81 



On Changes in the Proportloxal Elastic Limit of N^ickel 

 Steel, With a Note on Calibration of Testing Machines. 



W. Kendrick Hatt.* 



The variabiHty of the proportioual ehistic limit of metal due to over- 

 strain ami its subsequent recovery after a period of I'est, or proper anneal- 

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

 Bauschinger, Professor Gray and Mr. Muir. 



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

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

 periments had two ends in view: 



1. To calibrate the testing machines of Purdue University, in com- 

 parison with the testing machines of the government testing laboratory of 

 the Watertown Arsenal, and those of the I'niversity of Illinois. 



2. To study the variability of the proportional elastic limit and yield 

 point of this special nickel steel. 



The proportional limit here mentioned is that limit beyond which 

 stress ceases to be proi>ortional to strain. The yield point spoken of below 

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

 nut an increase in stress. 



CALIBPATIOX. 



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

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

 load on the scale lieam 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 l)e accurately 

 measured. Calil>ration by means of these l)ars 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 caliln-ated by the 

 dead Aveight method. The bars can be preserved and used from twne 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 and J. H. Jascha, senior students in Purdue Uni- 

 versity, 1901. 



