) AND AVIATION INTERMEDIATE MAINTENANCE (AVIM) MANUAL FOR GENERAL AIRCRAFT MAINTENANCE (SHEET METAL SHOP PRACTICES)
CHAPTER 6 FORMING

AC43.13-1 ACCEPTABLE METHODS, TECHNIQUES, AND PRACTICES - AIRCRAFT INSPECTION AND REPAIR

FAA-H-8083-31 V1 Aviation Maintenance Technician Handbook – Airframe
Chapter 4. Aircraft Metal Structural Repair

AC65-15 Airframe and Power plant mechanics –Airframe Handbook
CHAPTER 5 AIRCRAFT STRUCTURAL REPAIRS

Hmmm... forgot to mention...

Aluminum Association FMA-17 Forming and Machining Aluminum.

FAA has a very stern perspective on parts fabrication... AC43-18 FABRICATION OF AIRCRAFT PARTS BY MAINTENANCE PERSONNEL.

Alas... forming combined with heat/heat-treatment can blow fabrication issues into the stratosphere.

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[WKTaylor]

Rb…

The [2] USAF T.O. websites [Tinker and Robins ALCs] open to the public contain a tiny fraction of knowledge generally available. For instance there are several T.O.s on bearings and bearing maintenance, a T.O. for just about every protective finish that is plated/flame-sprayed/etc, a T.O. for Advanced-composites repair, etc... that are mind-boggling.

The USAF website open to the public at Robins is...

https://www.robins.af.mil/About-Us/Technical-Orders/

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[rb1957]

yep, but they are open to the public; I'm surprised they aren't limited to US public, but I'll download what I can while I can !!

thx for the links.

another day in paradise, or is paradise one day closer ?