lifespan of aluminium beams

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

  1. Ad Hoc
    Joined: Oct 2008
    Posts: 7,773
    Likes: 1,678, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Again, without any details, this is assumption/speculation and misleading.

    If, just if, (since no details of the beam is given), the beam is an I beam of 50x6 flnage with 100x4 web, and another is the same but 200x4mm web.

    With the given deflection over the span, and assuming built in ends, (to hold the boat), beam #1 has a stress of 172MPa, and beam #2 a stress of 325MPa to produce the deflection from its original non-deformed shape.

    A stress of 325MPa is not a small amount, it is easily sufficient to cause a permanent deformation, into the plastic region, but, it is also very close to failure if loaded again; whatever the grade and temper (assuming normal 6000 series alloy, which ever type)

    Thus in the absence of knowing any details of the beams scantlings...there is little one can really say. The rest, like my assumptions above, is simply that..just assumptions and speculation.
     
  2. raf pali
    Joined: Mar 2012
    Posts: 71
    Likes: 1, Points: 8, Legacy Rep: 12
    Location: Australia

    raf pali Junior Member

    I didn't mention, the beams are round aluminium pipe of an estimated DIA. of about 10''/250mm. The wall thickness and type of AL, is impossible to tell. the beams are 2 only inserted into the hulls right across the full width through a fiberglass sleeve. The sleeve at the intersection with the hull is massively beefed up and it seems 1''/25mm thick.
    The catamaran was designed by Crowther and professionally build as a production boat. We can only assume, sizes and quality of materials, are right.
    Dye testing, I doubt it be possible as, the beams are painted. That I didn't know till my inspection yesterday. The paint work looks original from day one and not a quick amateur job to cover mishaps. I touched the beams all over where possible and seemed perfect. the fiberglass around the beams at the point where it enters the hulls looked perfect and original with no sign of stress nor putty cover or paint patches.
    And as for the speed and efficiency of this cat, have no idea if it improved after the bending of the beams as in the case of the wings of the Boeing 707 after the nose dive. Nice story though. I bet, engineers munched a lot of numbers in the attempt to reproduce the unexpected "improvement" after the accident.
    Thank you kindly.
     
  3. Ad Hoc
    Joined: Oct 2008
    Posts: 7,773
    Likes: 1,678, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Well, doing the same as before (above)

    If one tube is say 6mm thick and another is 10mm thick, the stress is 384MPa in the 6mm and 383MPa in the 10mm wall thickness.

    Thus, based upon the limited data you have supplied, I would suggest that the permanent set deformation you have, is not good news. Since the stress required to cause such a deformation is well into the plastic region and too close to any UTS whatever the alloy. Its only saving grace is that I suspect the ends are not full fixity.

    But, this is all supposition and assumption, with very little real hard evidence. Thus the choice is yours...
     
  4. CDK
    Joined: Aug 2007
    Posts: 3,324
    Likes: 148, Points: 63, Legacy Rep: 1819
    Location: Adriatic sea

    CDK retired engineer

    So the beams sag approx. 0.8% after 30 years of gravity, sharp temperature gradients and jolts in all possible directions. I consider that "excellent".

    For a long, slender object to remain straight after so many years, it should be slightly bent away from gravity before it is put in place. In steel structures for bridges and roofs that can be done by strategic welding and cutting, but that is not something you want to do with a perfect aluminum tube.
     
  5. SamSam
    Joined: Feb 2005
    Posts: 3,899
    Likes: 200, Points: 63, Legacy Rep: 971
    Location: Coastal Georgia

    SamSam Senior Member

    That's a 20' free span, between the hulls?
    Do both the beams sag equally, or does the one with the mast sag more?
     
  6. SamSam
    Joined: Feb 2005
    Posts: 3,899
    Likes: 200, Points: 63, Legacy Rep: 971
    Location: Coastal Georgia

    SamSam Senior Member

    That's a 20' free span, between the hulls?
    Do both the beams sag equally, or does the one with the mast sag more?

    If there is a hole in the beams anywhere, such as a drain hole on the bottom or an unoccupied bolt hole, you can get an idea of the tube's thickness.
     
  7. Baltic Bandit

    Baltic Bandit Previous Member

    My understanding of "mach tuck" was not the shock wave issue - but rather that as the CoL moved aft as speed increased, particularly with aircraft with high tail empenages - it became impossible to pull the plane out of a dive because there was not sufficient control throw to overcome the ever increasing acceleration caused by the ever steepening dive.
     
  8. raf pali
    Joined: Mar 2012
    Posts: 71
    Likes: 1, Points: 8, Legacy Rep: 12
    Location: Australia

    raf pali Junior Member

    22' is the width of the cat and the length of the beam.
    15' is approximately the free span within the hulls and the cord from which the 2'' sag was measured.
    the mast sits in a third beam half way between the two connecting beams. This central beam is resting on shelfs bolted on the inner hulls and is not a connecting structural member.
    The 2'' sag is about the same for both connecting beams.
     
  9. Petros
    Joined: Oct 2007
    Posts: 2,934
    Likes: 148, Points: 63, Legacy Rep: 1593
    Location: Arlington, WA-USA

    Petros Senior Member

    this is totally off topic, but the mach tuck is a complicated topic, not related to the location of the horizontal stabilizer (first aircraft it was identified in was the P-38 lighting, with a low horizontal tail). It has to do with shock wave formation that drastically changes the pressure on the surfaces, moving the center of lift aft so much there is not enough control power to bring the nose up. High pressure behind the shock wave that forms on the control surface itself can also result in inability to bring the nose up. wandering shock waves on the wings can also create stability and control issues.

    http://en.wikipedia.org/wiki/Mach_tuck

    High horizontal tail locations cause other control problems related to low speed handling, especially at landing (a bad place to have unpredictable pitch control). Horizontal tail always work better at low speeds when they are placed low on the aircraft, the "T" tail is a styling and marketing gimmick and almost always results in poor low speed handling. enough on this topic.
     
