15000 



10000 



5000 



Deflection 1 in inches. K 2 3 JS? 4 



Fio. 6. — DotiTiiiiuiition of relative elastic resilieiice. 



tion" column is coiiixiuted from the scale reading. It is placed next to the <;olumii of " Loads" for 

 convenience in ijlotting the strain diagram, which is done on the ruled squares at the bottom ot 

 each sheet. These i)lotted results fall in all cases on a true curve, similar to the one shown above. 

 The total area of tJiis curve 0. D. E., properly evaluated by the scales used, represents the total 

 nnmber of foot-pounds or inch-[)ounds of work done upon the stick before rupture occurred. This 

 is called tlie Total Gross-brraliiH) Resilience of the stick, and when divided by the volume of the 

 stick in cubic iuclies it givers appnjximately the total cross-breaking resilience of the stick in incii- 

 pounds per cubic inch of timber. (Fig. G.) 



A better criterion of toughness, or resistance to shock, is some detinite i)ortion of this strain 

 diagram area, as O P K, for example. This amount of resilience or spring can be used over and 

 over again, and is a true measure of the toughness of tin; timber as a working fpiality. To locate 

 the point 1', the following arbitrary rule has been followed. 



Draw a tangent to the curve at the origin, as O A. Lay oft' A G=iB A and draw O C. 

 Draw m » parallel to O G ami tangent to the curve. Take the point of tangency as the point P 

 and draw P K. The area P K is then called the Belutivc Elastio Resilience.* 



There is no "elastic limit " in timber as there is in rolled metals. In this respect it is like cast 

 iron. The point V is the point where the rate of deflection is 50 per cent more than it is at first, 

 and usually falls on that part of the curve where it begins to change rapidly into a horizontal 

 direction or where the deflection begins to increase rapidly. The areas of these curves are nunis- 

 nred with a iilanimeter aiul reduced to inch-pounds. Thus, if 1 inch vertically represents o,0()0 

 |)ounds and 1 inch horizont;dly reiireseiits 1 inch deflection, then 1 s(|uare inch represents .'),0(H)x 

 ! =."),000 inch-pounds. If the area P K is 1.73 square inches, then the corresponding resilience 

 is S,C."iO inch-i)ounds. This mealis that a weight of 100 pounds, falling 86.0 inches, or 1,000 pounds 

 lalling S.d,-) inches, would have strained the l>eani up to the point P or it would lia\-e deflected it 

 l.fiG inches, and the b<'a-m would have been then resisting with a force of 10,000 jjounds, since /' 

 falls on the iO,000-poun(l line. If this result — 8,0.")0 inch-pounds — be divided by tlic numbeiof 



' This term lias been eoiiieil to deliiie this )iarticular portion of the resilience which will hr used fur cumpariug 

 the relative elasticity or toughness of dift'erent timbers. 



14.-,(lO_No."8 2 



