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Bolt holes, burr and fatigue life effects

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SamP

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I've been told that not properly deburring can lead to decreased fatigue life in a bolt hole. I'm trying to go through the mental exercise of how burrs affect the fatigue life on a joint. Haven't found a good explanation yet on the internet. I understand how a crack that is cyclically loaded may grow, but that model doesn't transfer to the burr effects. A burr could be considered an external corner, which I don't believe is a stress concentrator.

Reading the discussion on how to best deburr prompted these thoughts. Some suggest the slight countersinking associated with spinning bits should be avoided since it decreases the bearing area. Rather, a file or chisel should be run over the surface. Since file doesn't go below the surface, it has to leave a sharp corner. At the worst case, there is a slight burr that still exists.

Thanks for your thoughts.
 

Marc Bourget

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Probably because you're not "grokking" that a drill penetrates by fracturing or tearing the metal (microlevel)

Deburring sufficient to remove the "tear" doesn't appreciably decrease the bearing area.

The metal is disturbed (moves) with loading. The tiny "cracks" have an opportunity to expand.

It's worth pointing out that reaming is a " good thing" I take into account - on more important joints like wing attach fittings, that AN bolts have a significant dimensional tolerance. So, I have a series of .0005" stepped reamers so I can match/slight interference fit. A Naval publication explains this gives significant increase in both overall strength and useful life of the assembly.

FWIW
 

TFF

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On an assembly, a burr is like a rock in your shoe. At minimum it’s going to cause problems putting it together. Once put together, the chunk is not matched like the other parts and is going to imprint it’s self when you tighten stuff up. It’s trapped in there where nothing is supposed to be. Can you be sloppy and it not crack? Yep. Would you want the best chance for it not to crack when you are a 1000 feet in the air? Yep. Just one of the many choices in building.
 

Victor Bravo

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I believe these principles also hold equally true with rivet holes. Although most airplanes are designed with a very large safety margin, traditional aircraft rivets are really supposed to be installed in holes that are deburred and reamed. The "dreamer" or drill-reamer is really the best possible tool for this.
 

cvairwerks

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It gets deep into fracture mechanics, but essentially a burr creates a very localized stress riser. Depending on a number of things, that riser can lead to localized cracking, at loads or fatigue cycles way under the design limits. Go thru the Aloha 243 investigation report for some analysis on crack and propagation from improper hole preparation.
 

b7gwap

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TL,DR :)

breaking sharp edges is extremely important, no matter the material, although some are more sensitive to it than others. Just don’t take so much that it becomes a chamfer and now you have reduced bearing strength and bolt bending issues to consider.
 

wktaylor

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Deburring [breaking sharp edges, and edge quality in-general] is a relatively mundane/boring subject/operation... but it's incredibly important for the long-term health of an aircraft... when cutting any metal.. and even composite and plastic materials... for an aircraft.

I have dragged many young engineers thru the subject... and in the beginning 'they get only about 25%' of the very important reasons for deburring' up front. In this discussion many important factors have not YET been mentioned.

Quiz... Anyone have other reasons for deburring and good-edge quality??? I'll check-in in a day or two..
 

geraldmorrissey

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My 1977 Cessna 150 had sawn edges on sheet metal detail parts, some poorly drilled holes and some generally crappy workmanship. It did spend time as a trainer and I sold it with 4,500 hours TT. Never saw a crack in the primary structure.
 

David L. Downey

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The last time I stuck my neck out and defined my experience as 40+ years int eh test world with one of the 2 major airframers I got pounded down by one of the well known posters so I have layed low since then. However, cvairwerks got it right: the difference in the deburred and as drilled worlds is simply the deburred structure has a chance to live up to the design assumptions and will usually outlive the ragged joint in a fatigue perspective. Not5e that few amateur designs are highly engineered and optimized and that is likely the reson that my fanaticism on deburring and edge finish are seen as absurd and objectionable.
The best tool I ever used for deburring was a specially made bit for a microstop countersink. the cutting angle was ground to somewhere between 175 and 180 degrees and the cutting edges were very sharp. when used with the appropriate pilot it left a burr-free hole with virtually zero loss of surface planarity.
After all those words, I still feel that step drilling with an abundance of clecos and final size drilling using only lubricated double margin bits results in holes that can almost be deburred by dragging a fingernail across the hole.
OK...fire suit on...
 

wktaylor

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GM, all this is a tiny hint where this topic can lead...

RE your 1977 C150... IF it was built in a typical production-line way, You ALSO didn't get significant corrosion protection [alodine and primer]… that cost a LOT extra!!

So, with all this 'rough sheet metal manufacturing you described', even though you may never have had cracks, I'll take a SWAG on this subject: You had corrosion Issues! Of course You MAY have maintained/operated your 'bird' in a relatively 'dry' climate with good air quality... but you were most certainly NOT near any large body of water... much less an ocean... and you kept it hangered and cleaned.

Hint. hint.
 

cvairwerks

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David... What programs were you an orange wire guy on? The NF-16 was one of the last -16's I built before going to the orange wire world for a number of years. :pilot: F-16, F-111, A-12, F-22 and a bunch of weird stuff was my world at that time.
 

trimtab

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Burrs indicate significant work hardening and/or phase transitions (especially in aluminum, titanium, and some other common alloyed materials). A discussion of why it is a problem can be found in Juvinall & Marshek. There is a list of changes and behaviors in a homogenous material that is locally stressed from machining operations, and it is not voodoo. It would be a few paragraphs to quickly summarize some of the high points, but you can find many easonable descriptions on google, and about 5 times that amount of poor descriptions.

