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Compression Failures as Defects 



rioducts Laboratory, Madison. Wisconsin. 



By L. J. MAHKWARLiT. Assistant Engineer in Forest Products. Forest 



When too great a pressure is applied to struts or columns of wood to bending usually fails first in compression, there are probably many 



in the direction of the grain, the fibers Vien.l or buckle at the weakest cases where compression in.iuries result without further damage to 



the tree. 



An example of a failure of this type is 

 shown in figure 2. The injury apparently 

 caused an abnormally rapid growth about 

 the region of failure. Such ridges or bumps 

 may be produced by numerous other causes, 

 and, t)\erefore, are not always indicative of 

 compression failures. 



The ends of the specimen shown in figure 

 2 were originally sawed even. The wood 

 about the failure was evidently confined in 

 the tree in a stressed condition, and when 

 given freedom expanded in the direction of 

 the grain beyond the uninjured material 

 farther out in the tree. The position of 

 failures on the north side of this and other 

 trees from the same locality indicates that 

 tlie damage was due to a storm from the 

 South. A count of the annual rings, to- 

 gether with a knowledge of the time the 

 material was cut, shows that in all proba- 

 bility this storm occurred about 189S. 

 E-xtr-jme carelessness and rough handling 

 of logs also produce 

 compression failures. 

 Such practices as 

 felling trees across 

 logs, dropping logs 

 from a considerable 

 height or unloading 

 from a car across a 

 single skid should 

 be avoided wherever 

 possible. Coinpres- 

 s i o u failures pro- 

 duced in the process 

 of logging will, of 

 course, be most 

 jirominent at the cir- 

 cumference of tlie 

 log and will dimin- 



FIG. 2— EXAMPLE OF COMPRESSION FAILURE FORMED IN LIVING TREE. 



"While some failures are 



PIG. 1— TELLOW PINE. 1— PIECE OF FLOORI NG CONTAINING COMPRESSION FAILURE. 

 AND 3— COMPRESSION FAILLRES FORM EH BY TESTING MACHINE. NOTE SIMI- 

 LARITY BETWEEN 1 AND 3. 



point, resulting in a 

 streak or wrinkle 

 on the face of the 

 material, which is an 

 indication of a com- 

 pression failure 

 (see figure 1). The 

 plane of compres- 

 sion failure usually 

 makes an angle of 

 about 60° with the 

 grain. These inju- 

 ries vary in size 

 with the kind and 

 condition of the ma- 

 terial and of course 

 increase with the 

 magnitude and dura 

 tion of the stress. 



so pronounced as to be unmistakable, others 

 are so minute that they can be detected only 

 with great difficulty. 



When a beam is loaded the fibers in the 

 upper half are in compression while those 

 in the lower half are ,in tension. In conse- 

 quence, failures similar to those found in 

 props and columns are developed in the com- 

 pression half of beams. 



Compression injuries not readily apparent 

 to the eye do not greatly impair the strength 

 of wood when subjected to slowly applied 

 loads, but the shock-resisting abilitj' of 

 wood of this character is seriously reduced, 

 complete failure occurring very suddenly 

 along the plane of injury. 



Severe storms cause great stresses in 

 trees, -especially those which are in exposed 

 positions. In this condition the windward 

 side is in tension, while the leeward side is 

 in compression. Since green wood subjected 



— 24 — 



FIG. 4— CHESTNUT CCASTANEA DENTATA). THE SECTION AT THE LEFT SHOWS \ 

 ^^iX'^-'^-'^'" FAILURE IN CROSS EI:NDING AT PLANE OF PREVIOUS CO.MPUESSION FAILURE. 

 THE ONE AT THE RIGHT IS A PIECE FROM WHICH PHOTOMICOGRAPH (FIG. 3) WAS MADE 

 SHOWING DIFFICULTY OCCASIONALLY EXPERIENCED IN DETECTING PWILURE THIS 

 PIECE WAS TWO INCHES IN WIDTH 



