The larger the cable, the longer it will survive thermal cycling. When copper is thermal cycled (rapid heating and cooling) it hardens. Everytime it cycles, it gets a bit harder, a bit more brittle, and degrades. The thermal cycling occurs when the cable flows more amperage than its rated for. The highest resistance points are at the terminals, so that heats up and becomes brittle over the years, hence why the ends tend to be so crusty, shielding falls apart, etc. This is a big reason why the ends are not supposed to be soldered. The reheating wont reflow the solder, but it will reflow the flux residue and the flux attracts moisture and corrodes the strands
Yes, I have noticed that in automotive starting circuits. I had worked with wiring in a refinery and never remember seeing similar wiring degradation. Now, it makes perfect sense. Refinery stuff was designed for continuous run. I didn't think it was possible for copper wire to just "wear out", until a mechanic friend let me disassemble a main battery cable that had a particularly nasty case of "battery cable rot". Brittle as all high heaven at the battery end. I blamed battery acid, outgassing, and wicking.
I wonder if the solder failures are due to using plumbing (acid) flux. Acid based fluxes ( like zinc chloride ) are well known in electronic applications to be a big no-no.
I re-did part of my glow plug system and liberally used 63/37 eutectic solder, as an electrically conductive sealant, intermetallic wetting agent, mechanical bonding agent, heat transfer agent, and water displacer ... and a bit of thin silicone sealant to wick up the wire. ( AWG #6 ) before final cover with heatshrink. I am talking first fluxed, then crimped, then soldered so the solder would wick in and fill any voids, just to make sure water couldn't get in later. I had done this as the previous connection was at the verge of failure and I was blaming water wicking into the previous crimp, initiating corrosion, which initiated localized hot spots...and I was attempting to nip a repeat of that in the bud by filling up all the voids with solder and silicone.
I used Rosin flux that's not hygroscopic or corrosive. It's made for electronics work.
I don't know if I did the right thing, I probably won't live long enough to find out. I have taken many very old things apart that were made this way and they were still sound as water hadn't got into it.
You just confirmed my suspicions on how such a massive battery wire could fail in such a manner. I could not see Corrosion alone causing what I saw. But work hardening does.
I now speculate a lot of our "intermittent duty" electronics fails for this very same thing.
