TIMBEE TESTING 305 



be determined from the data obtained in the 

 above test by plotting a graph of load against 

 deflection, usually called a stress strain diagram. 

 Such a diagram will show a straight line for the 

 initial readings which will eventually curve, 

 slightly if the wood is dry, perceptibly if it is 

 green. The point at which the line begins to 

 curve indicates the elastic limit, which may be 

 calculated in pounds per square inch from the 

 same formula as is used in calculating the 

 ultimate extreme fibre stress or breaking load, 

 viz. : — 



3 W L 



2 B H^ 



Where W = the load in lbs. applied at the 

 centre of the beam. 

 B = the breadth of the beam in inches. 

 H = the depth of the beam in inches. 



Hardness. Concerning the question of hard- 

 ness of woods, we have said something in a pre- 

 vious chapter. Hardness there described refers 

 to resistance to indentation, but there is also 

 another form of hardness which it is important 

 to determine, that is to say resistance to wear. 

 This is a very important consideration in wood 

 which is to be used for floors, wood paving, etc. 

 Tests for either form of hardness are made for 

 comparative purposes only. 



