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Metal fatigue

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In Reply to: RE: What kind of hookup wire does Magnepan use in Crossover, stranded copper or stranded aluminum? posted by pictureguy on September 20, 2012 at 20:59:33

Copper has no fatigue limit either although its fatigue strength is a bit higher than aluminum. Fatigue strength is the stress level that causes failure at some fixed number of cycles (typically 10 million cycles). The stress applied on the speaker hookup wire (aluminum or copper) is so low that the endurance time is well beyond the reasonable lifetime of the speaker. Unlike steel, failure will eventually occur but this is not a factor in choosing aluminum v. copper. There are obviously other factors that go into this decision - mostly conductivity. Cost is also not a consideration as aluminum is cheaper than copper. For power transmission lines the lower weight and cheaper cost make aluminum preferrable to copper but again fatigue strength is not a factor.

BTW, this thread has become much more erudite than the OP ever dreamed of.
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RE: Metal fatigue - pictureguy 12:29:31 09/22/12 (0)
I wanted to add something...perhaps 'picky'.
My understanding of fatigue limit it is that stress level under which the test piece has essentially no wear from the stress. ABove the fatigue limit, you being eating into the ultimate lifetime. In Bicycles, some riders complain of a bike going 'soft' after part of a season. This is nutty, given the high strenght steel they were talking about, the work hardened nature of same as well as the impossiblity of even the strongest rider exceeding the fatigue limits of the frame.
The strength limit question is why an Aluminum structure can't take (full) advantage of the lower weight material. It's gotta be built stronger to make up for no fatigue strength. Aluminum bikes are also generally made with large diameter tubes (at least 6000 series) in order to minimize the 'soda straw' effect. Walls must be a certain %age of diameter to help in this factor......

Too much is never enough
RE: Metal fatigue - pictureguy 20:07:04 09/21/12 (3)
yes:
My entrance to metalury was 25yrs as a semiconductor process guy in metals / thin films. I have a book on metalurgy for Bicycles here somewhere. That is in interesting read since it introduced me to some interesting insights about alloys.

Now, back to Aluminum. I don't know offhand if Aluminum self-passivates. IOW, will the oxide 'crust' on the surfact propgate thru the entire piece or shell over and become impervious to further oxygen incursion?
Silicon, for example, as used in semiconductors has a native oxide of 20 angstroms. That's IT at room temp. Steel...at least non-core10 alloys will simply eat all the way thru.....Look at that '56 Buick which spent a couple summers vacationing in Florida, for example...or a dozen winters in Chicago with salted roads.
I don't know where Aluminum lies on this scale.
The reason I bring this up is that The wires or ribbons of the panel itself are subject to lots of stress. I'd have to think thru whether it was mainly torsion or tension.....or as the ribbon flexes, it would be compressive on the bottom and tensile on the top? Anyway, Once you get micro flex cracks, is that IT, except for the time it takes to oxidize all the way thru? IF so, as the active conductor gets less and less cross sectional area, the current density goes UP as does the tendency to 'fuse' at very high levels......

And this is WAY beyond this thread, but do you thank Magnepan has some SEMs of ribbons or wires after extensive use ot answer nutty questions like that?
NOTE TO WENDEL::
Too much is never enough
- RE: Metal fatigue - neolith 07:01:18 09/22/12 (0)
  I agree with Andy, you bring up an interesting question. For sure, the aluminum wires corrode all the way through as this happened to a pair of MG-IIb's that I had. OTOH the ribbon's on my IIIa's were going on 30 yrs and would still be with me if I hadn't knocked over one of them causing it to rupture. I always thought that the aluminum oxide formed an impervious barrier to further oxidation but then why did the wire fall apart and why did the ribbons hold up? Ah, life's mysteries!
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- Interesting point, pictureguy ... - andyr 01:15:01 09/22/12 (1)
  I assume the "QR foil" used on the latest models (and some previous models) is much thinner than the "round wire" which it replaced? (How else does it get approximately the same driver resistance, when it is 1/8" wide?)
  
  So will we get wire failure in these models in 10 years? (Whereas my 30-year old, 30g mid drivers would've been fine ... except the glue degraded. :-(( )
  
  Regards,
  
  Andy
  - RE: Interesting point, pictureguy ... - josh358 13:16:01 09/22/12 (0)
    Not as far as I know. The quasi ribbon foil has been in use for many years -- it was first introduced in the midrange of the 20.1 -- and as far as I know, it hasn't deteriorated, though the adhesives used until ten years ago have. However, I haven't asked. In the case of the ribbons, the word is that they will eventually develop metal fatigue and fail if played at very high levels, but will last indefinitely if not. Judging by reports of levels and ribbon replacements on the forums, for most audiophiles, it will be forever. But Satie's would be at risk (though AFAIK, his haven't ever failed).
    
    You could figure out how the maximum deflection of the ribbons compares to that of the midrange and bass ribbons by looking at frequency and area, but I'm not sure the comparison would tell you anything, since the ribbons are pleated and tacked down. I gather from what people report that failures tend to occur at the points where they're attached and experience the most stress, though again, it's not something I've confirmed.

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