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Thread: Metal Fatigue in Tube Frames?

  1. #21

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    Quote Originally Posted by Eric Marsh View Post
    I am reading about best practices for maintaining old aircraft and it got me to wondering if fatigue is an issue in tube frame construction aircraft. I would think that they would be tougher than aluminum construction if for no other reason than that steel will flex more before failure but that's just a guess. Does anyone have any input on the subject?
    All the respondents below are correct to different degrees. You are concerned about a welded, steel, tubular, construction. Fatigue cracks initiate at points where high stress is concentrated and usually that occurs in the welds. To begin, the weld itself has the structure of a casting because it solidified from a molten state. Typically cast structures (welds) are about half as strong as the wrought material they hold together. Most fabrications are designed based on the strength of the welds if they will experience bending or tension loads. The shape of the weld is important. The point where the weld blends into the parent metal, at the edges and where the weld starts and stops, must be smooth. If the "toe of the weld" looks like it laps over and does not blend in smoothly, this is likely an indentation that will be a stress concentrator. Fatigue cracks like to start in these points. Close inspection of the welds for corrosion and stress points is all that's necessary. And if you find any suspect spots, blend them in with a file or grinder.
    I haven't run into any welded, aluminum tubing structures in the airplanes I have worked on. They have all been riveted assemblies where the rivets are loaded in shear. Both structures, steel and aluminum, are usually designed to be stiff enough to resist bending, and fatigue cracks are not a problem.

  2. #22

    Re: Metal Fatigue?

    A bit off-topic, but Jimmy Stewart and Marlene Dietrich starred in a movie based on this subject, " No Highway in the Sky": An aeronautical engineer predicts that a new model of plane will fail catastrophically and in a novel manner (the tail falls off!) after a specific number of flying hours. There are some interesting scenes of aircraft fatigue testing.

  3. #23
    Aaron Novak's Avatar
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    Quote Originally Posted by Morrie Caudill View Post
    All the respondents below are correct to different degrees. You are concerned about a welded, steel, tubular, construction. Fatigue cracks initiate at points where high stress is concentrated and usually that occurs in the welds. To begin, the weld itself has the structure of a casting because it solidified from a molten state. Typically cast structures (welds) are about half as strong as the wrought material they hold together. Most fabrications are designed based on the strength of the welds if they will experience bending or tension loads. The shape of the weld is important. The point where the weld blends into the parent metal, at the edges and where the weld starts and stops, must be smooth. If the "toe of the weld" looks like it laps over and does not blend in smoothly, this is likely an indentation that will be a stress concentrator. Fatigue cracks like to start in these points. Close inspection of the welds for corrosion and stress points is all that's necessary. And if you find any suspect spots, blend them in with a file or grinder.
    I haven't run into any welded, aluminum tubing structures in the airplanes I have worked on. They have all been riveted assemblies where the rivets are loaded in shear. Both structures, steel and aluminum, are usually designed to be stiff enough to resist bending, and fatigue cracks are not a problem.
    I aggree with most of this, except the touching of a weld with any kind of grinder etc. Welds are NEVER to be ground, dressed, covered up etc unless it is for bolt head clearance etc. There are multiple reasons for this, and if needed they can be elaborated on later.

  4. #24

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    Fatigue cracks in small lightly stressed personal planes are mostly limited to vibration around the engine such as the engine mount and exhaust.
    I can't recall weld cracks on the aircraft itself, in decades of inspecting. Some cracks from hard landings around the gear.

    In general, any reasonable weld will be good on a homebuilt. Some of the standards that apply to nuclear plants and spacecraft are not applicable to homebuilding. AC 43.13 is a source of info such as grinding, mentioned above.
    The EAA Museum has some samples of good and bad welds on display in a case. The bad welds look really bad.
    A reasonably good looking weld will be fine even if not perfect.

    A good weld is always better than a bad and undetectable composite or wood bond joint, in my opinion. Don't fear welds.
    Bill

  5. #25

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    The original question was about whether a steel tubular frame would be prone to fatigue failure and the answer is "not likely" because of the high endurance limit of steel compared to aluminum.

    A tubular structure is designed for stiffness which is a function of modulus of the tubes, steel at 30MSI and aluminum at 10 MSI. The stress level at which the structure is designed to operate is kept far below the yield point or proportional limit. But if you keep design stresses so low that fatigue never can happen, the plane will be very heavy or not fly. Most commercial transports have a warranted life where no primary structure will suffer fatigue cracking issues, somewhere around 60,000 hrs or more. To verify this, periodic inspections are done on critical areas using ultrasonics, dye pen or X-rays.
    On a GA plane, the design stress levels are much lower so fatigue of aluminum structure is not an issue unless there are localized high stress areas or poor workmanship which allowed stress raisers to initiate cracking. This is why edges of detail parts must be smooth and blended and holes deburred.

    If you have a steel frame and are concerned about fatigue, clean the frame down to bare metal, examine the tubes with a 10X loupe for cracking, check closely at welded joints for crack initiation, and if all looks good, prime and paint the frame to inhibit corrosion/rust.
    Bob H

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