Layers in broken metal

[rjlittlefield]

Let's start with the closeup stereo. This is about 1.5 mm high. I was impressed with the amount of layering visible in this piece of metal that I would have expected to be pretty homogeneous.



The story here is that today I was called to another house to fix a piece of equipment that was mostly not working but could be made to work sometimes if the power plug was held "just so".

It turned out that the wall socket needed replacing, but more importantly, so did the power plug. Replaced both, problem fixed, great.

However, the power plug problem was odd enough to attract my attention, because one of its prongs was quite loose in the plug. So I sliced apart the plug to see what had gone wrong.

Here's what I found: that prong had been bent far enough and/or enough times that it had simply snapped.



Here's the broken end of the prong, in its entirety. The stereo pair is the area outlined.



Again, I was surprised by the extent of visible layering in this piece of metal. Naively I would have expected it to be a lot more homogeneous. The cord and plug assembly superficially appeared to be high quality, but it seems that "under the covers", not so much.

I hope you find this as interesting as I did.

--Rik

[BugEZ]

How cords are assembled.

https://m.youtube.com/watch?v=UbDmZ5OljJI

Some discussion on the metals used in connectors here. Probably a copper-zinc alloy.

https://www.waytekwire.com/datasheet/Ho ... ctured.pdf

Regarding the layering you mentioned, that would come from the rolling process used to form the strip stock. The v shaped bands I think I see are due to the squishing taking place between the rollers.

In the third photo, I suspect the crack initiated on the left and propagated to the right. Was the plug cycled up and down in the receptical? Vibration?

Nice photos!

Keith

[rjlittlefield]

Thanks for the links to documentation!

In the third photo, I suspect the crack initiated on the left and propagated to the right. Was the plug cycled up and down in the receptical? Vibration?

The wall outlet was severely worn and the plug's prong was bent outward, across the flat, as if someone had attempted to "adjust" it to make it fit tighter in the worn socket. The plug was for a piece of powered furniture, essentially fixed equipment with a long cord that doesn't seem subject to vibration or up/down cycling.. It's always possible somebody bumped the plug sideways. I really don't know the history, since in a prior life the equipment was a rental unit with probably a long line of users. This is one of the rare things I've gotten to fix without the background of having broken it myself!

I agree about the origin of the layering. I was just surprised to see that it was so obvious, complete with lumpy inclusions. Now I'm tempted to sacrifice a couple of prongs from replacement plugs, just to see what they look like when broken.

--Rik

[ChrisR]

Missed this.
That reminds me of one of my metallurgy lecturers who often referred to "monkey metal", of vague composition, heat treatment and processing. "They chuck anything in the pot as long as it melts".

Different phases (structures depending on alloying element concentrations) solidify at different temperatures, then as said above get squished according to the flow going on in extrusion, or rolling, pressing etc. It's called "mechanical fibering".
The phases oxidize differently, which is why metallurgists acid-etch the surface to see the structure. In air, or the presence of other impurities, they can colour up strongly. Apart from the obvious "inclusions" (=grot) in that specimen, I think that's what you're seeing.
Local impurities such as sulphur, phosphorous, lead, can make metals quite brittle so small fractures appear preferentially.
The other thing copper based phases do is work-harden dramatically. That's the process where the dislocations in the crystal lattice ripple along to the grain boundaries when you bend it. When they’re all piled up, it makes the metal harder to bend, then it cracks.
In hot rolling, the grains can partially recrystallize.
If the specimen were rolled at too low a temperature, it would be more brittle, therefore.
Once you have a crack of course you get magnified strains around its tip, so the fracture process accelerates on further stress.

In other words, that's monkey metal!

Etching acids - note the use of DIC
and that this is much higher magnification
http://www.metallographic.com/Etchants/ ... chants.htm

[rjlittlefield]

Chris, thanks for the added information. It makes a nice explanation for some other observations & pictures that I haven't posted yet (and may never, because I'm not sure they're worth the effort).

To quickly summarize: I sacrificed a replacement plug and found that it broke only after much bending, giving what appeared to be a clean homogeneous face like what I expected. Then it occurred to me that the original broken prong still had a long unbroken section, though thicker = less rolled, and I could break that. Somewhat to my surprise, that was also difficult to break, and also gave a clean apparently homogeneous face. Then I dissected out the other prong from the original broken plug, and broke it in the same place as the original break. That was much more difficult to break than I expected, and gave another clean face.

I was sort of scratching my head about how to understand all that, but now I think the answer is provided by corrosion. I'm guessing that even final parts of the original break were exposed to a significant amount of ozone and other chemicals as a result of arcing inside the plug after it was completely broken, while people were jockeying the plug into positions that would make the equipment work. Then I suppose that, plus maybe some overheating that occurred just before the final break, colored up the surface as you describe. It all makes a nice consistent story, anyway.

--Rik

[brentbristol]

Interesting study. The history of a thing can be more interesting than the thing itself.

[Smokedaddy]

Interesting stereo image.