[ 35 



II. The Elastic Limits of Iron and Steel under Cyclical Variations of Stress. 

 By LEONARD BAIRSTOW, A.R.C.Sc., Wh.Sch. 



Communicated by Dr. R. T. GLAZEBKOOK, F.R.S. 

 Received April 20, Read May 13, 1909. 



INTRODUCTION. Since WOHLER'S original experiments* on the fracture of iron and 

 steel by repetition of stress, similar experiments have been made by independent 

 observers, and all agree in showing that neither the maximum tensile strength nor the 

 yield stress bears any simple relation to the range of stress which may be safely 

 repeated. 



The only theory of fatigue, i.e. of failure due to repetition of stress, which has 

 received serious attention was put forward by BAUSCHINGER. t According to this 

 theory, specimens subjected to repetitions of stress begin to be fatigued when the 

 stresses applied in each cycle are so great that the extension of the specimen is not 

 wholly elastic. 



It is the object of the present paper to supply direct experimental information 

 relating to suggestions made by BAUSCHINGER when applying his theory to WOHLER'S 

 results. 



Previous Work. BAUSCHINGERJ repeated WOHLER'S experiments in similar machines 

 and made the additional observation of the stresses at which the specimens became 

 inelastic. To determine the elastic limit in tension, the machine was stopped 

 periodically and the specimen allowed to rest for a few hours, after which extenso- 

 ineter readings were taken with increasing loads until a deviation from HOOKE'S Law 

 was found. The repetitions of stress were then continued, usually until fracture 

 occurred. 



As the present work is more comprehensive in its character than was BAUSCHINGER'S, 

 it was found necessary to adopt a generalized expression for his theories. Cases 

 occurred in which the usual terms "tension" and "compression" were unsuitable, 



* W. C. UNWIN, "The Testing of Materials of Construction." 



t J. BAUSCHINGER. ' Mittheilungen aus dem Mechcinisch-technischen Laboratorium der k. Technischeu 

 Hochschule in Hiinchen,' Heft XIII. 



t J. BAUSCHINGER, ' Mittheilungen aus dem Mechanisch-technischen Laboratorium der k. Technischen 

 Hochschule in Miinchen.' Gegriindet von J. Bauschinger. Herausgegfcben von August Foppl. Heft XXV. 



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