Archive image from page 400 of Department bulletin (1923-1929). Department bulletin departmentbul11261150unit Year: 1923-1929. 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 with
Archive image from page 400 of Department bulletin (1923-1929). Department bulletin departmentbul11261150unit Year: 1923-1929. 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 cur
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Keywords: 1920, 1923-1929, archive, book, bookauthor, bookdecade, bookpublisher, bookyear, drawing, historical, history, illustration, image, page, picture, print, reference, united_states_dept_of_agriculture, vintage, washington_d_c_the_dept_supt_of_docs_g_p_o_
