4 Mr. T. E. Stanton on 



initial section applied, and kept constant for 20 minutes. 

 The extension of the bar practically ceased after 1 5 minutes. 

 On increasing the load it was found that no further per- 

 manent set took place until the stress had reached 17*0 tons 

 per square inch, at which point the bar commenced to draw 

 out rapidly. At 17*40 tons another pause of 20 minutes was 

 made, and the test continued as before. It is seen that the 

 hardening effect of each load extends through practically 

 the same range of stress. 



In the same figure the dotted curve is the diagram for 

 a bar of the same length and sectional area as the first, but 

 loaded in a different manner. The same initial stress of 

 16 tons per square inch was applied, producing the same 

 permanent strain as before. The load was then kept 

 constant for 18 hours. On increasing the load no permanent 

 set took place until the stress had reached i8"8 tons per 

 square inch. After this the drawing out took place rapidly. 

 In this case, the hardening effect extended through an 

 increase of stress of 2*5 tons. A similar test was made on 

 a bar of the same material, the initial stress of i&j tons 

 being kept constant for 46 hours. In this case the hardening 

 effect extended through an increase of stress of 3*0 tons. 



It is seen from the diagrams that when permanent set 

 again takes place, after the load being kept constant, that 

 the amount of extension depends on the time during which 

 the preceding stress has been kept constant. 



Thus, in the tests of bars A and C, which were of the 

 same material and initial dimensions, the extension of A 

 for a stress of 20 tons was 1*125" in a length of 10". The 

 extension of C for the same stress, after having been subject 

 to a stress of 167 tons for 46 hours was "762", or 32% less 

 than that of A. 



In order to establish a relation betweeen the stress and 

 permanent strain by experiment it was necessary that the 

 elongation caused by any given stress should not be affected 



