14 



STEENGTH OF MATERIALS 



FIG. 3 



usually near the center, begins to diminish more rapidly than else- 

 where. This contraction of section intensifies the unit stress at this 

 point, and this in turn tends to a still greater reduction of section until 



finally rupture occu: 



The appearance of a 

 bar subjected to a test of 

 this kind is represented in 

 Fig. 3. The contracted 

 tion, AB, of the bar is called the region of striction. The contraction 

 of the section at which rupture occurs is usually considerable . 

 soft steel its amount is from .4 to .6 of the original area of the 1 



In Article 6 the unit elongation was defined as the ratio ! tin- 

 total elongation to the original length of the bar. It has been fund 

 by experiment, however, that the extent <>f the region of 

 depends on the transverse dimensions of the bar and nit ..n it^ length, 

 the region of striction increasing in extent as the transverse dini'-n- 

 sions of the bar increase. Consequently, if two bars are of equi\ .. 

 cross section but of different lengths, the region of striction will be 

 the same for both, and therefore the unit elongation will appear to 

 be less for the long bar than for the short one. On tin- <thcr hand, 

 if the two bars are of the same length, but one is thicker than tin* 

 other, the region of striction will be longer for the thick lr f and 

 therefore the unit elon- 

 gation of this bar will 

 appear to be greater 

 than for the other. 



The form of cross sec- 

 tion of test pieces sub- 

 jected to tensile tests 

 has also an important 

 influence on their elon- 

 gation and on their ulti- 

 mate strength. If a sharp change in cross section occurs at 

 point, nonductile materials, such as cast iron, will bivak at this se<y 

 tion under a smaller unit stress than they could otherwise cam'. 

 This is due to a greater intensity of stress at the st-eii-m wh.-i, 

 change in area occurs. 





FIG. 4 



