LAWS OF BONE ARCHITECTURE 189 



hence the name. As an example of shear, in figure 1 e let B 

 represent a block of an homogeneous material rigidly fixed in a 

 vertical plane and resting between blocks A and C, as shown, 

 with the left edge of A and the right edge of C in the same ver- 

 tical plane. A vertical force, P, applied to B through the 

 block A will be transmitted to block C by means of B and will 

 develop in B a shear equal to the force transmitted. Shearing 

 force is usually assumed to be uniformly distributed over the 

 area of the cross-section upon which it acts. 



If a straight bar of homogeneous material be rigidly fastened 

 at one and at the other a twisting force be applied, the condition 

 of stress produced is called torsion. This is a special type of 

 shearing stress. 



12. Elasticity is the property possessed by a material of re- 

 turning to its original dimensions and shape when the external 

 forces producing distortion are removed. 



13. Elastic limit. Every solid body is elastic within certain 

 limits, and the deformation produced in it is directly proportional 

 to the force producing it and to the internal force resisting it. 

 If the force producing deformation exceeds a certain limit the 

 distortions will increase more rapidly than the forces; and when 

 the load is removed the body will not entirely regain its former 

 shape and dimensions, but will have a permanent set. The 

 maximum unit stress up to which the deformation increases 

 proportionally to the stress is called the elastic limit. This 

 limit varies with the kind of material and the kind of stress, 

 tension, compression and shear. 



14' Ultimate strength is the maximum unit-stress which .is 

 reached previous to rupture when a body is tested to destruction 

 (as in a testing machine). 



15. The modulus of elasticity is the ratio — - , . -. and 



unit deformation, 



varies with the material and the kind of stress to which it is sub- 

 jected. Generally the modulus of elasticity used is that for 

 tensile stress, and is represented by the letter E. 



