DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. 11 
In all cases where it can not be determined satisfactorily by other 
methcds, representative pieces should be selected for impact bending. 
This test above all others most readily reveals brittleness in wood. 
But the test must be made, or at least the results and breaks reviewed, 
by some one experienced in this method of testing and thoroughly 
conversant with the mechanical properties of wood. 
COMPRESSION FAILURES. 
Compression failures may be due to abnormal stresses on the stand- 
ing tree (from a wind of unusual velocity, for example), to shocks 
in felling the trees, or to injury during the process of manufacture. 
Figure 2 shows a compression failure, probably caused when the tree 
was felled, in a section from an unfinished wing beam of Sitka spruce. 
As an example of injury during the course of manufacture, it might 
be mentioned that when a large number of wing beams, improperly 
piled, are transported 
on a car or wagon the 
weight and jar some- 
times cause such ‘ail- 
ures in beams near the 
bottom of the pile. 
The smaller com- 
pression failures are 
not easy to detect. 
They appear as small 
whitish wrinkles or 
irregular lines across 
the face of the piece, 
at right angles to the 
erain. A hand mag- 
nifier is often neces- Fie. 2.—Section from an unfinished wing beam, showing 
airy tho butile OWL NS caomadti eaten disreee meets 9 pn fee me 
finer failures dis- 
tinctly. The more pronounced failures appear as rather rounded 
ridges resulting from the “ buckling ” of the wood fibers under stress. 
Compression failures are quite detrimental to the strength of 
wood, particularly as regards bending strength and shock-resisting 
ability. Material showing compression failures must not be used 
in parts where strength is required. One visible small compression 
failure usually indicates the presence of others. 
Members with a small cross section are sometimes subjected to a 
rough test which makes the wood appear to be brash. It is well 
known that beams when placed in static bending characteristically 
fail first in compression, that is, in the fibers between the center 
(neutral plane) and the top of the beam. Hence, when a spruce 
longeron, for example, is supported at both ends and a load applied 
in the center, slight and practically invisible compression failures 
may result. Such failures appear as tiny whitish lines or wrinkles 
on the surface of the wood. If the member is then turned over and 
the load again applied until failure occurs, the break will be sharp 
and straight across with no splintering, typical of a compression 
break. This test should not be applied to softwood longerons, par- 
ticularly spruce, since the resulting breaks will nearly always be 
