HOW FAILURE UNDER STRESS OCCURS IN TIMBER. AQ) 
We would thus conclude that, even though the cohesion between the wood fibres and 
the medullary rays was equal to the cohesion between two wood fibres radially adjacent, 
the mere fact of the initial set would be sufficient to start the buckling in a tangential 
direction. 
The micro-photographs figs. 13-18 prove that what has been said of Oak applies 
equally to the otber woods under investigation. Fig. 13 is a normal transverse section 
of Ash and is comparable to fig. 9 (Oak), but, being to a three times greater magnifica- 
tion, it is evident that, while the medullary rays and wood fibres are not greatly 
different, the size of the tracheides is only about one-third of that of Oak. Fig. 14 is 
a corresponding section of Box, and this, being to the same magnification as fig. 13, 
shows that here the tracheides are greatly diminished in size, and that the wood fibres 
are smaller and more solid when compared with either Oak or Ash. Fig. 15 is a normal 
tangential section of Ash, exhibiting the sinuous form of the wood fibres, and becoming, 
as shown in fig. 16, the source of weakness when subjected to longitudinal stress. 
Figs. 17 and 18 are a corresponding pair of tangential sections of Box, and show 
that, in spite of the greatly multiplied number of tracheides, smaller in diameter of 
course, it is still the cohesion between the medullary rays and wood fibres that 
is at fault. 
LONGITUDINAL TENSILE STREss. 
Characteristic Failure and its Cause. 
When the longitudinal stress is of a tensile nature the fracture shows certain 
characteristics. In fig. 19 we have a set of Oak strips % natural size, which have 
been fractured by direct tension. The two upper specimens, one exhibiting its broad 
side, and the other its narrow side, were cut with their broad sides tangential, while the 
two lower ones were cut with their broad sides radial. Both (b) and (c) show a tan- 
gential face, and the fracture is a ragged one, tearing through wood fibres and then 
following the planes of the medullary rays, where evidently it meets with the least 
resistance. (a) and (d) are specimens with their radial faces exposed, and show an 
abrupt break extending more or less straight across this face. 
From this it appears that there is little or no slip between the annual rings or 
wood fibres radially adjacent, but that a considerable slip takes place between the 
medullary rays and their adjacent fibres. 
Figs. 20 and 21 prove that this is not peculiar to the Oak alone. ‘The latter 
represents an Ash strip, + natural size, and shows the slips along the planes of the 
medullary rays. Although no illustration is given, the radial face had the usual 
abrupt break. The former represents specimens of Boxwood, where, though the 
medullary rays are not nearly so prominent, the same kind of fracture occurs. (a) and 
(b) of fig. 20 exhibit tangential faces and the slip along the rays, while (c) with its 
radial face shows the tear across. 
