Fretting corrosion factor for lug design 8

[EdBundy]

Good day

I've got a question regarding a reference for fretting corrosion factors used in fatigue analysis. We're analysing a launcher structure, and thus far we've been using an old Aerospaciale reference for the factors (based on bush/no bush, hole and pin tolerance, finishing, etc.)

However, this source is quite old now, and surely more work has been done on fretting in the past 30-40 years?

Also, seeing that we're only analysing low-cycle (1000 cycles, undulating) fatigue, is a fretting factor even necessary?

I've checked the usual suspects, Bruhn (old, but still good), Nui, etc. but can't find any info on what fretting factors to use, and when they're applicable.

Any help in this regard will be greatly appreciated.

Ciao, Ed

 

[rb1957]

i'd prefer to design to avoid (eliminate?) fretting, rather than "fudging" my stress calcs in the expectation of fretting

 

[EdBundy]

Hi rb1957
That seems the first prize But alas, the structure is to be mounted on a helicopter, which will definitely induce some vibration at the interface, irrespective of load direction...

Thanks,
Ed

 

[KENAT]

Hmm, when I was working on a lug for a practice store, I don't remember this coming up. Of course the lug was looked at as a limited life item - it wasn't designed to be used over & over again.

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[tbuelna]

EdBundy,

The reason fretting is generally of concern is because the fretting usually results in material transfer and surface pitting on at least one part, at the contacting interface subject to relative motion that causes fretting. These surface pits are nucleation points where fractures are likely to propagate from. The fretting process is also self-propagating, so the longer it occurs the worse the pitting becomes.

Fretting is basically a micro level contact phenomena that occurs when the contacting surface asperity tips are mechanically welded and then broken apart repeatedly under cyclical loading conditions. The asperity tips mechanically bond together due to the high contact stress that occurs locally between them.

Fretting can occur with even the smallest relative motions at a joint. The best way to prevent fretting is to make sure that you never have any relative motion within your joints under any conditions. If that is not possible, other methods may involve using a sacrificial fretting barrier (like silver plating) that prevents cold welding between the joint surfaces, or designing with a very low contact stress. However, a contact stress that is below the fretting limit is usually not practical for most material/joint combinations.

As rb1957 suggests, the best approach is to design your joint so that there is no relative motion and thus is immune to fretting. If you do this, then your analysis should not require any knock-down factors for surface flaws due to fretting corrosion.

Hope that helps.
Terry

 

[EdBundy]

Hi Kenat, tbuelna

The literature I have covers fretting, it's causes, mechanisms, and how to design to avoid it, however due to legacy and external design constraints these cannot be avoided. The lug is on the launcher and not on the store itself, so every "launching" event will put a severe load on the launcher which is designed for a 1000-cycle life (hence the low-cycle fatigue).

If someone knows of a reference regarding fretting and the fatigue strength reduction associated with it, for low- and high-cycle fatigue, it would be appreciated.

Regards,
Ed

 

[rb1957]

i'd've thought you might have used non-metalic washers to prevent fretting, maybe shoulder bushings to create a "sacrifical" bearing surface. if these approaches don't work for your design, and since fretting is a sufrace degradation, maybe assume that 0.01" (0.02", 0.05" ??) thickness on each fretting surface is ineffective (ie analyze for a thinner lug) ?

 

[RPstress]

RAE Tech Note 182 by Heywood, and Heywood's 'Designing Against Fatigue' book may help. I also found reference to Heywood's 'Strength of Lugs in Fatigue' at

http://www.excelcalcs.com/repository/machines/pins-and-joints/pin-and-lug--static-and-fatigue.xls/

.

http://openlibrary.org/books/OL5867925M/Designing_against_fatigue_of_metals.

http://openlibrary.org/books/OL20358701M/Designing_against_fatigue

I no longer have a copy of Tech Note 182, but I remember it being reasonably thorough.

I suspect a good library may be necessary to find a copy. The I.Mech.E. or the RAeS libraries in the UK spring to mind.

The work on these things tends to be quite old (and none the worse for it). I suspect that in-house methods have been developed by various companies which may have been updated. Boeing's Detail Reliability Factor 'DRF' methods may address the issue. Unfortunately I last dealt with the DRF methods a while back and can't remember!

 

[KENAT]

EdBundy,

Is your launcher mounted to the helicopter via standard 14" or 30" lug pattern? If so I assume you've already looked at mil MIL-A-8591H.

Not sure it really gives anything of much help but I'm wondering if the pre-load generated by properly used sway braces is assumed to virtually eliminate movement between the lugs and hooks?

