81 



On Changes in the Proporteoxal Elastic Limit op Nickel 

 Steel, With a Note on Calibration of Testing Machines. 



W. Kendrick Hatt.* 



The variability of the proportional elastic limit of metal due to over- 

 strain and its subsequent recovery after a period of rest, or proper anneal- 

 ing, have been studied by investigators, among whom may be named 

 Bauschinger, Professor Gray and ^Ir. Muir. 



The writer records here the results of experiments on a special nickel 

 steel rolled for the purpose by the Bethlehem Steel Company. The ex- 

 periments had two ends in view: 



1. To calilirate the testing machines of Piu-due University, in com- 

 parison with the testing machines of the government testing laboratory of 

 the Watertown Arsenal, and those of the University of Illinois. 



2. To study the variability of the proportional elastii- limit and yield 

 point of this special nickel steel. 



The proportional limit here mentioned is that limit l)eyuiKl which 

 stress ceases to be proportional to strain. The yield point spoken of below 

 is that limit at which a sudden increase in the elongation occurs with- 

 out an increase in stress. 



CALIBRATION. 



A testing machine of ordinary screw type consists of a screw press 

 and a large platform scale. Tt is necessary, of course, to ascertain if the 

 load on the scale beam correctly indicates the pressure on the platform. 

 This is often accomplished by loading the platform with a dead load of 

 pig iron. For light loads the purpose might be served by a calibrated 

 spring. The use of nickel steel bars of high elastic strength furnishes us 

 with a spring of high capacity, whose deformation may be accurately 

 measured. Calibration by means of these bars may be readily effected 

 and relative errors in the machines detected. The absolute error may 

 be known by comparison with a machine that has been calibrated by the 

 dead weight method. The bars can be preserved and used from time to 

 time to detect changes in the machine due to wear of knife edges. This is 



"The main observations on which this note is based were carried out under the 

 author's supervision by Messrs. R. Hitt ;ind J. H. Jasoha, senior students in Purdue Uni- 

 versity, 190L 



