Pin routing of cracks? 4

[KirbyWan]

I had a disagreement with another engineer about the need to pin rout cracks. His opinion was that as a first step in repairing a cracked member should be stop drilling and pin routing it. His feeling was that the rough surface of the crack could initiate a new crack. I feel that only stop drilling (as a first step in a repair) is needed because the primary stresses are from one direction which would be normal to the crack direction and the stresses parallel to the crack would not be significant (assuming we are not taling about stresses caused by pressure in which case I would agree with him.)

I have looked at a couple of CMM repairs and they only stop drill before other steps, but I havn't done a thorough survey yet.

I was wondering what more experienced engineers who design aircraft repairs feel about this issue? I work mostly on commercial engine nacell systems, CF6-50 and 6-80 CMF56 and more rarely PW and RR.

I would also like to thank all the posters here. This forum is an excellent jump start to undersand all the small issues that could become big problems if not thought through.

-Kirby

K-State '06

 

[rb1957]

i'd agree that stop drilling is sufficient to revent the crack from continuing, if the stop drill takes out the crack tip, which can be beyond the visible crack tip.

at the same time, i think your collegue has a point, about removing the crack faces so that they don't rub against one another ... though i don't think this in itself is enough to restart crack gorwth from a sufficently large stop drill hole.

this is what we (well, i) call in the biz "an each way bet"

do you intend to do any crack growth calcs for a crack initiating at your stop drill hole ? ... this would be of more use than routing the crack faces.

 

[RPstress]

There are tales of cracks that've been stop-drilled repeatedly, and carry on growing the other side of the hole so that you wind up with a row of holes joined by the crack. (Seen a photo but never an actual example in front of me.)

Once you've got a crack the 'a' dimension for any further growth from otherwise small flaws in the hole surface is the small-surface-flaw-size plus the existing crack length, so theoretically the stress intensity is still high. The more you can do to smooth out the end of the crack the better, but it may still grow without a more general repair.

 

[bf109g]

The key to stop drilling is to remove the crack tip. Boeing is very specific about drilling a .25" diameter hole .1" beyond the visible tip, HFEC the hole, and if no crack is found, enlarge the hole to .31".

I've seen many run on cracks that were stop drilled with .125 and .19 holes. If you think about the Boeing method and the limitations of HFEC, it makes sense.

 

[Dan320]

Routing the crack, alodine, primer and sealant is a whole lot better than covering it with the doubler and hoping for the best.

 

[KirbyWan]

Dan320,

You think alodine primer and sealant is a better repair then a doubler or in addition to a doubler? I usually indicate alodine, primer and either sealant or adhesive in addition to a doubler, but I was just concerned with the material removal of the crack.

In the application I was thinking of (this was a couple of months ago, but for some reason this point has been gnawing at me for a while) A crack was found on the upper portion of a cf6-50 thrust reverser fixed structure on the cone web under the acoustic panels. There is a doubler directly behind the web for the first foot or so. The doubler was not however cracked.

This is what caused the disagreement, because it is difficult to pin rout a crack without affecting the doubler beneath it. Removing the doubler was not economical since it would have entailed disassembling most of the fixed structure including removing the main beam. Possible, but an additional 80 hours of repair work for a ~1 inch crack.

BF109,

Thanks for the reference I'll look that up.

RPStess,

I have seen that as well, as a matter of fact this area of the fixed structure is prone to cracking and I have seen reversers come in with multiple stop drills on a crack line, though not in the same spot as I was working on for this repair

RB1957,

I have not done crack growth calcs. I'm still fairly new to this and am not familiar with crack growth calcs. Is this used if I want to stop drill it and leave it without a doubler repair? Would this still be used if I repaired the crack with a douber? Send me a reference so I can review it. Thanks.

And thanks to each of you for responding.

-Kirby

Still learnin'

 

[rb1957]

it sounds as though there is a crack in a non-structural web, there is a doubler on the web, but not particularly relevent to the crack (no?). or maybe there is a crack in a web supported by a doubler, and the doubler is still intact (yes?).

could the crack have been some previous damage, repaired by the doubler ?

if the web is not-really-structural, stop drill (like bf109g says, big dia and crack check), consider placing an inspection on the web.

if the web looks structural, stop drill and cover with structural doubler.

the calcs for crack growth are (in theory) not difficult but in practice quite difficult ... do you have stress data, a fatigue spectrum, ... that's why i would consider a visual inspection maybe every 6 months, every year ... just to watch the damned thing.

good luck !

 

[Sparweb]

I have a "hall of shame" that I keep on a corner of my desk and one of the examples is a repeatedly stop-drilled crack that grew to be 3" long before being permanently repaired.

I think the suggestion above to stop-drill beyond the visible crack is a good one because the crack can be longer than it looks. Your experience with other webs in the area having multiple stop-drills bears that out.

I might consider something like "pin routing" if the web was subjected to a lot of shear. If it was just tension then the two surfaces won't rub together, but if there is shear in the panel then they could, no matter which way the crack points. The same argument might be valid for cycles of compression.

But then, that opens up the question of why it matters if the crack faces rub together? Contact stresses and fretting might be present occasionally but I can't think of a mechanism where they would initiate new cracks or make the existing one worse. Perhaps if the shear and/or compression makes them buckle out of plane with each other, then a scissor effect speeds up crack growth. Just speculation.

Trying to route the crack without scratching the doubler sounds impossible. Damaging the doubler just starts a new crack in IT and now it's going to take less time for both to fail. Stop-drilling the crack in the web will be hard enough to do without putting a divot in the doubler. In fact I would just expect the doubler to be nicked in the process.