By deburring, you remove discontinuous hardness, strain, geometry, and phase elements from the local area, making crack propagation through purely mechanical means or through chemical means far less (hundreds of times less often) likely. It's a big deal in common materials like aluminum, and really a big deal in others like titanium. Steels often are far more forgiving for chemical and microscale defect stress risers, but are not invulnerable.
 

Dan Thomas

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I see burrs as being like the ragged edge of a torn sheet of paper. It's much easier to start a tear from a ragged edge than from a clean edge.
Cessna wasn't too fussy about deburring or edge finish. I even found the little disc (that sometimes pops out as the bit breaks through) stuck under the shop heads of rivets, telling me that no deburring had been done. Found a bunch like that on a 210 spar, a highly loaded cantilever spar, during an airworthiness directive inspection to look for cracks in the lower spar flanges. I suspect that the cracking addressed by the AD emanated from those sharp-edges rivet holes.
 

wktaylor

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'Orange wire' is 'code' for....?

Come-on guys… there are lots of other equally important reasons to deburr and remove scratches on edges and surfaces/holes...
 

wsimpso1

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This mechanical engineer sees a bunch of individual issues that are taught to mechanical engineers:
  • All cutting edges form chips by deforming the material ahead to failure - the work hardening in chips and burrs is maximum for that material;
  • All cutting tools make small cracks in the wall of the hole;
  • Reamers make smaller cracks than drills;
  • Burrs can interfere with fastener straightness in the holes;
  • Burrs can interfere with rivet setting and joint preload;
  • Burrs can cause sharp edged dents in rivets and joint material, starting cracks;
  • Burrs can cause excess joint settling, which reduces joint preload in service;
  • Burrs can interfere with joint closure;
  • Burrs can introduce new "cracks" when imbedded in fasteners, holes, and joint contact surfaces.
The up shot is that burrs can make for poorly set fasteners, poorly clamped joints, overstressed fasteners, and much fatigue induced of both fasteners and clamped joint materials. Leaving raised rims, chips, burrs, etc is bad practice. Then have confidence in the rather high FOS designed into riveted joints.

Billski
 

wktaylor

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wsimpso1 … Your post #19 [Saturday at 6:43 AM] is excellent summary of the mechanical effects of leaving burrs and sharp edges in/on/between metal parts... for factors You listed… but life and durability depend on freedom from corrosion.

Burrs, chips, trapped debris between pieces structure with their sharp edges and gaps are natural moisture traps and/or passages... and have a remarkable lock on dirt/grime/fuzz/mold/fungus. And inevitably the burrs, chips and other trapped debris are BARE = un-treated for corrosion resistance.

Thoughts.

#1... Proper attention to deburring/edge sanding creates a smooth/minimal exposure of [and removal of specs/flakes] of bare-alloy aluminum, creating a tight/minimal leak joint that is electrically conductive. Likewise little/no deburring is a set-up for corrosion and EE-EL systems problems.

#2... You can't paint a sharp edge! Due to surface-tension finishes... alodine, primer/paint, sealant/etc... will flow-away from sharp corners/edges and cannot reliably adhere to these details.

#3... Sharp edges, without corrosion protection WILL corrode. This is a function of ragged bits of bare metal, and the continuous replenishment of oxygen and moisture. Oddly one aspect of aluminum corrosion is that the aluminum-oxide corrosion film is actually soft/porous... which entraps moisture like a sponge to further aggravate corrosion. AND the product of corrosion also generates a swelling/lifting force... as evidenced by lifting/flaking/spalling of the aluminum. IF corrosion is trapped between parts, the swelling force is aggravated since for every 0.001 corrosion into a part surface there is net growth of the corrosion layer [below/above the original surface] of 2X-to-3X...

Example1: 0.002 deep corrosion into a surface produces a 0.004-to-006 layer of corrosion... hence 0.002-to-0.004 net swelling [compression] between trapped surface... which can generate very high tensile forces thru rivets/bolts. Also... all moist corrosion is now setting tightly to the adjacent material. Beware... trapped mold/fungus/fuel/oil in these areas is a 'hot-house of ' anaerobic-organism corrosion.

#4. Even the judicious use/placement of moisture hole/gaps-drains [0.38-Dia and larger] demands high quality deburring and good/adherent corrosion treatment helps to keep the drain open [FOD/grime accumulates on burrs/chips] and flow-away the puddled moisture and isolate bare metal from the flow and abrasion at/around the hole.

#5. The combination of Burrs, chips, trapped debris, dirt/grime/fuzz/mold/fungus... and of course metallic corrosion... can cause weird/dangerous problems for electrically bonded/grounded structures and systems. The necessary electrical-bonding between parts that proper/clean/protected structure provides [fastener-hole-to-rivets/bolts-to-fastener-hole] is vital for protecting from arcing/sparking due to static and the reliable/low resistance between parts for low/high voltage current return and... gasp... lightning. IF lucky, then ‘simple’ electrical problems occur... for example failure for electrical/electronics to operate at-all. IF unlucky... then arcing and sparking and fuel or oil is a really bad combination.

#6...My favorite Topic... Chips/shavings, burrs and razor-sharp edges are a cutting/slicing/puncture hazard to skin, fabric, clothes, rubber [hoses, tanks, etc], plastics, sealant, wire-insulation jacketing, etc. I also include cut-ends of safety wire, cotter-pin ‘arms’, break-off tabs, and other sharp-pointy ‘broken’ parts... etc in this hazard. The scars on my hands and arms attest to how severe injuries can be to skin... and the exposure to filth/bacteria/fungus has made several of these open wound injuries hard to heal. Imagine the hidden harm to vulnerable materials. Anyone every have a close call with a fragment of safety wire... or some-other clipped piece of metal [shaving, etc] flickering into an eye?

See how open this topic is for a LONG/deep discussion??? What-else? Yes, there is more [out of the box thinking needed]. I have to go back-to work.
 
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