What properties to change to reduce fatigue failure?
What properties to change to reduce fatigue failure?
(OP)
I have an application where a braided copper cable is breaking during a sinusoidal sweep vibration test, at the termination points. Changing the temper of the wire is the easiest option to improve this condition. Redesigning the parts to move the resonant frequency is not an option right now. My initial thought was to change the temper to get a higher elongation at fracture so the part is more ductile but I think now that since this is a fatigue failure below the tensile strength that the elongation value is not really relevant. So would using a material with a higher tensile strength be better to resist crack propagation, even if the ductility is lower? Or something different altogether? I’ve only been able to find fatigue strength values for “annealed” and “half-hard” material, but not the other tempers that I can select from.





RE: What properties to change to reduce fatigue failure?
Pictures of the design and the failed parts would likely help a lot.
2 down, 18 to go
RE: What properties to change to reduce fatigue failure?
TTFN (ta ta for now)
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RE: What properties to change to reduce fatigue failure?
First move would be to eliminate solder and used crimps.
RE: What properties to change to reduce fatigue failure?
RE: What properties to change to reduce fatigue failure?
RE: What properties to change to reduce fatigue failure?
http://www.homedepot.com/p/Gardner-Bender-4-fl-oz-...
or, similarly, a few drops of very thin epoxy applied to the braid at the termination could modify and soften the woutransition.
Also, it would protect the copper from air, which can have a negative effect on fatigue life of some materials
A very simplified FEA model might be revealing. But probably just indicate what you already know. where the braid will break.
But perhaps a serendipitous improvement might emerge, like re-orienting the braid, or installing a "pigtail" connection of another material at each termination.
Can the Item Under Test be observed during the test, to perhaps identify the frequencies doing the damage, and the mode shapes?
Modest Work hardening the copper by over bending or tensile loading might boost the fatigue strength.
https://cdn.intechopen.com/pdfs-wm/30476.pdf
4 down, 20 to go.
RE: What properties to change to reduce fatigue failure?
RE: What properties to change to reduce fatigue failure?
RE: What properties to change to reduce fatigue failure?
A MINOR reduction in wear/damage rate might result if copper wires have nickel [Ni] or tin [Sn] or silver [Ag] coating which will change friction by improving fiber-to-fiber lubricity.
Example of a wire system that needs special coatings for improved long-term fatigue-wear protection, is thus...
High strength carbon steel 'aircraft quality flight control cable' [MIL-DTL-83240] has strain hardened carbon steel filament wires that have a zinc coating or tin-over-zinc coating applied. The zinc [on the steel] is for basic corrosion resistance; the premium control cable [usually specified 'just because it much better'] has tin-coating applied over the zinc-coating for improved lubricity... and which also seals the zinc coating for better corrosion resistance. When each steel fiber is coated this way, the fatigue durability of the wire rope is substantially better than just zinc-coated wire-rope [any helical twisted 'construction'].
Although this wire rope example is substantially different than copper wire braid, the 'similarity of example' may hold-up, even though electrical grade fine copper wires are far more damage prone that steel wires. OBVIOUSLY, ONLY testing would be able to evaluate whether any of these coatings will provide relief from Your problem.
NOTE.
Braided copper electrical shielding wire spec that I've worked with for 'special shielding' problems is as follows...
A-A-59569 BRAID, WIRE (COPPER, TIN-COATED, SILVER-COATED, OR NICKEL COATED, TUBULAR OR FLAT). [Replaces obsolete QQ-B-575]
Regards, Wil Taylor
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