422 MR ANGUS R. FULTON ON EXPERIMENTS TO SHOW 
Microscopic SECTIONS. 
Radial sections of specimens tested in tension do not give us enlightenment, as they 
exhibit only an abrupt break of the fibres. ‘Tangential sections, however, show the 
characteristic slip between the cells of the medullary rays and the adjacent tissues, 
proving the weakness of their adhesion at that part. 
Fig. 22 is a tangential section of Oak fractured by longitudinal tension, while 
fig. 23 is a similar section of Boxwood. 
CROSS-BREAKING. 
One naturally looks for similar results where similar stresses are induced, as in the 
case of timber used as beams and tested by means of cross-breaking. 
A typical example is that of fig. 25, which shows an Oak beam fractured by this 
means, the tangential face being exhibited. The beam was laid on a radial face, 
supported at two ends and loaded in the middle. The ragged break on the tensional 
side of the beam is similar to what we have already noticed in connection with the 
direct tension experiments. 
The specimen of box seen in fig. 24 also shows the same characteristic, and is even 
a better example on the tension side of this kind of break than the specimen fractured 
in direct tension. 
The fracture of beams which have been laid on a tangential face, and which are 
shown exposing a radial face, is sharp, with no evidence of slip taking place by one 
annual ring sliding past another. 
Fig. 26 shows this in the case of Oak, and fig. 27 in that of Ash fractured under 
similar conditions. In addition, the upper portion shows the bulging out tangentially 
due to the compression of the applied load and the induced compression along the axis 
of the beam. This is not usually so pronounced. 
The micro-photographs from beams fractured by cross-breaking are similar to those 
obtained from direct stress experiments. Fig. 29, from a section on the upper or 
compressive side of a beam, exhibits features similar to those in the first stage of an 
ordinary compression test. 
Fig. 30 shows an enlargement of a portion cut from the lower or tension part of a 
beam, and since the section is a tangential one, it makes it quite clear that the parting 
of the tissues follows the groups of ray cells. 
Fig. 31 is given for the purpose of showing the effect of end compression on a test 
piece which contains the centre of the log, and is arranged to exhibit a transverse 
section. The movement under stress is a tangential one similar to what would be got 
by a twisting moment combined with an end thrust, one portion of the test piece being 
rotated past the other, but having the centre of the tree as the common axis of 
rotation. 
