320 



REPEATED STHKSSKS. 



The author has found that a precisely similar relation 

 exists in the case of permanent compressive strains, as 

 for tension, by testing copper compression cylinders 

 and plotting the logarithms of the strain and stress. 

 'The mean results obtained were : 



Similarly, testing a short piece of Lowmoor iron in 

 -compression, the author finds the following values to be 

 given : 



Which agree fairly well with the figures obtained from 

 tension tests. 



EEPEATED STKESSES. 



1 68. A very large proportion of engineering 

 structures and parts of machines are exposed to stresses 

 which may be regarded as unvarying in both their 

 magnitude and their direction ; that is to say, they 

 are subjected to steady loads, and, so long as these loads 

 .are so chosen as not to strain the material beyond the 

 elastic limit, they will have no deteriorating effects, no 

 matter how long they are allowed to remain on the 

 material. In other words, time, so far as we are able to 

 tell, has no material effect on the condition of a substance 

 subjected to steady loads within the elastic limit. 



