IRON AND STEEL UNDER CYCLICAL VARIATIONS OF STRESS. 



49 



application of the load, gradually appeared due to repeated applications of a range of 

 stress slightly greater than the safe range. This extension continued for some time, 

 the dotted line GH representing the slow yielding of the specimen. When the 

 adjustment of the elastic limits was complete, H represented the final extension, and 

 no further increase of length occurred due to further repetition of stress. 



STRESS 



TONS 3d IN 



AXLE. STEEL 



O.z O.4 



EXTENSION Mi 



Fig. 6. 



The point J was similarly obtained by repeating a cycle of stress having a less 

 maximum value than that which produced the extension H. At E, which corresponds 

 to the maximum safe stress during reversals, no extension occurred. 



The points H, J, and E are evidently on a continuation of the curve BC, and when 

 cyclical variations of stress are considered there is no break, in the curve at B, 

 corresponding to the static yield-point. 



Above this latter point the whole extension is produced by the maximum stress 

 only, independently of the range of stress, which may be zero. As HJE is continuous 

 with BC, it seems possible that an extension such as GH may be produced by the 

 repetition of a cycle of stress in which the range is less than the safe range. 



Should further experiments bear out this contention, it seems that very great care 

 must be exercised in the use of materials having a higher maximum stress than that 

 corresponding to alternations of equal and opposite stresses. 



Below the static yield-point, iron and steel appear to be capable of maintaining an 

 unstable condition for a considerable time against cyclical variations of stress which 

 ultimately produce a considerable change of length. The first application of the 

 maximum stress in a given cycle may show only a scarcely measurable extension, 

 in spite of the fact that extension of thousands of times the amount may be obtained 

 without any change in the cycle of stresses. It would appear from this that 

 BAUSCHINGER'S definitions of the "primitive" and "natural" elastic limits are in 



VOL. OCX. A, H 



