HOW FAILURE UNDER STRESS OCCURS IN TIMBER. A419 
(c) A tangential stress is a stress acting parallel to the annual rings, and 
therefore perpendicular to the medullary rays. 
Our direct forces will, as far as possible, be such as produce stresses 
in these directions. 
(2) In the case of a beam with its axis horizontal and the medullary rays in its 
cross-section set vertically, we have stresses induced in a longitudinal direction 
as in (@), and a horizontal shear stress which is along the plane of the annual 
rings. This latter stress will be called a tangential shear stress. 
(e) Similarly, if the beam is laid with the medullary rays horizontal, the horizontal 
shear stress is along the plane of the medullary rays, and will be called a 
radial shear stress. 
LONGITUDINAL COMPRESSIVE STRESS. 
(a) Characteristic Failure of Timber. 
If we consider a test piece of timber subjected to a longitudinal compressive stress 
sufficient to cause rupture, we find that deformation takes place in a direction 
tangential to the annual rings, and that failure occurs by a local buckling of the 
fibres over the whole of a plane which is perpendicular to the annual rings and 
inclined to the direction of pressure. ‘This is true for all timbers, true whether 
the medullary rays are more or less strongly marked, and true whatever the 
proportions of the cross-sections subjected to this compressive stress. 
As already mentioned, the proportions of the cross-sections adopted in these 
tests were generally in the ratio of 2 to 1. Fig. 2 is a block of Oak cut with 
broad longitudinal sides tangential, and arranged so as to exhibit one of these 
sides § normal size. It shows this local buckling taking place in two 
planes more or less equally inclined to the direction of pressure, each fibre being 
displaced tangentially, but not in a radial direction. Fig. 3 is a block of Ash 
similarly cut and similarly placed, but here the buckling is shown taking place in a 
single plane, and this is the more general case. As is always the case, the fibres 
are displaced tangentially, that is, in the plane of the paper, and the slip extends 
across the greatest breadth of the test piece. To emphasise this fig. 4 is shown. 
The broad longitudinal sides of the Ash piece are cut radial, and one is exhibited. 
With the buckling the fibres have moved tangentially as before, or in a plane 
perpendicular to the paper, and there is no movement in the radial direction. 
The same characteristic is illustrated in the case of Boxwood by figs. 5 and 6. 
There two views of two separate blocks, cut with the broad sides tangential and 
radial respectively, are shown, and the slip in the tangential direction is evident, 
with no movement in a radial direction. 
