STRENGTH OF MATERIALS 243 



the metals used in construction and their approximate weights 

 per cubic inch. 



Metal Wt. per Cu. In. 



Cast Iron 260 



Wrought Iron 281 



Steel 282 



Copper 317 



Brass and Bronze 307 



Lead 409 



Tin 263 



Aluminum 096 



291. Factors of Safety. In building a machine or a 

 structure of any kind, care must be taken not to subject any 

 part to a stress that would strain it beyond its elastic limit. 

 The usual practice is to divide the ultimate strength of the 

 material by some number depending upon the kind and 

 quality of material and upon the nature of the stress. This 

 quotient is called the factor of safety. The factor of safety of 

 any material is the ratio of its ultimate strength to the actual 

 stress to which it is to be subjected. 



Suppose the actual tensile stress on a rod 1 in. square is to be 

 10,000 Ibs., and we have found by testing that the ultimate tensile 

 strength of a material of this kind is 70,000 Ibs. Then the factor 

 of safety for this material would be 



70,000 _ 

 10,000 



The rod when stressed 10,000 Ibs. will then have a factor of safety 

 of 7. 



As has been stated, a force acting suddenly is called a 

 shock and does more damage than the same force gradually 



