286 STRENGTH OF MATERIALS 



239. Flexure tests. Flexure tests are usually made by supporting 

 a rectangular piece at both ends and loading it in the middle, can- 

 being taken to guard against local failure at the supports and at tin- 

 point of application of the load. This local failure may be pre\ vnird 

 by inserting some kind of metal plate between the beam and the 

 knife-edge. The deflections of the beam for specified loads are mea>- 

 ured by means of a deflectometer, usually measuring to .01 in. or 

 .001 in. From the data obtained from a test, a strain diagram may 

 be drawn by plotting loads in pounds a> ordinates and detleelions in 

 inches as abscissas. Tin- til>er >tress for any load within the elastic 

 limit is determined, for central loading, from the formula 



and the modulus of elasticity from the formula (Article 66) 



PI 9 

 SD. 



The formulas used to determine the fiber stress in the case of 



flexure of beams / l -' = -!/,) an- true only within the elastic limit 



of the material. They are used, however, to determine the fiber stress 

 beyond the elastic limit, although the \y approximately true 



beyond this limit. Tin- value of the fiber >trevs at rupture as deter- 

 mined by the formula is usually de>ignated as the modulus of rupture 

 (Article i',.".): it is expiv ed in Ik/in. 2 . 



(>n account of the peculiar >tructure of timber the character of 

 the fracture due to a failure in flexure is rather difficult to pi- 

 In case the specimen is free from knots and the Liiain is parallel to tin- 

 length of the piece, failure from concentrated central loading is likely 

 to take place either on the tension or the ei.mpiv-- . or both. 



It may happen, however, even in the case of such a j <-imen as 



indicated, that failure will IK- due {<\ horizontal shear. In Mich 

 shearing takes place along the spring gmwth of one < .nual 



rings. This may have been weakened previously by wind shakes. 



If part of the beam is sapwood and part heart wood, the fracture 

 will be influenced thereby, due t<> the difference in tl ' h of 



the two portions. A cross grain may cause a failure due to splitting. 



