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KILN DRYING HANDBOOK 37 
STRESS DETECTION 
The detection and the relief of the shrinkage stresses causing case- 
hardening, checking, and honeycombing are among the most im- 
portant of the kiln operator’s duties; they require special skill and 
close application. The usual method of detecting the presence of 
these stresses, which commonly are called casehardening stresses, is 
to cut a stress section from an average board. Such a section should 
be cut at least 2 feet from the end of the board, and should be about 1 
inch long in the direction of the grain. It must be slotted somewhat 
as shown in Figure 8, the exact number of slots depending upon 
the thickness of the board and upon the preference of the individual 
operator. Often it is desirable to cut off several stress sections, 
slotting them variously. The direction in which the individual 
prongs turn and their relative lengths tell the story. If the outer 
ones turn out, it is an indication of tension in the outer layers. 
If they turn in, there is compression in the outer layers. 
Figure 8.—Typical stress sections. 1 represents a green board; 2 indicates tension in 
the surface, typical of early stages of drying; in 3 drying has progressed enough 
further than in 2 to make the shrinkage of the interior balance that of the surface; 
4 shows typical casehardening ; 5 reveals slight reversal of the stresses in 4 by treat- 
ment to relieve caSehardening; and 6 is the finish board, free from stress. The 
changes in the length of the prongs have been exaggerated slightly for clearness 
Slotting the stress section into prongs frees groups of layers, which 
had been locked in a common restraint, allowing each group to make 
a new adjustment within itself. The tension side of each will imme- 
diately contract and the compression side will stretch, just as a spring 
under tension or under compression will change its length when the 
deforming pressure is removed. In doing this the prong will be 
bent, the amount of the bend depending upon the thickness of the 
prong and upon the magnitude of the stress originally present. The 
side that was originally in tension will become concave and the one 
originally in compression will become convex. The comparative 
values and the distribution of the drying stresses can be judged by 
the relative bending of the several prongs, especially when they all 
turn outward. 
When some prongs of a stress section turn inward, however, the 
relative bending can not be judged so well, since interference is 
