The traditional Katana did have a fail-safe to keep edge chipping from spreading beyond the immediate impact area. The yakiba (literally "fired edge") or hamon ("edge design" or "temper" line) pattern was partially esthetic styling, but primarily it afforded projections or partitions (ashi) of softer martensitic steel to project at right angles down into the hardened edge steel. If you had a chip, these would help to contain it into a smaller area.
I've had a rare opportunity to do tamashigiri (test cutting) for years with an old, fairly tired, Koto katana. My teacher deems this sword "too poor" to be treated as a collectable so we get hack stuff up with it (!). This poor old blade has seen incredible abuse. Among other things, we cut 8" to 10" diameter Banana tree logs propped on a length of iron pipe. A few not-so-advanced students have somehow managed to hit the pipe several times, don't ask me how. The sword has a few tiny edge nicks, less then 1/16" deep, and no cracks. It has bent a few times when someone cuts through a particularly thick log and doesn't make it through. It's amazingly tough!
