GEOPHYSICAL RESEARCH — BECKER. 22 1 



GEOPHYSICAL RESEARCH. 



ELASTICITY AND PLASTICITY OF SOLIDS. 



George F. Becker, U. S. Geological Survey, Washington, D. C. Grant 

 No. 172. (For first report see Year Book No. 3, p. 80.) $7,500. 



Investigations for the purpose of determining experimentally a 

 relation between stress and strain were begun early in November 

 in the Washington Monument. A 3- inch tube 480 feet in length 

 had been attached to the steel framework which forms the elevator 

 shaft. The tube was closed at the top by a plate, to which the 

 specimens to be tested were attached. At the bottom the tube 

 projected through the ceiling of a small room that had been built as 

 a temporary laboratory. In this little observing station were placed 

 various pieces of physical apparatus, such as storage batteries for 

 light and a meter-bar and cathetometer to be used in making the 

 measurements. 



The general plan of conducting the experiments was to suspend 

 two wires 1 mm. in diameter in the tube at a distance of about 3 mm. 

 apart. L,oads were applied to one of the wires, the other one serving 

 to indicate the linear expansion of the strained wire due to changes 

 in the temperature. A mark was placed near the lower end of each 

 wire, and the distance between these marks, which is evidently the 

 total increment corresponding to any particular load, was measured 

 with the cathetometer. The plan was first entertained of using copper 

 wires and annealing them electrically. This plan was finally aban- 

 doned on account of the difficulty of protecting the copper against 

 rapid oxidation, and also on account of a considerable lack of uni- 

 formity in the cross- section of the specimens. It was therefore decided 

 to subject them to overstraining, a process which hardens the mate- 

 rial and raises the elastic limit. This treatment proved to be unfor- 

 tunate, as the wires did not completely regain their equilibrium after a 

 period of two months had elapsed. The relative motion of the two 

 wires became negligible, however, for short intervals of time, and 

 systematic observations were begun in April, the loads employed 

 ranging from 25 grams to 600 grams. The total increment in length 

 produced by a load of 600 grams after an interval of time of ten 

 minutes was found to be 0.9 cm., a strain of 1 in 15,000. 



The deviation from Hooke's law, even with these small strains, is 

 clearly perceptible — a result never before established experimentally, 

 so far as I know — but the exact determination of the stress-strain 

 relation requires the use of larger strains. Successive loads were then 



