578 



BELL SYSTEM TECHNICAL JOURNAL 



sufficiently dissimilar to the metal to accomplish good clamping 

 without scoring the surface of the clamped portion. Furthermore, 

 the use of the phenol fiber blocks provides a means of automatically 

 recording the breaking of the specimen since the specimen is insulated 

 from the machine and may be employed to break an electrical 

 monitoring circuit. The deflected end of this specimen is held between 

 two fingers (i^i.2) which have a vertical cylindrical half section. The 

 cylindrical portion is in contact with the specimen. This is necessary 

 in order to compensate for the angular movement of the reciprocating 

 arm {A) in relation to the specimen. One of these fingers (Fi) is 

 fixed and the other is movable {Fo) but bears against the specimen with 



Fig. 2 — Sheet Metal Fatigue Machine. 



a tension provided by compression spring (5p). This permits some 

 slight movement of the specimen in relation to the fingers which is 

 necessary by reason of the change in free length of the specimen as 

 the reciprocating arm moves backward and forward. The deflection 

 of the specimen is determined in two ways, first by measuring the 

 movement of the reciprocating bar and also by observing the specimen 

 in operation by means of a stroboscope. In this way the static and 

 dynamic deflection of the specimen may be measured and for all 

 practical purposes, these have been found to be the same within the 

 range of deflection and speed used in this investigation. 



The speed of the machine is approximately 1,500 r. p. m. It is 

 necessary to adjust the speed of the machine to the material under 

 test since if this is not done the machine may be operated near the 



