44 



Prof. 0. Keynolds and Mr. J. H. Smith. On a [Mar. 5, 



" On a Throw-testing Machine for Keversals of Mean Stress." 

 By Osborne Keynolds, F.K.S., and J. H. Smith, M.Sc., 

 Whitworth Scholar, Victoria University. Received March 5, 

 —Bead March 20, 1902. 



(Abstract.) 



The present research, which was carried on in the Whitworth Engi- 

 neering Laboratory of the Owens College, Manchester, was undertaken 

 at the suggestion of Professor Osborne Reynolds, who proposed an 

 investigation of " repeated stress " on the following lines : — The stress 

 should be direct tension, and compression of approximately equal 

 amounts, such tension and compression being obtained by means of 

 the inertia force of an oscillatory weight. The rapidity of repetitions 

 should be much higher than in the experiments of Wohler, Spangen- 

 berg, Bauschinger,' and Baker — in fact, ranging as high as 2000 reversals 

 per minute. 



In the apparatus employed a weight is supported vertically by 

 means of the specimen to be tested, and the upper part of the 

 specimen receives a periodic motion in a vertical direction by means of 

 a crank and a connecting rod. The inertia of this weight gives a 

 tension at the bottom end, and a compression at the top end of the 

 stroke, the change from tension to compression being gradual. The 

 specimen and parts are guided by suitable bearings placed in a vertical 

 direction. The motion was made vertical in order to reduce the 

 friction of the bearings to a minimum. The stresses can be changed 

 by varying the diameter of the specimen, the load, and the speed of 

 revolution of the crank. In order to enable one to calculate the 

 stresses in the specimen, the centre of the crank shaft must be at rest, 

 and the crank must move with uniform angular velocity. These con- 

 ditions are obtained when the crank shaft is driven by a constant 

 turning effort, if the moving parts of the machine are balanced, and if 

 at the same time the total energy of the moving parts is invariable. 

 The apparatus was therefore designed to satisfy these conditions as 

 approximately as possible. 



The apparatus was driven by the low-pressure engine of the triple 

 expansion experimental engines, and had a speed indicator and a 

 revolution counter attached. A great amount of trouble was experi- 

 enced in lubricating the machine and in keeping the fluctuations of 

 velocity small. 



The specimens employed were carefully prepared, and with a few 

 exceptions were of constant length and diameter. They were, in most 

 cases, annealed before testing. 



In conducting the tests the reversals for rupture were estimated 



