﻿KILN DRYING HANDBOOK. 



27 



cut up several stress sections with varying numbers of slots. The 

 direction in which the individual prongs turn and the relative lengths 

 of the various prongs tell the story. If the outer prongs turn out, 

 it is an indication of tension in the outer layers. If they turn in, 

 there is compression in the outer layers. Cutting the section into 

 prongs disturbs the balanced state and allows each prong or group 

 of layers to make a new adjustment within itself. The compression 

 side of each prong will immediately stretch and the tension side 

 will contract, just as a spring under tension or compression will 

 return to its original length when the deforming pressure is re- 

 moved. In doing this the prong will be bent, the amount of the 

 bend depending upon the thickness of the prong and upon the 

 amount of stress originally present. The side which was originally 

 in tension will become the concave side and the one originally in 

 compression will become the convex one. The amount and distribu- 

 tion of the drying stresses can be judged by the relative bending 



Fig. 5. — Typical stress sections. 1 represents a green board ; 2 shows tension in the 

 surface, typical of early stages in the drying ; 3 shows drying has progressed farther 

 and the shrinkage of the interior has balanced that of the surface ; 4 shows typical 

 casehardening ; 5 reveals slight reversal of stresses by treatment to relieve case- 

 hardening ; 6 is the finished board free from stress. The changes in the length of 

 the prongs have been exaggerated slightly for emphasis. 



of the several prongs on each side, especially when the prongs all 

 turn outward. When they turn inward the relative bending can 

 not be judged so well, since they interfere with each other. In such 

 cases it may be advantageous to cut the section into a larger number 

 of prongs, thus reducing the amount of curve in each prong and 

 permitting comparison of the relative lengths of the individual 

 prongs. If they are thin enough there will be but little difference 

 in stress between the two sides of each prong, and the state of stress 

 will be indicated by the change in its length. All prongs in tension 

 at the time of cutting will shorten, and those in compression will 

 relieve themselves by lengthening. The top ends of all the prongs 

 will form a curve, and the shape of this curve will indicate clearly 

 the state of stress. If it is convex or high in the center, it indicates 

 tension in the outer layers and compression in the core. If low in 

 the center, the reverse is indicated. 



So far only general indications at the time of sawing the sections 

 have been considered. If they be now set aside in a warm place 

 they will soon dry down to an approximately uniform moisture con- 

 tent, the actual amount depending upon the temperature and humid- 



