Hand Tools Archive

Stress vs strain

Patrick Chase
What you're describing is a strain test, i.e. how far can you bend the knife before it fails.

Strength is usually specified in terms of stress, i.e. how much force does it take to break it.

There are two problems with the test you describe:

1. The maximum strain that a beam can withstand depends on its thickness.

2. The test as described doesn't distinguish between elastic and plastic strain, so a softer material that deforms plastically would appear to last longer even though it's useless as a knife once it starts to deform. Softer materials will therefore appear to do better.

I suspect that the only thing you can really conclude from the test you witnessed is that the PM blade probably had a thicker spine and/or was harder. I'm also not convinced that that test has any value at all as a predictor of the object's performance as a knife.

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