Copper Fracture and Oxidization

[NJAus]

Hi All,

I am looking at a fracture surface on a copper commutator bar. I was wondering the following about the fracture surface which is brittle in nature(see attached picture):

1) Is the discoloration on the fracture surface due to varying levels exposure of the surface to the atmosphere (suggesting a crack grew over time and the copper has varying oxidization) OR;
2) Fast fracture but the surface discoloration is due to arcing on the commutator bar which has resulted in varying oxidization rates on the surface ?

Thanks

 

[mrfailure]

I would suspect from the fracture morphology that this was a fatigue crack, with discoloration being oxidation, especially the green discoloration. Initiation is definitely on the bottom but the oxidation suggests the crack was present for some time before final fracture occurred. Of cours, this is just first impressions - you would need to do some more evaluation to fully understand this fracture.

 

The green is a copper oxide hydroxide, suggesting there was moisture and somewhat elevated temperature.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

Sorry, it is copper carbonate hydroxide.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[Dhurjati Sen]

Dear NJAus,

The surface where the material has failed shows enough dimples to suggest it is a ductile fracture, as it ought to be for copper except where SCC takes place.

As suggested by mrfailure, it being a fatigue crack, requires further study as follows:

1. What is the composition of the bar?

2. What was the environment it was exposed to?

3. Metallography of the fractured surface and also the adjacent (good) surface.

Regards.



DHURJATI SEN

https://www.nace.org/people/dhurjatisen

 

[NJAus1]

A bit more details

a) The material is cold worked copper
b) It was subjected to a large centrifugal force that increased rapidly as the motor sped up.
c) There is a v-shaped notch where at the crack origin which would give rise to a stress concentration proportional to the centrifugal force.
d) Upon further review, arcing is not possible nor plausible in this situation. Could the blue markings be due to hear from the energy released when cracking? This would suggest very little energy absorption has occurred?

I would like further clarification as to why it is a fatigue failure? I cannot see any obvious beach marks at the origin. The surface does somewhat resemble a cup and cone failure surface but there is no visible necking.

 

[mrfailure]

You have a minimum of 5 visible beachmarks that I can see in your picture - these are the rounded-edge portions of the fracture that for the most part are visible by change in coloration. These indicate edges of the crack front at different times and usually are defined by change in load. These features are most commonly (but not always) associated with fatigue crack propagation. Flow pattern indicates cracking first initiated at the center of the notch. The system you describe is cyclically loaded, so fatigue is actually a logical mechanism. You should really ask yourself the question how a rapid overload failure would be logical, as you are suggesting, if you pursue that path. Again, all of this comes from thoughts examining an individual picture - you really should send it to a metallurgical lab performing failure analysis if you want to confirm.

 

The question you would want to answer with SCC is whether ammonia compounds could have been present; these are the Achille's heel of copper alloys.
I would not strongly suspect SCC in this case, because I do not see much fracture surface degradation.
I do not see any characteristics that would contradict fatigue fracture. The rough middle portion would be the final failure by ductile overload.



"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[NJAus1]

No ammonia compounds are present as this is inside a motor, however it is cooled by external air.

As I move further away from the crack surface I would expect less oxidisation as the exposure to the atmosphere is less. The colour seems inconsistent? I would expect a darker oxidisation colour at the crack origin and lighter as you move further away.

I thought the crack propagation surface would be somewhat smooth with beachmarks? Fast fracture surface would be granular. This was causing me a bit of confusion.

Is there a reason why it is granular as the crack propagates?

I can see the middle section has cup/cone fracture surface but there is no necking as I would expact with ductile failure?

The car would be subject to a cyclic load due normal use. Occasionally it will be subject to an overload, and at the time of failure it was subjected to a large increasing overload.

 

[Compositepro]

The blue tint looks like high temperature oxidation of the surface. Perhaps it heated due the lower area for current flow before the final break. It would help to have a side view and view of the original geometry of that part. The crack surface is not flat and has an interesting geometry that seems to be caused by something about the original shape. Heat tint takes time to develop, so the last section to break may have been the hottest but did not get any tint.

 

[NJAus1]

attached is a side view (I don't think it is the same bar unfortunately), definitely not flat.

 

The blue tint looks like high temperature oxidation of the surface.

I see it as green, but either way, green or turquoise blue indicate forms of copper carbonate hydroxide, a corrosion product. It is not heat tint, a term that is not associated with corrosion.

The lines demarcating zones of progressively lighter shading are 'arrest' marks that indicate intervals when the crack was not growing. The cracked surfaces oxidize; and the intensity is a function of exposure time. Beach marks are a defining feature of fatigue cracks.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[Compositepro]

Re: heat tint on copper:

https://www.eng-tips.com/viewthread.cfm?qid=194516

 

[WKTaylor]

Is this the alloy make-up for the specified commutator bar...?

MIL-B-19231 BARS, COPPER, SILVER-BEARING (FOR COMMUTATORS)

3.2 Chemical composition.

3.2.1 Copper. The bars shall contain not less than 99.90 percent
copper (including silver) ,

3.2.2 Silver. The bars shall contain not less than 0.085 percent
silver (25 Troy ounces per ton).

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

Of course heat tint is a thing, but I stand by my comment.
In this fracture surface there are clear indications of the involvement of moisture. Any possible heat tint is only incidental.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

'Heat tint' would be better labeled as 'high temperature oxidation in air'.


"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[Compositepro]

Yes, that is the definition of heat tint. Environmental corrosion could not possibly explain the picture. Every surface of the part is clean except this one stage of the fracture surface that shows the colors and gradations in colors typical of heat tint.

 

It may resemble 'heat tint', but was heat involved?
Until we know more, all we can say is 'discolouration, probably caused by oxidation'.
Clarity in terminology is essential, especially when communicating the story to civilians.


"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[Compositepro]

Heat tint is probably a far more familiar term to "civilians" than your words. When you see it, it is a clear indication that heat was involved. It is commonly seen on almost anything that gets hot: chromed or stainless exhaust pipes, cookware, soldered copper pipes and wires, and copper wires at loose terminals that carry much current.