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Elastic versus inelastic deformation.

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Al Sekela

In Reply to: RE: Yess, but....... posted by RickeyM on August 24, 2010 at 06:45:58

Elastic deformation will not cause material failure, but it also will not absorb mechanical energy.

Follow Ups:

RE: Elastic versus inelastic deformation. - pictureguy 23:47:40 08/28/10 (2)
Aluminum is crumby stuff and has NO fatigue strength. Each stress cycle 'eats' into its lifetime.
To the extent that these CLD materials 'stress' the aluminum, they have a finite life.
That's one reason aluminum bicycle frames weigh very close to the steel they replace, not the ratio of weights of materials. Aluminum objects under stress, especially in safety related designs, must be very conservatively rated. I realize that doesn't apply to this material.......
Vibrating aluminum? I'm prejudiced and don't like it.
Too much is never enough
- Remember that the next time you take air transport. - Al Sekela 15:40:10 08/30/10 (1)
  Airplanes are made of aluminum and they vibrate. They vibrate a lot.
  
  The trick is to keep the stresses well below the onset of plastic (inelastic) deformation.
  
  Remember the deHavilland Comet? They had rectangular windows and broke apart from cracks starting at the window corners. The sharp corners increased the stresses in the aluminum body. You will not find windows with sharp corners on any airplanes flying today.
  
  Aluminum skins on constrained layer dampers work fine because the stresses are low.
  - RE: Remember that the next time you take air transport. - pictureguy 00:04:56 08/31/10 (0)
    airplanes are extremely conservatively designed and maintained like you wouldn't (or me, either) believe. Hard Landing? Let's have a look.
    1000 hours? Let's have a look.
    All sorts of inspections and maintenance by airframe hours. As a matter of fact, they have a design lifetime, in no small part CAUSED by the properties of Aluminum. Aluminum has NO FATIGUE strength. period. Doesn't matter too much the alloy, either. Though, for bicycle use, the 6000 series can be hardened to T6, which is about mild steel tubing.
    The Comet cracks to which you refer started in the HIGHEST STRESS part of the design....the hard corners of the ports. Agreed. Boeing knew this. Problem was found during repeated pressure cycling of a fuselage in a water tank. As the failure occurred, it became impossible to pressurize. BTW, the descendent of the Comet is STILL FLYING as the Royal Navy Nimrod, the Brit equal to the P3 Orion, sub chasing planes both. But, as a commercial plane, it was finished.
    
    Now, I'm being a shit. Aluminum has some properties which lend itself to this kind of use. It isn't under very much stress in this application. The aluminum will probably handily outlast the stuff it's bonded too and the excursion of the pieces is very small. About 30 years ago I rode an early Aluminum framed bike. Vitus by name. Awful. Wimpy, soft and had the lateral stiffness of a screen door. Rode awful, too. Later, I tested a Canondale. Stiff, efficient but no 'give' like good steel. I'd buy Titanium if I could afford it and still rode 5k miles a year.
    
    My personal approach to 'mods' will be the most conservative. Wooden frames of the lightest design consistent with proper stiffness. Mounting and suspension features I'm simply not going to discuss...yet. and NO screws thru the driver OR metal bridgework stand. ALL brass or stainless hardware. The crossover will remain dead stock until the next round. NO goopy stuff or CLD will be applied until I do driver rotation tests on the modded panels....or probably ever.
    
    Class Wednesday......Woodshop, of all things. This is a maximum effort and I want to have reframed drivers in about a year. Starting with basic woodshop, this being the starter class.
    Too much is never enough
    - http://en.wikipedia.org/wiki/Hawker_Siddeley_Nimrod (Open in New Window)

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