lifespan of aluminium beams

Discussion in 'Materials' started by raf pali, Jan 4, 2014.

  1. raf pali
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    raf pali Junior Member

    Do aluminium beam on a 30 year old catamaran suffer fatigue? How long are they supposed to last?
    Are new beams required or the original beams might be still ok?
    Are folts, other than evident kinks, cracks and blush, visually detectable?
    Thanks
     
  2. gonzo
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    gonzo Senior Member

    You can look for stress cracks with products like Magnaflux
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Fatigue is related to the applied load and the number of cycles. Then, the stress range, the manufacturing quality, the in-service environment, has there been a nick or repair done...has the beam been overloaded during its life etc etc etc etc....impossible to say.
     
  4. raf pali
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    raf pali Junior Member

    Many thanks.
    It sound as 30 years old aluminium beams might be still ok providing no physical damages were applied. that means, age itself doesn't deteriorate the properties of the material, or does it?
    Cheers
     
  5. raf pali
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    raf pali Junior Member

  6. CDK
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    CDK retired engineer

    Age itself has no effect whatsoever on aluminum. There may be some weakening from fatigue in parts that were repeatedly subjected to forces they were not designed for. A telltale are cracks in the oxide skin around fasteners.

    In general, the material is very resilient. The wings of aeroplanes are subjected to incredible dynamic loads, yet remain firmly attached to the fuselage after half a century of service.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Any Dye-pen will do:
    http://www.fidgeon.co.uk/product.php?id=682

    That's because the design has taken fatigue into account and there is a regular maintenance checks to enhance this level of acceptance.
     
  8. upchurchmr
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    upchurchmr Senior Member

    +1 for Ad Hoc's answer.
    The required design and analysis for extended life in an aluminum aircraft is a big cost of the huge price for aircraft. Perhaps and equal cost is the maintenence over the life.
    This assumes you want the minimum weight. Older aircraft were much heavier that they needed to be.

    On the other hand, the way you use the aircraft severly affects life also. The US Navy had some specially modified F-16's built and used them much heavier than expected. They were retired after several years instead of the expected 30 years.

    The only practical thing is inspection for cracks as has been discussed above.
     
  9. gonzo
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    gonzo Senior Member

    Magnaflux makes a dye that glows with ultraviolet light. It works with aluminum, plastic and ceramics too. Check their website for availability in your area.
     
  10. raf pali
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    raf pali Junior Member

    Thank you all gentlemen. I had a close look at the beams yesterday and as far as I could see they are excellent all over. No visible imperfection, no corrosion, no dents, no cracks but, they sag slightly, some what as 2''/50mm on a 20'/6M span on a uniform curve. This downwards curve, is an obvious sign of deformation due to overloads applied in the past. Is this bend a prove of strength loss?
    Thank again.
     
  11. jonr
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    jonr Senior Member

    Aluminum will always fail - eventually. But something like 10,000 cycles to near max load.
     
  12. gonzo
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    gonzo Senior Member

    Cracks may not be visible to the eye, hence the Magnaflux dye system. It is a cheap way to test them.
     
  13. Petros
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    Petros Senior Member

    typical way to test aluminum is with dye penetrate products, some well equipped auto parts stores even sell such chemical kits. Usually a spray on dye, plus a developer, or UV light to make the cracks visible.

    the permanent (inelastic) deflection of the beam does not weaken it if there was no buckling or structural damage. It is difficult to imagine the beams were loaded up so much they yielded the metal to take on a new shape. Check that the mounting points did not slip or deflect.

    There is an interesting story of an early Boeing 707 that was on a trans Atlantic commercial trip. the crew did not notice the auto pilot kicked off while at cruise altitudes (someone bumped the control column in the cockpit when the flight crew was studying an air chart). the plane slowly picked up speed in a shallow dive until it reached near sonic speed, which causes a lot of buffeting on the aircraft. It than suffered "mach tuck" where the flight controls become ineffective from the development of shock waves over the surfaces, making it nearly impossible to level off the flight and reduce the airspeed. with all of the crew pulling up on the control columns together they managed to level the flight off after losing over 20,000 ft, and the high "g" pull-up made them lose one of the engines (of the four). They managed to land safely on three engines, and transferred the passengers and their luggage to other aircraft. A careful inspection found the wings were permanently deflected up-ward about 6 ft at the tip, but they found no damage, just deformation. After the engine mount was repaired and a new engine installed it was flight tested and found it checked out safely. The plane was put back in commercial service and served a long service life. During its following service however, it was discovered that particular aircraft burned 3 percent less fuel than other 707. somehow the deformation improve the flight efficiency.

    If your beams are merely deformed or deflected, and have no damage, they will be plenty strong.
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    That's now wholly correct.

    It depends upon the axis of loading and the axis of the permanent set. If the applied load is in a different axis, it could be a considerable difference in strength. Also how much deformation....in other words, how far into the plastic zone....and of course the alloy itself, which grade and temper.

    Otherwise, just speculation and supposition....
     

  15. Petros
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    Petros Senior Member

    This is true of course Ad Hoc, but the description of the amount of deflection did not sound like it would change the loading enough to make a difference. far more critical it seems to me is if there was any hidden damage that caused the deflection, rather than it simply being bent a little where there is no damage to the beam.
     
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