MECHANICAL PROPERTIES OF WOODS GROWN IE" UNITED STATES. 15 



WORK TO ELASTIC LIMIT. 



Work to elastic limit in static bending is a measure of the work 

 which a beam is able to resist or the shock which it can absorb 

 without being stressed beyond the elastic limit as determined under 

 slowly applied loads. 



WORK TO MAXIMUM LOAD. 



Work to maximum load in static bending represents the ability of 

 the timber to absorb shock with a slight permanent or semi-permanent 

 deformation and with some injury to the timber. Wood, especially 

 in small sizes, can be bent somewhat beyond its elastic limit with only 

 slight injury if the load is removed at once. Work to maximum load 

 is a measure of the combined strength and toughness of a material 

 under bending stresses. Superiority in this quality is the character- 

 istic which makes hickory better than ash, and oak better than 

 longleaf pine, for such uses as handles and vehicle parts. Many 

 species yield butt cuts that exceed upper cuts in combined strength 

 and toughness, hickory showing this characteristic most markedly. 

 The superiority of butt cuts of hickory to upper cuts for ax handles 

 is well known to experienced woodsmen. 



IMPACT BENDING. 



The impact bending test is made upon a beam 2 by 2 by 30 inches 

 over a 28-inch span. A 50-pound hammer is dropped upon the stick 

 at the center of the span, first from a height of 1 inch, next 2 inches, 

 etc., up to 10 inches, then increasing 2 inches at a time until complete 

 failure occurs. The deflections of the specimen are recorded on a 

 revolving drum by a pointer attached to the hammer. This pointer 

 also records the position the specimen assumes after the shock. 

 Thus data are obtained for determining the various properties of the 

 wood when subjected to shock. 



FIBER STRESS AT ELASTIC LIMIT. 



Fiber stress at elastic limit is the greatest stress to which a timber 

 may be subjected under impact loading and recover immediately. 

 Fiber stress at elastic limit in impact is approximately double the 

 fiber stress at elastic limit in static bending. This is an expression 

 of the fact that a small beam, if suddenly strained, bends approxi- 

 mately twice as far to the elastic limit as when loaded slowly. (See 

 also fiber stress at elastic limit, p. 13.) 



WORK TO ELASTIC LIMIT. 



Work in bending to the elastic limit in impact is a measure of the 

 ability of a timber to absorb shock and recover therefrom imme- 

 diately and without injury. The values apply only to resistance 



