330 



II. — Tension Tests. 

 The strength of most woods under direct tension is so 

 high compared with its strength in other ways, that wood 

 seldom fails in this manner in actual structures, and so these 

 tests are not of such practical importance as the others. A 

 number of tests were made upon specimens turned in the 

 lathe to a diameter of about IJ in., the length of the speci- 

 men being 22 in. over all and 10 in. inside the grips. The 

 stretching of the wood under tension was measured between 

 two points, originally 8 in. apart, by means of a Goodman 

 extensometer, enabling measurements to be made to the 

 __!__ of an inch, and from these readings the modulus of 

 elasticity was computed. 



III. — Shearing Along the Grain. 



The failure of wood by shearing 

 or detrusion along the grain is one of the 

 commonest methods of failure in actual 

 structures, but the determination of the 

 shearing strength depends a good deal 

 upon the way in which the experiments 

 are made. The method adopted is the 

 standard method in the United States 

 Forestry Department. The test piece is 

 cut in the form of a block 2 in. square 

 and 16 in. long (as shown in fig. 2). 

 This is pierced by two morticed holes, 

 each J in. wide and 2 in. deep, one at 

 each end of the block in directions at 

 right angles. The block is held by a 

 bolt passing through the centre. A 

 steel key, J in. wide, is passed through 

 each mortice in turn, and pulled until 

 it forces out the wood at the end of the 

 mortice. In order that the key may 

 press evenly over the wood a small steel 

 bearing piece is fitted into the bot- 

 tom of the mortice; this has a 

 curved back upon which the steel 

 key presses. 



When the mortice cuts across the rings 

 of growth at right angles the shearing 

 resistance is much less than when the 

 direction of the mortice is parallel to 

 the rings. In the tables append- 

 ed, however, the average results are 

 given. 



Fig. 2. 



Form of Test 

 Piece- for Shear- 

 ing Experiments. 



