﻿DECAYS AND DISCOLOEATIONS IN AIRPLANE WOODS. 



11 



In aJl cases where it can not be determined satisfactorily by other 

 methods, 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 

 onversant 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 fail- 

 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 

 grain. A hand mag- 

 nifier is often neces- 

 sary to bring out the 

 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 



Fig. 2. — Section from an unfinished wing beam, showing 

 a compression failure in Sitka spruce which probably 

 occurred when the tree was felled. 



