katanas break with stab. its pretty simple. they were made for deep slicing. there thin brittle blades.
They are not too brittle, actually, nor thin. The outside and edge of a katana are made from hard, brittle metal so it can be very sharp. The core, however, is a softer more flexible temper to allowing it to bend without breaking.
Here's examples of different ways to Katanas are made like this:
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loginThe clipped, one-sided point ensures it is not as good as a thruster as a longsword, for example....but it can still thrust effectively. The truth is, if Katanas broke when thrusted into a person, the swordsmiths would have made them without points!
Sorry, I like history.