1902.] Throw-testing Machine for Reversed s of Mean Stress. 45 



from the mean speed and the interval between the time of attaining 

 full speed and breaking. In many cases the test could not be carried 

 out without interruption, and for this reason the specimens had to 

 rest, sometimes for a few days. The effect of these periods of rest 

 on the total reversals for rupture was investigated, and found to be 

 negligible. 



In carrying out the tests it was found that for a series of tests in 

 which the range of stress was being lowered (the specimens were of 

 constant diameter), the limiting range of stress was more rapidly 

 approached by this apparatus than by that of Wohler. Since the dimi- 

 nution of range was obtained by diminishing the speed, this suggested 

 that the limiting range of stress varied with the speed. 



It is possible to use six different loads for the machine, and therefore 

 to repeat a test with a given range of stress at six different speeds. 

 It was observed that when more than one million reversals were 

 required for rupture the rate of change of reversals with range of 

 stress was very great indeed, and the author for this reason decided to 

 limit in general the tests to one million reversals. 



In the case of mild steel, six sets of tests were carried out corre- 

 sponding to the six different loads which could be applied to the 

 machine; six sets of results, similar to those of Wohler, were ob- 

 tained ; these results were plotted, and the range of stress for rupture 

 with one million reversals was obtained at six different speeds. The 

 results were : — 



Eange of stress for Keversals per 



rupture with 10° reversals. minute. 



20-9 1337 



20-1 1428 



19-2 1516 



18-1 1656 



15-2 1744 



12-4 1917 



The mean result of statical tests for the mild steel employed in these 

 experiments was : — 



Yield stress 17-12 tons 



Maximum stress 24-54 ,, 



Breaking „ 20-47 „ 



Percentage elongation 30 



In the case of cast steel, four sets of tests were carried out. The 

 results obtained were very little different from those of mild steel. 

 Thus the important conclusions arrived at are : — 



1. The reversals for rupture with a given range of stress diminishes 

 as the periodicity of the reversals increases. 