Note: the scrap that I keep on my desk is the damaged doubler. The skin it was attached to had a crack at least 4.5" long. With each stop-drill of the skin this doubler was nicked by the drill bit - about 7 times! Eventually the doubler developed its own crack. The doubler was an antenna reinforcement, designed for a whip antenna, but the operator installed an 18" long blade antenna on it instead. Flying through icing, the pilot reported a "buzzing" sound....


Steven Fahey, CET

 

[Dan320]

Normally there will be a doubler regardless of routing or not. If you already have a doubler installed and don't want to remove it then there is probably not much else to do but to leave the crack as it is. Some doubler repairs I've seen did not include routing instructions. A narrow crack is difficult to fill with sealant, and consequently corrosion may occur later on. That's my view.

 

[WKTaylor]

Guys...

These are typical cracks:

from a hole in a sheet: --o----

from a hole-to-a-hole and out both holes -o------o--

from an edge to a hole thru the hole: |---o-

This are typical SD schemes for the noted cases (small SD Holes):

from a hole in a sheet: O--o----O

from a hole-to-a-hole and out both holes O-o------o--O

from an edge to a hole thru the hole: |---o-O

This are typical SD schemes for the noted cases large SD Holes):

from a hole in a sheet: ( )--o----( )

from a hole-to-a-hole and out both holes ( )-o------o--( )

from an edge to a hole thru the hole: |---o-( )

If a crack is fully captured from SD-Holes-to-SD Holes, or SD Hole-to-fastener hole or SD Hole-to-an edge, then the crack has effectively becom an OVAL SLOT. The smaller the aspect ratio of the rounded-end slot [length/width] then the more-effective the stress concentration (Kt) reduction.

Hence the bigger the SD Hole, the wider the effective-slot, then the better.

Adding solid rivets [or Hi-Loks] in the SD Holes (to keep the "slot-ends rigid) will also boost stop-drilled crack life dramatically (inhibit crack-growth from the SD Holes).

If a well designed repair patch is installed (accounting for the slot and patch Kts, loss of base-metal crossection in various orientations, eccentricities, etc) then damage (re-cracking) may be retarded indefinitely.

Structure without a patch, or a poorly designed patch, will inetively begin cracking, again, and again and again.

Now to the problem of allowing crack faces to continue rubbing is basically a moisture retention (cappillary action), fretting and corrosion issue. Moisture plus microscopic rubbing leads to the "smoking effect typically seen in cracks (how most are visually detected). Without edge treatment and sealing the area can become a corrosion problem.

If the SD hol is treated with alodine then it WILL migrate down the crack faces and aggravate corrosion.

Pen-routing along the length of the crack, deburring, corrosion treatment [alodine/primer] of the bare aluminum and sealing will generally eliminate corrosion as a secondary issue. CAUTION: without pen-routing the crack, applying corrosion protection to the edges and gap is impossible.

CAUTION: in some high-yield and high-hardness materials, the edges fretting together can work-harden and/or create "microburs"... resulting in damage to adjacent doublers and/or spider cracking.

NOTE: the flexing of structure is why edges of parts must NEVER be butted together.

NOTE: stop-drilling ends, slotting-along and deburing cracks is also an excellent way to prep weldable materials for weld-fill repairs.

Regards, Wil Taylor

 

[KirbyWan]

Howdy all,

Wil,

You state:

If the SD hol is treated with alodine then it WILL migrate down the crack faces and aggravate corrosion.

I thought alodine, being a chemical conversion coating to produce a stable oxide layer, would help prevent corrosion. What's the difference here? Is the rough surface of the crack the problem? Or is this a chemical Maginot line that allows the least protected area to be the focus of corrosion attack?

 

[WKTaylor]

Kirbywan...

Pleez don't hole me acountabl for mi speling...

Most chemical conversion coatings are by nature acidic so they etch a microlayer of the aluminum surface to form the stable oxide [or chromate-oxdide] compound/film that is adherent when it dries.

The problems with cracks are multiple...

If/when acidic compounds get trapped in the crevice, corrosion will fluorish, by many "mechanisms": acid, trapped moisture, O2 depletion, no paint coat etc...

Cappillary action and differential air-pressure leakage will suck-in new contaminants and moisture thru/along the crack.

Conversion coatings only form/adhere readily on bare aluminum. The crack faces are dirty and oxidized.. so coating formation is unlikely.

Conversion coatings are soft and somewhat life limited (sacraficial). If coated crack faces rub together, then the coating goes away. If separated by enough of a gap for air, then they go-away by sacrificial action... unless coated with primer... but if the gap is too-small, viscous paints will likely "bridge" tiny-gaps" and form pockets for moisture traps.

Regards, Wil Taylor

 

[DRW75]

Hey folks, sorry to hijack, but - can someone describe pin routing for me? thanks

 

[KirbyWan]

This is where you use a very small (1/8") diameter bit to remove the material along the crack line. Thus the original crack is completely removed, the ends by stop drilling and the length of it by pin routing.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[swall]

And wouldn't you also have the issue of proper post Alodine rinse? You would not be able to rinse the faying surfaces of the crack.

 

[Dan320]

Swail: Maybe one can use an Alodine-pen. No rinsing needed then.

Dan

 

[DRW75]

KirbyWan,

Thanks... that makes perfect sense. pin-routing. its quite literal!

I assume that the bit (1/8") would be some kind milling bit that would be typical for side cutting and not a typical twist bit that you would use for plunging?

 

[KirbyWan]

DRW75,

Yep. I'm not as familiar with the tools as our craftsmen, but there is a difference in the type of bit from a standard twist drill. Perhaps someone who has more experience in acomplishing repairs then I do could elaborate on the type of bit.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.