Uniform Stress and Permanent Strain. 7 



applied was kept constant for 66 hours, the hardening has a 

 marked effect on the next permanent sets, which were each 

 20*7 less than that due to the loads. 



/o 



The above tests were all made on the same brand of 

 B.B.B. iron, the only variation being in the value of the 

 constant C. which, for bar No. 2, was 40*30, and for bar No. 

 7 was 40*3 5. 



Experiments were also made on some bars of mild 

 steel, the results for which are shown in Fig. 2. In these, 

 with a time duration of stress not exceeding 30 minutes, the 

 value of k was found to be \. The results are also plotted 

 for the case of a mild steel bar, in which the initial stress was 

 kept constant for 18 hours. 



Similar experiments made on nine specimens, cut from 

 a long bar of Crown BB iron, did not give such consistent 

 results. This was probably due to the varying hardness of 

 the material in different portions of the bar. 



In the case of bars Ri and R9 taken from these, and 

 tested with successive loads at intervals of 20 and 15 

 minutes respectively, the value of k was found to be '275 

 for Ri, and "265 for R9. The combined results for the 

 other seven bars, each tested with one load only, gave '265 

 as the value of k. 



Several other experiments were made on iron of different 

 qualities, including some on Low Moor iron. The results 

 showed that in the case of iron of a soft and fibrous nature, 

 the same law of permanent set held, but when the iron was 

 harder and showed a crystalline fracture the relation could 

 not be expressed in the above form. 



If p = stress on initial section of bar, then, assuming 

 the density to remain constant and taking the formula 



p = Ce l (1) 



we have 



Ce l . . 



A=— (2) 



