and Poissons Ratio j or Spruce, 873 



curvatures of this surface the values of Young's Modulus 

 and Poisson's Ratio were deduced. The curvatures were 

 obtained t'rom measurements of the slope of the surface in 

 the planes of the principal curvatures. In the case of the 

 curvature of flexure, this was deduced from the slopes at 

 the ends of a 1 in. length symmetrically situated with 

 respect to the central normal cross-section and the knife- 

 edges. Also the curvature in the plane of the central normal 

 cross-section was obtained from measurements of the slope 

 near the edges of the beam. Actually the load was increased 

 by equal amounts and the slopes measured after every 

 increase. The slopes were then plotted against the load, 

 and from the slopes of the resulting lines the curvatures, 

 and thence the elastic constants, were calculated. 



In addition to the elastic constants, the elastic limit stress 

 and modulus of rupture were recorded for each test piece. 

 The elastic limit was calculated using the value of the 

 couple corresponding with the point where the load-curvature 

 line ceased to be straight. The modulus of rupture was 

 taken as equal to the stress at the elastic limit, multiplied 

 by the ratio of the breaking couple to the couple at the 

 elastic limit. 



The timber experimented upon was obtained from the 

 R.A.E. in 1915 in the form of four large balks, which 

 represented the good average type of spruce which was 

 available for aeroplane manufacture early in the war. The 

 balks will be denoted by the letters A, B, C, and D, and a 

 photograph of the end of balk A is reproduced in fig. 1 

 (PL XXII.). Micro- photographs of normal, axial, and tan- 

 gential sections for balk A, magnified about 40 diameters, 

 have previously been published *. 



All these photographs are typical of the remaining three 

 balks. The balks were straight grained and free from 

 knots and shakes, and were well seasoned before the test 

 pieces were prepared from them. 



In the notation used in the paper, the direction of the 

 grain is denoted by ZOZ, the direction perpendicular to this 

 and normal to the annual layers by XOX, and the direction 

 perpendicular to both the above, which is thus tangential to 

 the annual layers, by YOY. The symbols E^, E^ and E are 

 used to denote Young's Modulus in the direction XOX, 

 YOY, and ZOZ respectively. Also the symbols o-^, a^. 



* Phil. Mag. June 1921. 



