Art. 8. 



ELASTIC LIMITS. 



its upper end; if a weight W be suspended from it, it will 

 stretch; at the same time a resistance is developed in it which 

 increases, as the bar becomes deformed, until it balances, or holds 

 in equilibrium, the external forces acting on the bar and the 

 deformation ceases. 



A STRESS is the internal resistance de- 

 veloped in a body by forces deforming it. 

 A stress may be imagined to be carried 

 from molecule to molecule as the links of 

 a chain transmit a force by acting in op- 

 posite directions upon each other, so that, 

 at any imaginary cross section as pq, 

 Fig. 3, the internal forces are acting in op- 

 posite directions and away from the sec- 

 tion. In this way, the effect of the weight 

 W is transmitted through the bar to its 

 support at the top, producing a stress at 

 every cross section equal to W. It is plain F'g. 3. 



that, at the ends of the bar, the stresses will have opposite direc- 

 tions, just as the external forces have opposite directions. 



In Fig. 4 the directions of action of af w 



the external forces and stresses are reversed 

 the bar is in compression-, the bar of Fig. 

 3 is in tension. 



8. Elastic Limits. If, from the bars 

 of Figs. 3 and 4, the loads be removed, they 

 will return to their original dimensions or 

 shape, provided the stresses have not ex- 

 ceeded a certain limit called the elastic 

 limit. It has been found by experiments, 

 that for steel stresses and deformations are 

 directly proportional to each other within a cer- 

 tain limit called the limit oj proportionality. 

 (HOOKERS LAW). Practically, these two limits are the same, 

 although the true elastic limit is below the limit of proportion- 

 ality (21). 



When the stress has exceeded the elastic limit, some of the 

 deformation is permanent and the body is said to have taken a set. 



The elastic limit of any material depends upon its grade and 

 quality, and is much more sharply defined in wrought iron and 



Q] 



Fig. 4. 



