Wing spar fatigue pattern

Discussion in 'General Experimental Aviation Questions' started by Stoyan, Apr 24, 2019.

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  1. Apr 24, 2019 #1

    Stoyan

    Stoyan

    Stoyan

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    Hello, could it be told what is the cause of failure in this picture would it be fatigue or overload or crack?

    https://ibb.co/Sf9PyV7

    [​IMG] Untitledasdasd.jpg
     
  2. Apr 24, 2019 #2

    pictsidhe

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    Would be a lot easier with a better photo. Zoom right in so we can see the texture of the failure. In general, fatigue has a rippled pattern.
     
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  3. Apr 24, 2019 #3

    D Hillberg

    D Hillberg

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    typical over load. no fretting solid grain structure and lots of bent metal .
     
  4. Apr 24, 2019 #4

    BoKu

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    I think it could be told, but not from this photo. But a moderately hi-resolution photo of the fracture faces would show the presence or absence of the "beach marks" characteristic of fatigue failures.

    That said, there are all sorts of interesting things going on here. From the finished and painted ends of the aluminum (?) strips, it appears that this is likely a splice between right and left or inboard and outboard wing panels.

    The rusty finish of the bolts suggests that they are not typical aerospace hardware. That doesn't mean that they aren't quite strong, though; they could be Class 10.8 or Grade 8. But the rusty black oxide shows that they are not cadmium or zinc plated like normal aircraft bolts. Also, the many threads beyond the nut and the many washers suggests that they have the sort of long threaded portion characteristic of common hardware, without the granularity of unthreaded lengths typical in AN-type hardware.

    Can you tell us more about the circumstances of the photo, and where it came from?

    Thanks, Bob K.
     
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  5. Apr 24, 2019 #5

    wsimpso1

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    tough to tell from the photo. You appear to have four broken surfaces on this side and four more on the other side that are hidden. The top two surfaces are well lit but not of enough resolution to see w the surface in detail. The bottom two surfaces are in shadow and we can see nothing of them. And the four faces on the other side we can see only a tiny bit of one of them. So, yeah, we can not be sure of anything on this failure.

    The commentary on beach marks, well, they might or might not be present on the two well lit but poorly focused faces, and the other six for that matter, but we can not see them anywhere near well enough to tell.

    In general, if you suspect fatigue, you survey all of the fractured faces either with an optical microscope and oblique lighting or with an electron microscope. Find beach marks, and you had fatigue in that portion of the crack growth. The microscope survey also allows you to find defects or other sites for initiation of cracking, chevron marks indicating rapid crack progression and brittle fracture, or cup and cone surfaces indicating single overload.

    But with that low-res photo with much of the surfaces in shadow or blocked, naw, not much I will say about it.

    Billski
     
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  6. Apr 25, 2019 #6

    Jimstix

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    Without microscopy backup, it does look like tensile failure (too much P, not enough A). Ragged grain, no visible beach marks - and right through the bolt hole at minimum cross section.
     
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  7. Apr 25, 2019 #7

    BBerson

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    Could be some aluminum corrosion in the bars. Looks like a typical crashed wing separation.
     
  8. Apr 25, 2019 #8

    TFF

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    The hard to see side looks like there may have been a crack at the top. Smooth spots. The grainy stuff is not cause but effect. The bolts are rusty some. Holes could have been off and stressed the aluminum when the bolts were installed. What kind of plane? Better overall picture?
     
  9. Apr 25, 2019 #9

    wsimpso1

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    Understand that a slow growing crack reduces cross section that is carrying load, a little tiny bit per load cycle, until the remaining cross section can no longer carry the load, and then the catastrophic failure occurs. The fatigue and crack growth only gets things started. For instance, the fracture surfaces close to the original surfaces near the top of of both top pieces could be fatigue or even stress corrosion cracking, then the remaining surface was created during the catastrophic failure...

    Likewise, we could have all kinds of stuff going on in the hidden and shaded portions.

    One other thing to know about this case. There were four individual pieces bolted on and they all failed along eight faces. Cracking and section reduction could have been initiated on any of those eight surfaces, overloading the remaining surfaces, resulting in accelerated cracking, more reduced section, and the eventual sudden failure. You would not need to develop cracking in all eight surfaces, only one could initiate this failure... Alternatively, a sudden overload could fail all eight pretty much together...

    From one low resolution photo, all this experienced engineer will tell you is "Yeah, it broke". Microscopy of all broken surfaces is needed if you are to state with certainty about single overload or fatigue or stress corrosion or metallurgical flaws or nicks or design flaws or...

    Billski
     
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  10. Apr 25, 2019 #10

    PiperCruisin

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    Here is what you might look for in an analysis. The V striations point to the crack initiation site (bottom right) and in the upper left you can see some "beach marks" that indicate fatigue crack propagation.

    upload_2019-4-25_9-24-55.png
     
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  11. Apr 25, 2019 #11

    Dan Thomas

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    Corrosion on bolts implies moisture. Steel and aluminum in contact with each other, with moisture present, experience galvanic action, and the result is corrosion pitting in the aluminum. Pits in aluminum represent stress risers, where repeated stress will result in cracking.

    Cheap bolts don't have the best cladding on them.

    In Canada we have to do five-year inspections on all metal fixed-pitch propellers. The regs demand a close inspection of the bolt holes in the propeller hub, looking for corrosion, and old props often fail that. Rusty hardware did it. Corrosion pitting in the hub of a propeller is a fantastically dangerous affair, considering the huge stresses on a propeller, which is probably the most highly stressed part of the whole airplane.
     

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