304 



FORMS OF TEST BARS. 



absolute suddenness, but in the present instance the load 

 was put upon it as quickly as possible and at once 

 removed, and the measurements taken as before. 



The curves show very well the difference exhibited. 

 The curve marked S tt is the load-strain curve for the slow 

 loading, and that marked Q a is the corresponding curve 

 for the rapid loading. S,, and Q b are the curves showing 

 the actual calculated stresses and strains for the same 

 two bars. 



The two latter curves show the line becoming nearly 

 vertical towards the end of the test, indicating an almost 

 plastic condition, as indicated by the stress on the metal 

 being more and more nearly constant. 



The slowness in taking permanent set becomes of 

 great importance when the amount of set is used as a 



B 



PIG. 155. 



measure of the compressive load, as is the case in the 

 copper crusher gauges used for determining the explosive 

 pressures in the bores of guns. Here the copper cylinders 

 are compressed by the pressure of the gases, and exactly 

 similar cylinders are compressed to the same extent by 

 measurable loads, and the compressive force in this way 

 found. It is obviously important that the test loads shall 

 be applied at a rate not very dissimilar from those due to 

 the gas pressures. 



159. Influence of the Forms of Ends on Strength and 

 Deformation. On Fig. 155 three forms of tension speci- 

 mens are shown; of these A is a parallel turned bar 

 with cheese heads, the length between the shoulders 

 being, say, 12in ; C is of the same material and diameter, 

 but having practically no length, the reduction in diameter 



