Test blocks taken from different parts of the same log may show very decided differences in the manner of failure, while blocks that are much alike in the size, number, and distribution of the elements of unequal resistance may behave very similarly.

The direction of rupture is, according to Jaccard, not influenced by the distribution of the medullary rays.[7] These are curved with the bundles of fibres to which they are attached.

In any case the failure starts at the weakest points and follows the lines of least resistance.

The plane of failure, as visible on radial surfaces, is horizontal, and on the tangential surface it is diagonal.
[Footnote 7: This does not correspond exactly with the conclusions of A.Thil, who says ("Constitution anatomique du bois," pp.

140-141): "The sides of the medullary rays sometimes produce planes of least resistance varying in size with the height of the rays.