Hand Tools

Re: there must be an engineer in the house

Michael Bulatowicz
There is, though I haven't been contributing here yet.

Strength is a loosely defined term in mechanical engineering. The definition depends on the context.

Toughness is a measure of energy absorption, but is often tested under conditions that try to simulate worst-case behavior and so the measurements aren't necessary relevant to a given situation. A Charpy impact test, for example, simply doesn't provide enough information to draw firm conclusions about a steel beyond how it can be expected to perform in a similar test.

Why? Two big reasons. First, it's always done with a well defined notch in the test specimen to provide a large stress concentration and make sure it's going to break where you want it to. Second, it's an impulse load with a much shorter impact time than one would get in woodworking operations (unless you dropped the tool). A poor analogy would be to think of the steel like Silly Putty: brittle under a rapid strain rate (stretch it quickly and it snaps) but extremely ductile under a slow strain rate. The relevant quantity there is the material's strain rate hardening exponent (which would be extremely high for Silly Putty--much higher than any steel near room temperature).

Unless I am misreading David's posts, what he is calling strength is a multifactorial combination of properties. Part of it sounds much like the "proportional limit" which is related to the yield strength but not quite as well defined. Both are correlated to hardness, as he states.

Hardness vs toughness is not an absolute tradeoff. A fine grained example of a particular steel often exhibits much greater toughness than a coarse grained example of the same, at identical hardness. Keeping grain size fixed, there is indeed a definite tradeoff.

As an example, when I was involved in metal fatigue research as an undergrad, we were testing steel the the manufacturer told us had a typical yield strength of 80 ksi with typical industry standard processing, but their process for refining the grain pushed the yield strength to roughly double that without losing ductility. Our tests agreed and showed a significant increase in fatigue life as well.

Fatigue life is also correlated to carbon content: all else being equal, higher carbon content leads to higher fatigue life.

So, off the top of my head I'd describe David's "strength" property as a combination of a high proportional limit, high fatigue life, and ductility at the appropriate strain rate.

It's a vast subject and I'm not a metallurgist, so I'll stop there for now.

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