. Department bulletin. Agriculture; Agriculture. 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 win
. Department bulletin. Agriculture; Agriculture. 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
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