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The Janka hardness test is a test used to measure wood's resistance to denting. The test involves pressing a steel ball that is 0.444" in diameter halfway into a piece of wood and measuring the force required.

The 0.444" ball intentionally has a 1 sq cm circular diameter but I have two questions about this method:

  1. Generally why is a steel ball used rather than just a flat 1 sq cm anvil? It seems like the ball might give a better representation of the wood strength across a larger cross section of the wood but is there a well defined reason for this?
  2. Is the force required really even measurable in pounds per square centimeter (odd combination of units there I know)? Considering there will potentially be some additional friction and compressive forces at play. i.e.: white oak has a janka hardness of 1360 lbf or 616 kg. Would it be reasonable to say it takes around 8774 PSI to dent white oak since 616 kg per sq cm is equal to 8774 lbs per square inch?

In regards to solar mike's comment about side friction. I get that a sphere will have very little to no side friction once pressed in halfway but wouldn't there be additional friction prior to that point? The area of the cross section of the ball is 1 sq cm but the area of the sphere is 4 sq cm so the half being pressed in the wood is actually 2sq cm. I'm unsure what part this plays as the area of a nail for example isn't nearly as important as the area of the tip of the nail. The side may provide some additional friction but ultimately only the cross section of the nail is penetrating the wood.

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  • $\begingroup$ If you use a flat 1 sq cm anvil, will you taper the sides so that there i little to no side friction? Also, if we change to your standard, how many existing results will become invalid? $\endgroup$
    – Solar Mike
    Nov 23, 2023 at 14:34
  • $\begingroup$ @SolarMike: I'm not advocating for one way or another just wondering if there is some clear explanation for the steel ball. I bet it would have been much easier to setup tooling with an anvil so it seems likely the steel ball was intentionally selected for a reason. $\endgroup$
    – jesse_b
    Nov 23, 2023 at 14:36
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    $\begingroup$ the ball does not cut the wood ... the anvil cuts the wood $\endgroup$
    – jsotola
    Nov 23, 2023 at 15:57

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The steel surface needs to start and remain parallel to and tangent to the wood surface. This is automatic and easy to re-create with a spherical steel surface. The first part is unnecessarily difficult with a flat surface: the second part is impossible with sharp corners.
Both conditions can be met with parabolic/hyperbolic/thumb shaped probe: when considering adhesion forces, non-spherical probes may be used, but steel ball bearings and spherical probes are a standard item.

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