Janka Hardness Scale
The Janka hardness test measures the hardness of wood. It measures the force required to embed an 11.28 mm (0.444 in) steel ball into wood to half the ball's diameter. This method leaves an indentation. It is a good measurement technique to determine the ability of a type of wood to withstand denting and wear. ItÊ¼s also a good indicator of how resistant it is to sawing and nailing.
The hardness of wood varies with the direction of the wood grain . Testing on the surface of a plank, perpendicular to the grain, is said to be of "side hardness." Testing the cut surface of a stump is called a test of "end hardness."
The results are stated in various ways, which can lead to confusion, especially when the name of the actual units employed is often not attached. In the United States, the measurement is in pounds-force (lbf). In Sweden it is in kilograms- force (kgf), and in Australia, either in newtons (N) or kilonewtons (kN). Source: Wikipedia