304 W. T. LEE BUILDING OF SOUTHERN ROCKY MOUNTAIN? 



time. Some indication of the behavior of rock material under continnons 

 load is given by an experiment conducted a few years ago by Mr. H. D. 

 Ayres in collaboration with the late Dr. George F. Becker and myself. 

 A steel tape — cross-section, 0.0162 X 0.6365 centimeter; length, 2T.906 

 centimeters — was subjected to a load of 10 kilograms (less than one- 

 twentieth of the breaking load) for a period of 126 days and then re- 

 leased. On account of certain defects beyond our control, we were imable 

 to decide whether or not the tape returned to its original length, but the 

 following results on the increment in length of the tape with time, due 

 to the load of 10 kilograms, show that substances like steel do not neces- 

 sarily reach equilibrium Avhen subjected to continuous stresses well within 

 the so-called elastic limit. The results are contained in the following- 

 table and platted in figure 6 : 



Load. 10 kilograms. 



. ■- ■ ^ , 



Time Total increment, 



i""terval. wave lengths. 



5 minutes; 4.39 . 82 



1 hour 440.12 



1 day 440.89 



10 ilays 441.99 



19 days 442.42 



HO days 442.69 



.39 day.s 442.88 



49 days 443.04 



60 days 443.23 



80 days 443 . 51 



101 days 443.68 



114 days 443.82 



126 days 443.93 



No load, following load of 10 kilograms. 



/■ " s 



Time Total increment, 



interval. wave lengths. 



5 minutes 4 . 31 



1 liour 3.89 



1 day 3.00 



10 days 2.63 



19 days 1..32 



30 days • 1 . 08 



40 days 1.00 



50 days 0.87 



60 days 0.85 



79 days 0.61 



100 days 0.46 



108 days 0.44 



112 days 0.50 



Curve A of figure 6, which represents the data in the first two columns 

 of the table, shows that the tape would have continued to stretch for a 

 much longer interval of time. On the other hand, the data in the last 

 two columns of the table, represented by B in the diagram, shows that 

 the tape had almost returned to its original length at the end of 112 days. 

 The temperature of the tape during these tests was 27.1 degree centigrade 

 (80.8 degrees Fahrenheit). 



It was found by means of a least square solution, using 12 observations- 

 between the 101st and the 126th days, inclusive, that tlie tape lengthened 

 during that time at the rate of 0.00944 wave lengths per day. Consid- 

 ered as an extension, we have, 



0.00944X5876X10-^ , ^ ,, , 



e' = r:,,,,^. = 198 7 72 X 10"^^ per day. 



