1890.] The Rupture of Steel by Longitudinal Stress. 



363 



Kew comes last, as the most continental position, with the greatest 

 variability and the highest amount of range. This latter is due to 

 the greater prevalence of high temperatures there than elsewhere. 



III. " The Rupture of Steel by Longitudinal Stress." By Chas. 

 A. Carus- Wilson. Communicated by Professor G. H. 

 Darwin, F.R.S. Received March 10, 1890. 



(Abstract.) 



This paper gives an account of experiments made with a view to 

 determining the nature of the resistance that has to be overcome in 

 order to produce rapture in a steel bar by longitudinal stress. 



The stress required to produce rupture is in every case computed 

 by dividing the load on the specimen at the moment of breaking by 

 the contracted area at the fracture measured after rupture ; this stress 

 is called the " true tensile strength " of the material. 



It is well known that any want of uniformity in the distribution of 

 the stress over the ruptured section causes the bar to break at a 

 lower stress than it would if the stress was uniformly distributed. 

 Hence anything that causes want of uniformity is prejudicial ; for 

 instance, a groove turned in a cylindrical steel bar will produce want 

 of uniformity, and will consequently be prejudicial, the stress at rup- 

 ture being lower according as the angle of the groove is more acute. 

 The most favourable condition of test might appear to be that in 

 which a bar of uniform section throughout its length was allowed to 

 draw out freely before breaking, since in this case the stress must 

 be most uniformly distributed. 



Experiment, however, shows that the plain bar is not always the 

 strongest. So long as the want of uniformity of stress is consider- 

 able, owing to the groove being cut with a very sharp angle, the 

 plain bar is stronger than the grooved bar ; but, if the groove be semi- 

 circular instead of angular, the grooved bar its considerably stronger 

 than the plain, in spite of the fact that the stress is more uniformly 

 distributed in the latter. 



It would seem, then, that we can strengthen a bar over any given 

 section by adding material above and below it, the change in section 

 being gradual ; but such an addition of material cannot strengthen 

 the bar if rupture is caused by a certain intensity of tensile stress 

 over the ruptured section ; the added material cannot increase the re- 

 sistance of the ruptured section to direct tensile stress, but it can 

 increase the resistance to the shearing stress. 



The resistance of a given section of a steel bar does not, then, 

 depend on its section at right angles to the axis, but on its section at 



