IRON AND STEEL UNDER CYCLICAL VARIATIONS OF STRESS. 37 



The Present Experiments. Owing to the discrepancies just mentioned, further 

 experiments appeared to be necessary in order to determine the laws of variation of 

 the elastic limits ot materials when subjected to cyclical variations of stress. An 

 indication of the general results of the experiments will most clearly illustrate the 

 course of the work. 



It is found that, after a sufficient number of repetitions, iron or steel is capable of 

 adjusting itself to variations of stress, cyclically applied. When this adjustment is 

 complete, the specimen is found to have become perfectly elastic throughout the 

 whole cycle, and fatigue does not occur. 



This adjustment to a given cycle is possible because the limits of elasticity are not 

 fixed, but can be raised or lowered by repetitions of stress. 



During the adjustment of the elastic limits to a given cycle of stress a change ot 

 length occurs in the specimen which is the same as the extension observed in an 

 ordinary tensile test when the yield stress is exceeded. For cyclically applied stress 

 a similar extension occurs even when the maximum stress in the cycle is less than the 

 static yield stress. 



The greater the extension of the specimen during adjustment, the greater are the 

 amounts by which the elastic limits are raised. 



This power of adjustment is limited, and if the range of stress in the imposed cycle 

 is sufficiently great, the specimen becomes or remains inelastic, and work is performed 

 during each cycle. This work is expended in moving portions of the crystals 

 relatively to one another, and is probably associated with microscopic slip-lines which 

 gradually develop into cracks, ultimately causing the fracture of the specimen. 



Definition of Elasticity for Cyclical Variation of Stress. The definition of elasticity 

 in a specimen subjected to repetitions of stresses is most conveniently expressed in 

 reference to the cycle. If the elongation is proportional to the stress over the 

 whole cycle, the specimen is elastic and the stress elongation diagram becomes a 

 straight line. 



When the specimen is inelastic, the straight line is replaced by a figure which 

 closely resembles the familiar magnetic hysteresis loop. Such a loop EFGH is given 

 in fig. 1 , c, and will be in future referred to as a hysteresis loop. 



The Testing Machine and Extensomeler. -The testing machine was specially 

 designed for the work, and was made in the Engineering Workshop of the National 

 Physical Laboratory. For tensile loads the machine is of the usual type of single- 

 lever testing machine for static tests. To obtain compressive stresses, a second 

 system of knife-edges is used and the beam prolonged past the tension zero, so 

 that when the jockey is moved back farther than the zero the specimen is put into 

 compression. 



Cyclical variations were produced by attaching two scale pans- to the beam and 

 automatically applying and removing the scale-pan weights. One of these weights 

 regulated the maximum compressive stress, and the other the maximum tensile stress. 