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[TVP]

There is considerable coverage of lugs, as well as fatigue and fretting in the book Fatigue of structures and materials by Jaap Schijve (Springer, 2004), but I am not sure if all three are discussed together. My version is at home, so I will have to check later, but in the meantime you can check some of the contents, etc. from Google Books:

http://books.google.com/books?id=Xj...r&dq=Fatigue+Schijve&cd=1#v=onepage&q&f=false

 

[EdBundy]

Good day all

Thanks for the valuable input. To clarify somewhat, the lug is for the launcher which is attached to a carrier, and not to the EMRU directly (14" btw).
RB1957, we've used non-metallic washers in the lightly-loaded out-of-plane direction, however in-plane the loads are such that non-metallic materials all fail (and getting info on the fatigue of non-metallics was quite an exercise!)
RPStress, the age of the references isn't really a bother, since we still use Bruhn extensively And as far as I know, Peterson is still the de facto standard for stress concentrations.
The Schijve reference looks quite promising, I've only had a look on google books but I'll try to find one in a local library. If good, it'll probably become the next addition to the company library. From the quick look on Google books, it also seems that fretting isn't a major concern at <10^5 cycles, but I'll have a closer look at that...

Thanks for all your help, guys (or gals, if that's the case)!

Gratefully,
Ed

 

[SAITAETGrad]

Jaap Schjive's book does indeed cover fretting corrosion of lugs in Chapter 15 including a short discussion hinting on how to analyze for an accepted occurence of fretting. In the second edition (2009), the general poor fatigue limit performance of lugs due to the high (>8) scf fretting corrosion is discussed.

Also included is a discussion on design and process tricks to extend the fretting corrosion capability of the lug. For example, a slotted hole can greatly reduce the contact area of the lug and therefore reduce fretting corrosion.

I would also suggest tracking down two of the references used by Schjive in his book:

White, DJ, "Fatigue Strength of Small Pinned Connections Made from Alloy Steel FV520" B. Proc. Inst. Mech. Engrs. Vol 182 Part 1 (1968)p.615.

and...

Schjive, J. "Fatigue of Lugs", Contributions to the Theory of Aircraft Structures (1972) pp. 423-440.

 

[thgirw]

Hello,
The question from Ed is really nice and the replies made me think, let's say re-think what we are doing in our design. Briefly to explain: we have a lug in which a bushing is installed to the lug with an interference fit. Bushing itself is coated with some kind of dry lubricant that is what is also proposed by "tbuelna" (coat the surface with silver plating).
However silver, dry film are in principle lubricants, don't you think that they may reduce the strength of the joint? Think it in this way please: we are supposed to design an interference which should not have any movement but we are using some coating which eases the movement.

Thanks

Zet

 

[tbuelna]

thgirw,

The function of silver plating in this instance is not as a lubricant, but as a sacrificial fretting barrier. The mechanism of fretting is basically where the microscopic surface asperity tips on the mating surfaces cold weld (or diffusion bond) together under the high local contact pressures and then break apart repeatedly under cyclic loads. This results in material transfer to one part and pitting on the other.

The way silver plate, or other dry film lubricants, reduce fretting is by creating a "contaminated" boundary between the mating surfaces that prevents the cold welding from occurring. An good analogy would be trying to weld two metal parts that had grease, paint, rust or oxide on the surfaces.

Your comment about designing for an interference fit that has adequate friction such that any movement is prevented is the correct approach. If such a joint condition is possible, then silver plate would not be of benefit. But I believe the poster said his particular joint would experience some relative motion, so that is why I suggested a fretting barrier.

Hope that helps you.
Terry

 

[WKTaylor]

Guys...

A valuable tool for retarding initiation of fatigue cracks, SCC, galling and FRETTING is shot-peening**.

NOTE. I have also seen some benefits of shot-peening for slowing initiation of EXCO on exposed grain-ends [machined steps in extrusions, etc].

Per SAE HS-84 (2004) "Manual on Shot Peening".

9.18 Galling and Fretting
Shot peening reduces detrimental effects of galling,
fretting, and fretting fatigue by cold working the surfaces and placing the surface into compression.

**WARNING: Poor peening-practices, dirty shot media or embedded shot residue allowed to remain in-place can be very detrimental. However good peening practices (prep, application and post-cleaning), combined with fresh "clean" media provide very positive/measurable benefits.

ESDU 90031 "Fretting Fatigue", ESDU 89004 "Effect of fretting on fatigue strength of aluminum alloys" and ESDU Guide to the effect of shot-peeing on fatigue strength" provide baseline knowledge of fretting and mitigation techniques [especially aluminum alloys]. There are also several ESDUs on titanium fretting, fretting-fatigue, etc.

Regards, Wil Taylor

 

[WKTaylor]

EdBundy... for data overload...

Also, try...

http://www.dtic.mil/dtic/search/tr/index.html

Search " fretting and fatigue "



Regards, Wil Taylor

 

[WKTaylor]

DTIC...

DOT/FAA/CT-93/2 Literature Review and Preliminary Studies of Fretting Fatigue Including Special Applications to Aircraft
Joints

Regards, Wil Taylor