Scott, there is another way of testing hardness, right?
When wood scientists and wood science textbooks talk about wood hardness, it's almost universally (from my experience anyway) a reference to Janka hardness. However, there's another test known as the Brinell hardness test, named after Swedish engineer Johan August Brinell. The ASTM test for Brinell hardness is ASTM E10, but please note that this test is specifically for metallic objects - there is no ASTM standard for measuring the Brinell hardness of wood.
What's the difference between the two?
As we discussed, the Janka test is where they take a steel ball and push it in one half the depth of the ball. They note the pressure it took to get to that depth. It's basically a force rating. But with the Brinell, the force is a constant. The test involves pushing in a steel ball at a fixed pressure for a fixed period of time. A measurement is made of the diameter of the indentation which is then used to calculate a formal rating. It's more of a measure of the "give" of the wood.
There is another variable-the type of ball used. Brinell is expressed as an "HBW" where the last letter indicates the type of ball used - 'W' for tungsten carbide, 'S' for hardened steel.
So the Brinell is a calculation? It's not simple like Janka, an actual measured pressure number?
I once was told that every formula in an article cut the number of readers in half, so I hesitate to get more technical. In simple terms, Brinell hardness is calculated by dividing the test force by the surface area of the indentation. But if you want the actual math….
The test (to determine HBW 10/100--which means using a tungsten carbide ball of 10mm and a pressure of applied load of 100kg) is conducted as follows: The ball is brought into contact with the surface of the test specimen; the test machine head should move quickly enough such that the full load (100 kg) is reached within 1-8 seconds. This load is held for 10-15 seconds (possibly longer if the material "exhibits excessive plastic flow") and released. The diameter (in two locations - one along the grain the other across the grain, 90° to the first measurement) of the indentation is then measured. The Brinell Hardness number is then calculated as:
Where: HBW = Brinell HardnessF = nominal force (kgf)D = diameter of the ball (mm)d = diameter of the indentation (average of the 2 measurements in mm)
If that formula makes your head hurt, again it's essentially the test force divided by the surface area of the indentation.
And what does that mean for wood?
As an example, if the average indentation diameter was measured as 5.6 mm, then HBW 10/100 would be 3.7, which is about the average for red oak.
You told me previously that the Janka for northern red oak is 1290 lbs. Is there any relationship?
Do you mean that if someone reported Brinell hardness for a species, could we convert it to Janka, or vice-versa?
Unfortunately, there is no such conversion factor, although some have tried to develop one. For example, Turkish researchers measured Janka hardness and Brinell hardness for eastern beech and then used linear regression to determine how closely they were related. The good news is they found that the values were related closely enough that "the Janka hardness of the wood specimens of eastern beech species could be converted into Brinell hardness and vice versa."
And the bad news?
The bad news is the underlined part - a different formula must be determined for every species.
I guess the wood industry will stay with Janka for the moment, but it's great to understand the difference. Thank you!