[lake-view]

> Maybe my cardiologist is just trying to make me feel good, but he says my leads will likely last 30-50 years. Intuitively that seems unlikely, but we'll see. It's got to be one of the most engineered cables in existence.

One of my favorite learnings in school was about the "Endurance limit".

Some materials, like aluminum, will eventually fail under cyclic loading even at tiny, tiny loads. This was a big problem when they built the first passenger jets. Other materials, like steel, have a threshold at which they can be cycled indefinitely without issue.

For something like a pacemaker, I like to imagine they dialed the materials and forces to be within such a threshold so you can keep on ticking!

[tengwar2]

Re passenger jets - I imagine you are thinking of the Comet 1? That was a more complex failure than is generally known. In brief, they did know about fatigue life at the time, and had ways of retiring aircraft before it was an issue (safe-life design, apparently introduced in the 19C for steam engines despite their being iron and steel). Ok, now you will be thinking "square windows, stress concentrators". Almost all pressurised aircraft use windows with angled corners in the cockpit. There isn't an intrinsic bar to square windows, and in fact the original design would probably have been ok. That used glued installation, avoiding stress concentrators. However a production engineer changed the design to use riveted installation, which caused the well-known problem with hull failure. Still, that would have been discovered if DH had not managed to resist government pressure to do fatigue testing on the pressure hull (because they were racing Boeing to be first to market, and fatigue testing takes time). They actually had the apparatus for repeatedly pressurising the hull in a bath, but only used it for testing static pressure.