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Hard = brittle

tough = springy

Duh.

Springs are low pass filters to an impulse. Any engineer can take the suspension system of a car, and do a spring/dashpot model complete with equations. That mathematics shows how all that works.

Interesting anecdote...

Years back when I was teaching at UVa, I had a couple of PhD candidates in the physics department who followed me to train kobudo in another city. We were living on budgets and still liked our toys. So my boys (Andy and Dave) took it upon themselves to research how we might make our own bos for kobudo training.

They found someone who would turn the wood and make them from scratch. So the next thing was to pick the right wood.

Andy and Dave did the research, and found all the properties of various woods. There really are many different parameters by which wood is judged in terms of strength and durability. You have to be a bit of a materials science expert to appreciate it. So they took their data. Then they ordered all kinds of wood. Then they had the carpenter make the bos.

The thing is, we had all these bo on bo kumite, so we got to put some of this wood to the test. And I wasn't very gentle...

As I recall, these were some of the woods we chose.

• Walnut
  
• Maple
  
• Mahogony
  
• Purpleheart
  
• Greenheart
  
• White oak

There were other classics that we chose, but I can't remember them all right now.

Anyhow... Then we got our toys and started playing yakusoku kumite.

You know what? Eighty percent of the bos shattered in the first two weeks. All these wonderful hard woods that we thought would last because they were so hard... didn't. They were hard alright. They didn't dent. But I could shatter those babies in a New York second. Damn shame, because some of them were exquisitely beautiful.

The two winners? Purpleheart and... plain old white oak. The purpleheart appeared to be a possible long-term winner. Unlike the white oak, it never dented. It was heavy and hurt when you hit someone with it. The white oak on the other hand would get little dents in it.

Then one day the beautiful purpleheart bo shattered.

My white oak bo lasted almost 20 years. Then last year I was doing a demo in class where I had a student do an upper block and I was showing a Yamani Ryu style overhead attack. Believe it or not, I shattered BOTH bos. Go figure... I'm damn lucky nobody got hurt.

There is one bo that I still have which survived that experiment. The carpenter was able to split a white oak in half, route out the center, and glue in a 3/4 " steel rod. When he glued it all back together again, most people can't tell that it's unusual.

Until they do yakusoku kumite with me...

That bo has lots of dings in it, but it's still in my arsenal.

- Bill

I was there for that one. I think everyone had a surprised look about that result.

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I was dreaming of the past...

If we can define "Springiness" as "the ability of a given material to absorb and return deformation without collapse", then I would categorize this quality into THREE categories:

1: Those materials which DO NOT exhibit VISIBLE deformation under load, but the deformation can be measured. (Example, piezo-electric crystals in phonograph needles.)

2: Materials which DO exhibit VISIBLE resistance to deformation under load: (Example, common coil or leaf springs.)

3: Materials which exhibit OBVIOUS deformation under load, and show no tendency to resist. (Malleable substances such as gold or lead.)

Now, having broadly drawn those categories, it's really just a scale of observation and usefulness.

All solids have SOME resistance to deformation, even liquids do, but the ones that exhibit "springiness" are toward the HARDER end of the scale.
They are able to "store" the deformation forces and "release" them later.
(There are springy liquids too, exhibiting the low-pass filter characteristics BG mentioned.)

Perfect brittleness would mean absolutely NO ability to accomodate deformation, but how finely can that be measured?
It's possible that even diamonds can exhibit deformation, given enough size.

Perfect springiness would mean complete return to original form, regardless of deformation, and we have that too: "smart metals" like used in some eyeglass frames.
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Here's what springs do:
They absorb deforming forces by converting them to potential energy.
They then release the deforming forces by converting them back to kinetic energy.

Pretty well ALL solids and liquids have this characteristic -- it's just the universal conservation/conversion of energy.

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I don't see any "dichotomy" between springiness, brittleness, hardness or softness.

Rather, I see the scale in which we observe, or make use of, these properties of ALL matter, including complex mechanical sytems (including bio-mechanical.)

~N~

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The music spoke to me. I felt compelled to answer.