240 APPLTKD SCIENCE 



For example, if we take a bar of wrought iron 1 sq. in. in section 

 and subject it to a tension of 5000 Ibs., it will be found to elongate 

 .02 in.; if a tension of 10,000 Ibs. be applied the elongation will be 

 .04 in.; if a tension 15,000 Ibs. be applied it will be .06 in.; for a 

 tension of 20,000 Ibs., .08 in., and for a tension 25,000 Ibs., 10 in. 

 When, however, the next 5000 Ibs. is added, making a total stress 

 of 30,000 Ibs., it will be found that the total elongation is .14 in., 

 which shows that the elongation is increasing more rapidly than 

 the stress. 



The point at which the elongation begins to increase more 

 rapidly than the stress is called the elastic limit. 



III. When the stress is sufficiently great a strain is pro- 

 duced which is partly permanent; that is, the body does 

 not spring back entirely to its original shape when the stress 

 is removed. This lasting part of the strain is called a set, 

 and when a body is strained sufficiently to give it a permanent 

 set it is said to be strained beyond its elastic limit. 



IV. When a still greater stress is applied to a body after 

 the elastic limit is reached, the strain rapidly increases and 

 the body is finally ruptured or broken. Many materials, 

 such as iron and steel, after the elastic limit is reached, act 

 very much like molasses candy. When pulled they stretch 

 and draw down thinner and thinner until finally they break 

 apart. The machine designer must remember that the 

 stress should never exceed the elastic limit of the material, 

 because when a bar is thus stressed it is very unsafe and is 

 likely to break. 



V. A force acting suddenly, such as a sledge hammer blow, 

 is called a shock and causes greater injury than the same 

 force gradually applied, because of the velocity or speed of 

 the blow and the effect of its sudden application. Familiar 

 examples of steel subjected to repeated stresses and shock 



