### ELI5:How Knife blades become dull from cutting much softer objects

At the knifes edge, the material is very thin and over repeated use (lots of pressure being applied) its thin edge will deform on a microscopic level, even if it is a stronger material. Given enough use it will be noticeable. Included some comments from previous posts.

"It's incredibly interesting. Say, you apply a certain amount of pressure on a certain material with a piece of metal, with an edge 1cm wide. And, say, you are applying 1kg per square cm, not enough to penetrate said material. Now, make that piece of metal thinner, and you are applying the same total pressure, but more pressure per square cm. Make it very very thin, and you are applying lots of pressure on a very small area. In that case, if the material you are using is harder than the one you are using it on, it'll penetrate said material."

• gnualmafuerte

"Take a stainless steel knife, and take a slab of meat. Which one is sharper and stronger? The knife right? The meat is not hard enough to scratch the knife.

Now hold the knife in a vice and beat the crap out of it with the slab of meat. The knife bent in half, but how can that be if the meat is softer than the knife?

That's basically what's going on as the edge on a knife gets duller (excluding things like minerals, sand, dirt, grit, bones, etc wearing the edge down) is that an edge is not perfectly straight or smooth. It's made of many microscopic daggers of metal like fine little teeth that grow from the edge.

As you cut things, these little metal teeth will start to bend to the left and right even though they are harder than the material they are cutting. When this happens you end up with the edge having microscopic mis-allignments of the teeth on the edge.

This is easily corrected by steeling the blade. You've seen chefs do this by scraping the blade up and down a metal rod which takes the bent teeth of the edge and stands them back up straight. This doesn't really sharpen the edge by removing material, but by straightening the material already there. Once this is done the knife is sharp again.

Barbers may also do this by stropping a blade with leather before using it. The leather takes the place of the steel and re-aligns the teeth on the edge of the blade. They may also use a stropping compound which is a fine abrasive which also removes a small amount of blade material and polishes the edge, making a more comfortable shave.

After a lot of use, or rough use, the edge itself may lose it's shape and at that point it needs to be re-honed or sharpened. The less material removed to get the blade sharp again, the longer the blade will last."

• kodack10