126 SECTIONAL ADDRESSES. 



tests are sufficient to show that its strength is enormously greater 

 along the grain than across it. How, then, is an engineer to calculate 

 the strength of a wooden member? There is no theory, in a form 

 available for the engineer, by which the strength of members made 

 of an anisotropic material can be calculated. 



I fancy I may be told that such a theoi-y is not required — that 

 experience shows that the ordinary theory is quite near enough. How 

 utterly misleading such a statement is I will try to show by a few 

 examples. Suppose a wooden tie or strut is cut from the tree obliquely 

 so that the grain does not lie parallel to its length. In practice it 

 is never possible to ensure that the grain is accurately parallel to the 

 length of the member, and often the deviation is considerable. How 

 much is the member weakened? This comparatively simple problem 

 has been of immense importance in aeroplane constiniction, and, thanks 

 to the researclies made during the war, can be answered. The solution 

 has thrown a flood of light on many failures which before were obscure. 

 If the tensile strengths of a piece of timber are, say, 18.000 Ib./sq. hi. 

 along the grain and 800 Ib./sq. in. across it (radially or tangentially) 

 and the shear strength is 900 Ib./sq. in. along the grai]i— these figm'es 

 coiTespond roughly with the strengths of silver spruca — then if a 

 tensile stress be applied at any angle to the grain tlie components 

 of that stress in the principal directions must not exceed the above 

 strengths, or failure will occur. Thus we can draw curves limiting 

 the stress at any angle to the grain, and similar curves may be drawn 

 for compression stresses. These theoretical curves have been checked 

 experimentally, and the results of the tests confirm llicm closely, except 

 in one particular. The strengths at small inclination to the grain fall 

 even faster than the theoretical curves would lead \is to expect. The 

 very rapid drop in strength for quit-e small deviations is most striking. 



Similar curves have been prepared for tensile and compressive 

 stresses inclined in each of the thi'ee principal planes for spruce, ash, 

 walnut, and mahogany, so that the strengtlis of these timbers to resist 

 forces in any direction can now be estimated reasonably accia-at-ely. 



As a second example consider the strength of plywood. Plywood 

 is the name given to wood built up of several tliicknesses glued 

 together \vith the grain in alternate thicknesses running along and 

 across the plank. The result of this crossing of the grain is that tlie 

 plywood has roughly equal strength along and across the plank. Ply- 

 wood is generally built up of thin veneers, which are cut from the 

 log by slicing them off as the log revolves in a lathe. 



Owing to the tnper in the trunk of the tree and to other irregularities 

 in form, the grain in the veneer rarely runs parallel to the surface, 

 but genei'ally runs through the sheet at a more or less oblique angle. 

 As a consequence the strength of plywood is very variable, and tests 

 show that it is not possible to rely on its having more than hnlf the 

 strength it would have if the grain in the veneers were not oblique. 

 It is therefore obviously possible to improve the manufacture enor- 

 mously by using veneers split off, following the grain, in place of the 

 present sliced veneers. The superiority of split or riven wood over 

 cut wood has been recognised for ages. I believe all ladders and ladder 



