The whole tool is very hard. I forget what tool steel alloy he uses - he actually told me when I called to ask about the risk of distempering on the grinder. They are heat treated to probably somewhere between 68 to 70 RC. The Young's modulus is probably somewhere around 30 maybe 40 times Ten to the Sixth Power. It's pretty inflexible.
So when the tool experiences chattering, the flexing can take the form of a wave thus creating multiple places along the tool where the Modulus of elasticity is being challenged. The shock of the first fracture can propagate along the tool causing fractures instantly where the flexing is also too high. Ergo lots of pieces.
Thompson uses either 10V or 15V tool steel (AISI A-11 if you want to use generic; I think 10V is trademarked by one of the steel manufacturers). This is an air-hardening tool steel.
All steels have practically the same Young's modulus--this is stiffness, or "springiness per unit area", if you want the qualitative description. For a given size of steel bar, regardless of whether it's tool steel, low carbon steel, etc., if you clamp on the bar with a vise, then, push on the bar with the same force, all steels will deflect by the same amount. Stainless steels have a slightly smaller modulus (springier) than regular steels. In customary English units, most steels have a Young's modulus of 30 million psi, or 200 gigapascal in standard SI units.
The
yield strength of the steel is the resistance to permanent deformation--how much you push on a staple or a paper clip before it takes a permanent bend. This is also measured in psi or Pa.
The
ultimate strength is the amount of force necessary to cause the steel bar to break (not just bend, but break). (This is a simplification, but you don't want me to quote lots of material science and metallurgy at you
😱)
Elongation to failure is how much you can stretch the steel before it breaks. This is a measure of the
ductility of the material. As an example, cast iron typically is not very ductile, but taffy is very ductile. Unhardened steel is a lot more ductile than hardened steel, but hardened steel usually has a lot higher yield strength than unhardened.
Hardness is a test. The Rockwell hardness test for steel is similar to the wood Janka hardness test. In essence, a small hard ball (usually carbide) is pressed with a fixed amount of force into the steel surface. The harder the steel, the less the permanent indentation, and the higher the Rockwell number. People use the Rockwell C-scale for hardened tool steels; a good quality kitchen knife might have hardness in the 55 range in Rockwell C. A less expensive knife blade (swiss army knife stainless, for example), might have a 48 hardness in Rc. The Thompson tools are hardened into the 60's; probably 63 or so. The Rockwell scale is not linear; a change of, say, 3 points, does not mean the same proportional change at various points in the scale.
Hardness is correlated with ultimate strength.
Toughness is resistance to fracture. It's how much energy can be put into a material before it breaks. The toughness test is usually swinging a big honking pendulum into a small section of steel with a defined geometry. Generally,
harder steels are less
tough.
Hardening is a process where the steel is heated to some high temperature, then, quenched (rapidly cooled) to "freeze-in" the material phase for higher strength. It is then "tempered" (re-heated to an intermediate temperature, slow-cooling) to help give the material a bit more toughness.
If you managed to read thru this without falling asleep, congratulations! I hope this answered your questions!
Again, Raul, I'm glad you suffered no injuries during your incident. If I were Doug Thompson (but I'm not, I'm a nerdy engineer), I would want to see the broken tool to do nerdy material science analysis. I've seen analysis of a bike frame (steel frame, Cro-Moly, double-butted, brazed with investment-cast lugs) that failed in fatigue after almost 100,000 miles of riding. That was a really cool analysis! (But my bicycle-riding friend had a colleague who was a professor of materials science, with access to electron microscopes, metallurgy labs, etc.--He too had no injuries when the frame broke.)
