MECHANICS OF MATERIALS 



69 



called elastic limit of the material. This elastic limit varies 

 with different materials. It is the largest unit stress which 

 the material will stand and still return to its original state 

 when the force causing the deformation is removed. The 

 word "strain" is sometimes confused with "stress." Strain 

 means the same as deformation. The expression "stress 

 and strain" is synonymous with "stress and deformation." 



When a body is stressed beyond its elastic limit, the 

 amount of deformation produced by a given force is not in 

 proportion to the magnitude of the force, and the body 

 receives a permanent change of shape, the extent of which 

 depends not only upon the magnitude of the force producing 

 it, but also upon the length of time during which the force 

 acts. For example, a tensile stress may become so large 

 that the material will continue to stretch as long as the 

 force is applied or until an actual break occurs. 



Forces acting to produce tension or compression always 

 produce shear along "-certain planes. This is illustrated in 

 Figure 96. These shearing stresses are zero along the planes 

 at right angles to or parallel with the longitudinal axis of 

 the body, and are at a 

 maximum on planes 

 cutting the axis at 45 . 



Forces applied sud- 

 denly. The forces so far 

 considered are assumed 

 to have been applied 

 slowly. It should be 

 understood that forces, 

 whether tensile, com- 

 pressive, or shearing, 

 produce far greater 

 stresses if they are 

 applied suddenly. A weight dropped from some consider- 

 able height upon a beam may cause it to break, when if it 

 were brought on slowly the beam would carry it. gome 



Fig. 96. Direction of shear in a body 



which is subject to tension or 



compression 