  10. Petros
    Joined: Oct 2007
    Posts: 2,934
    Likes: 148, Points: 63, Legacy Rep: 1593
    Location: Arlington, WA-USA

    Petros Senior Member

    you are still only talking about 1 percent deflection, not bad for the age of the hull (we should all be so lucky!). It seems to me if the beam was overloaded in use would it be bent upward? How would it get overloaded downward? from sitting on the trailer with unsupported amas perhaps? If that is the case it likely was not from overload but rather "creep", a slow sagging of the member over long periods of time at low loads. The metal atoms actually slowly slide over each other over time. Each atom trades electrical bonds with the next atom in line. This too does not affect strength, only shape (presuming loading direction and conditions have not changed).

    It they beams are deflected downward, would that not result in less heel for any given reach (0.64 degrees less). I can not see this as affecting anything in any significant way. Presuming there is no damage at the beam mounts, I would buy it just the way it is. If you really want to move it back you can always set up a winch/block and tackle with weights and bend them back to their original position. If it were mine, I would forget about it.
     
  11. Barry
    Joined: Mar 2002
    Posts: 1,851
    Likes: 505, Points: 113, Legacy Rep: 158

    Barry Senior Member

    Petros,
    just happened across this thread and found your "story about the 707" which resulted in a 6 foot wing permanent deformation but was still flightworthy and more efficient with the wings at a new 5 1/2 degree up angle, when static.

    I am intrigued and looked high and low on the internet to find any information on it.

    Can you supply a link to this?
     
  12. Petros
    Joined: Oct 2007
    Posts: 2,934
    Likes: 148, Points: 63, Legacy Rep: 1593
    Location: Arlington, WA-USA

    Petros Senior Member

    I heard it during a lecture perhaps 30 years ago by Dr. Jan Roscomb, professor of aeronautical engineering at I think Kansas state university. He was generally considered the most important "guru" of aircraft stability and control at the time, and has many personal stories about little known incidences. Not likely this kind of thing would ever get published by either Boeing nor the airline, they are very sensitive to public perceptions by airline passengers. He told many other interesting stories of accident investigations that he was privy to. This kind of hand-on experience is hard to find in published literature, and is often lost when such people retire and pass away.
     
  13. rwatson
    Joined: Aug 2007
    Posts: 6,163
    Likes: 495, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    Considering that the normal wingtip deflection is 6 feet in operation


    http://www.pprune.org/tech-log/218693-747-wing-tip-deflection.html

    and

    http://flightaware.com/squawks/view/1/1_year/new/30452/Video_of_B787_wing_flex

    , and claimed a permanent tip deflection was created of 6 ft appears to be a bit of a co-incidence


    I presume it was
    Pan Am Flight 115
    http://en.wikipedia.org/wiki/Pan_Am_Flight_115

    N711PA, Boeing 707-121,

    The airframe certainly was used for another 20+ years
    "N711PA, Boeing 707-121, Clipper America, notes: C/n 17590, delivered to Pan Am on 16 October 1958. It was the first Pan Am aircraft to fly trans-Atlantic jet service - 28 October 1958. It was later renamed Clipper Mayflower. In April 1965, this aircraft was converted to Boeing 707-121B standards. It was purchased on 21 February 1975, by Pan Ayer. After being operated by a number of firms, including Turk Hava Yollari, the airframe was finally scrapped in August 1984, at Taipei, Taiwan."

    http://www.logbookmag.com/databases/articles.asp?ID=127&CatID=47
     
  14. Barry
    Joined: Mar 2002
    Posts: 1,851
    Likes: 505, Points: 113, Legacy Rep: 158

    Barry Senior Member

    When I read the story about this 707 that had been under such a load that the wings had permanently deformed 6 feet, AND THEY LET IT CONTINUE TO FLY, I was and still am unsold.
    7000 series aluminum can have an ultimate tensile strength of 83,000 psi (572mpa) and a yield of 73,000 psi (503mpa)
    In order for the wings to be deformed permanently 6 feet would mean that some structural members below the neutral axis would have been stressed between 73k to 83 k psi which would cause work hardening and that even the smallest stress concentration caused by the smallest crack could easily take the stress concentration above UTS, ultimate tensile strength, which upon further cycling by continuing to fly could cause a catastrophic failure.

    Re: creep, which is the movement of the molecules along grain boundaries of the metal ( as compared to the interchange of electrons of atoms), creep is more significant when metal is highly loaded at elevated temperatures. In the catamaran case, the low loading of the beam when it is stored and the low temperatures at which it probably sat at, does not lead me to think that the deformation of the beams is caused by creep.
     

  15. Phil Westendorf
    Joined: Jun 2012
    Posts: 35
    Likes: 1, Points: 0, Legacy Rep: 20
    Location: Saginaw, MI

    Phil Westendorf Junior Member

    I am curious if you have checked with the boats manufacturer on it's design specifications of the beams in question? If you look at any aluminum over-the-road trailers made in the USA of Aluminum all have a "bow" in them. It's obvious the trailer's are designed that way by design relative to loading and repeated stress, etc.

    I am only suggesting the 2.00" over 15' might be by design???? Is there any way to contact the boats manufacturer?
    Just another $ 0.02 worth.

    pw
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.