ELA>TK PKoPKKTIKS oF MATERIAL- 15 



For ductile materials, such as wrought iron and mild steel, the 

 . ion extends over a length six or eight times the width of the 

 piece. Consequently, if the test piece has a form similar to one 

 of those represented in Fig. 4, in which the length An is less than 

 six or eight times the width of the piece, the flow of the metal is 

 dned and therefore its ultimate strength is raised. This has an 

 important bearing on the >trength ^i riv- 

 eted plates subjected to tensile strain. It 

 has been experimentally proved that such 

 plates will stand a greater tension than 

 plates of uniform cross section \vh--e 

 sectional area is equal to the sum ^i the 

 sectional areas between the rivet holes. 



le 10 the ultimate strength 

 was maximum 



stress to the original sectional area of 



.ir. It is e\ in what precedes, therefore, that the unit 



elongation and the ultimate strength are not absolute quantities, but 



I on the form of the test piece and the conditions of the test. 



Ills reason it is absolutely essential that the results of any test 



be accompanied by an accurate des .f the circumstances under 



i'-d. The elastie limit and modulus of elasticity, 



n the contra r dependent of their method 



of determination, and therefore more accurately define the elastic 

 properties of any material 



The tensile strength of long rods is affected in a way different from 



ceding. Since no material is perfectly homogeneity, the 



longer the rod the greater the chance that a flaw will occur in it one* 



where. If, then, hy numerous tests of short pieces, it has been deter- 



i how much a material lacks of being homogeneous, the strength 



of a rod of this material of any given length can be calculated by 



means of the theory of probabilities. Such a theory has been worked 



out by Professor Chaplin* and verified experimentally. 



If one dimension of a bod ty small compared with the 



i example, in long wires or very thin plates, the body 



uy. Mag., December, 1880; aim Proc. Eng. Club, Philadelphia, 

 March, 1882. 



