What properties to change to reduce fatigue failure? 1

[UkraineTrain]

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.

 

[Tmoose]

geometry easily trumps superb materials a lot of the time. Can you add a couple of short lengths of heat shrink tubing to effect a strain relief ? Add mass to the middle of the cable to change the "tuning?"

Pictures of the design and the failed parts would likely help a lot.

2 down, 18 to go

 

[IRstuff]

We've successfully used OFC cabling for repetitive flex applications.

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[SwinnyGG]

Are you using soldered terminations?

First move would be to eliminate solder and used crimps.

 

[MintJulep]

Oxygen Free Copper is not already using.

 

[UkraineTrain]

We are already using oxygen-free copper here. The cable is crimped on one end and soldered on the other. We have seen failures on both connections. From what I've found, this material is available in different annealed conditions which only vary the amount of elongation at failure, but tensile strength stays at 200MPa. I have a request in to the supplier to see if there are other options.

 

[Tmoose]

If, for hard-to-fathom without pictures and drawings reasons, I could not make the changes I suggested on 11 May, I'd expect a few coats of this stuff at at the termination would change things for the better -

http://www.homedepot.com/p/Gardner-Bender-4-fl-oz-Liquid-Electrical-Tape-Black-LTB-400/100119178

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.

 

[hendersdc]

Can you strain relieve the cable? A couple of zip ties to hold it down could work wonders.

 

Tmoose is exactly correct. Fatigue is best avoided by better design, and by mitigating or eliminating the driver when possible.

 

[WKTaylor]

Generally speaking IF wire-braid is failing due to vibration resonance, then its a mass/mechanical interaction. Changing wire braid materials... except for increased stiffness and/or mass... probably won't produce effective results.

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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