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Construction Maintenance 102

Summary: deburr; file and polish the edges; no oversize holes.

You know the effect a scratch on glass has when you try to break the glass along a line. In engineering terms that scratch is called a stress-riser. The lesson is that scratches, burrs and other sharp edges lessen the strength of a structural member.

For an interesting experiment, take a piece of aluminum, perhaps .020" thick, and cut yourself a piece 4" x 4" using Aviation Shears. If you look closely you'll see the serrations along the edge left by the shears. Take a good magnifying glass and you will see that the edge truly has been sheared - ripped, in plain terms - giving that rough grey colored surface. Those tiny rips, most of them invisible to the naked eye, are trouble. They are the beginning of a failure.

For proof, file one edge of the piece smooth. Now try to tear that edge. Likely you won't succeed. Now try the other edge, the rough edge. Likely it will tear. Same material, just unfinished. If you have trouble tearing the unfinished side, magnify the shearing effect by cutting a small nick, and try again. In fact, the serrations at the edge of the Saran Wrap box do just that - they provide the tiny fissures, the stress-risers.

If you read the literature on shearing you will see that one should file off about 1/16" of the edge which was sheared, just to be sure that none of the tears produced by the shear extend into the material. You can't just cut along the line and leave it like that to be mounted on the airplane. You must cut over-size and then file down the edge to the final size.

Another eye-opening experiment which you must do before you build or repair an airplane is to see the effect on the bending strength of a drilled tube .

To do this, take about a 12" length of an aluminum tube and clamp 4" of it in a vice, using some "V" blocks made of wood. Stand on a bathroom scale, note your weight, and then press down on the tube. Your loss in weight is equal to the force you are applying. Once the tube has bent about 10 degrees, note the scale reading. The difference in readings ( say 200 - 130), 70 lbs, multiplied by the distance between the end of the clamp and where you were pushing ( say 6") gives you the load applied. In this case the number is 70 x 6 = 420 inch pounds.

Now the interesting part. Take that same tube, or a new one, and drill a 1/8" hole (like for a couple of rivets, right through both sides of the tube. Repeat the experiment of measuring the bending moments applied. Align the holes to be at the top and bottom - i.e. vertically.

When I did this using a 5/8" tube, of .049" wall thickness, made of 6061T6 aluminum, I found that it required about 550 in. lb. to bend it.

With the rivet holes drilled into the tube it snapped catastrophically at 350 in.lb. before bending noticeably! This particular experiment surprised me enormously. Everyone should do this once - on the ground. Using " x .035" tubing the numbers were 210 in. lb. clean, 120 in.lb. with holes vertically aligned, and 180 in. lb. with holes horizontally aligned. That loss in strength was 43 %. Again, with the holes aligned vertically the tube fractured before any noticeable bending took place.

The lesson is that holes drilled into tubes weakens them tremendously, more than 35 % of the bending strength is lost. Not only do two 1/8" holes remove 1/6 (18%) of the material in the circumference of a 1/2" diameter tube, but the holes have sharp edges, which cannot be deburred on at least 2 sides. In addition, the diameter of the tube measured across the holes is now less than 0.500" actually closer to 0.490". That is not much of a difference, but if you consider that the bending strength of a beam varies with the cube of the depth, then the new height of the tube at the cross section has only (.49/.50), or 92% of the original strength, everything else being equal. Even done properly, drilling holes into load-carrying structure does not strengthen the structure. Only holes drilled along the neutral axis of a member do not greatly affect the load-carrying ability of the member.

What good are all those close tolerance bolts, rivets and other fasteners for which you pay dearly if the holes you drill are not also close tolerance? Just how to do this can be found in an article on my website, www.hoftec.com under Construction - "How to drill a round hole".

The bottom line is that if you are working on stress-sensitive parts of the structure - wing attachments and all control surface attachments, linkages, fittings - anything which is flight critical and which undergoes changing loads and vibration, it is absolutely essential that all the attaching hardware be free of cracks and scratches, that holes are drilled in prescribed places, that holes are properly deburred, and that the fit of the fastener is tight. Sloppy holes and attachments wear quickly and can cause deterioration of strength, corrosion, and in the case of control surfaces, flutter. All of these are bad news and must be dealt with aggressively.

Frank Hofmann, AME EAA Technical Counselor

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Frank Hofmann
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