Janka hardness

The hardness of wood varies markedly from soft balsa to hard ironwood with pine, oak, and maple in between. It is measured either by determining the force needed to push a hard ball (diameter = 0.444 in) into the wood to a depth equal to half the ball s diameter (Janka hardness) or by the initial slope of the force vs. penetration-depth curve (Hardness modulus). Average values of Janka hardnesses for typical woods are listed in Table 13.1. The data are from Green et al., (2006), and are for penetration transverse to the tree axis. The values are for moisture contents of about ten percent. The first set of five items are hardwoods, while the second set are softwoods. To roughly convert Janka hardnesses to VHN multiply by 0.0045. 

D. W. Green, M. Begel, and W. Nelson, Janka Hardness Using Nonstandard Specimens, Research Note FPL-RN-0303, Forest Products Laboratory, U.S Dept, of Agriculture, Madison, WI, USA (2006). 

In the absence of brittleness of wood and with the described differences in results according to variable number of layers acted on by the ball, attempts to correlate hardness test results with other strength parameters cannot succeed. Only after lengthy testing of various tree species, did Janka propose the following formula ... 

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