I don't pretend to understand this, but they could be saying that it's not infinite, just very, very large. According to the S/N curve on Wikipedia, it looks li

I don't pretend to understand this, but they could be saying that it's not infinite, just very, very large. According to the S/N curve on Wikipedia, it looks like staying in the plastic region for steel means you effectively get enough cycles that the part will "effectively" be immortal (just maybe not going to last until the death of our sun at MHz cycles). An analogy might be with transistors - if you run them at high temps, you might get ten years out of them. If you're running them at room temps, you might not be able to see signs of anything other than sporadic failure even over long periods of time.