Ml; .1. Ml IK ON Till; RECOVERY OF IRON FROM OVERSTRAIN .9 



In experiments on !i virgin piece of which the first table given ulx>\r is typ'u-al.the 

 time eli-int-nt dors not enter, for there is no perceptible creeping until the yield point 

 is all but reached. 



Slow Recovery of Elasticity with lapse of Time. 



l'..-f"iv proeri'din^ t-> di-si'i-ilic tin- i-tl'.-.-t ,,(' -.p.-.-ial tivatm.-nts on recovery from 

 tensile overstrain, I give two instances of the slow recovery from overstrain \\ith 

 lapse of time, similar to the examples already given by Professor EWINO.* 



The curves in Diagram No. Ill A. illustrate this slow recovery for a specimen of 

 1 inch round steel rod, which has been strained to or very little beyond its yield- 

 point. The material had an ultimate strength of 37 tons per square inch of original 

 area, the total elongation being almost 23 per cent, on an 8-inch length. The yield- 

 point was well defined and occurred at a stress slightly under 27 tons to the square 

 inch. The readings from which the various curves have been plotted are given in 

 the table on p. 11. The curves were obtained in the usual manner, the stresses 

 being plotted as ordinates, and the corresponding extensions as abscissae. 



Curve No. 1 of Diagram IIlA. is a record of the primary test of the specimen. It 

 shows that HOOKE'S law has been obeyed up to a load of 26 tons to the square inch ; 

 and that before 27 tons there was a well-marked yield-point. This load of 27 tons 

 per square inch was kept on for two minutes, by which time rapid stretching had 

 ceased, as was shown by the beam of the testing machine remaining stationary. 

 There was still a slow creeping, which probably would have continued for hours or 

 days, becoming however slower and slower. Curve No. 2 represents a test performed 

 as shortly after the removal of this load as the remeasurement of the diameter and 

 the calculation of the reduced area would permit : it illustrates the semi-plastic 

 condition of the material immediately after overstrain. In the plotting of this, and 

 all subsequent curves in the diagram, it will be noticed that the origin for the 

 measurement of extensions has been displaced ; this was merely to keep the curves 

 distinct and to facilitate comparison. 



Curves Nos. 3, 4, 5, and 6, obtained at succeeding intervals, illustrate the gradual 

 recovery of the elasticity lost by the overstrain. This recovery will be noticed to be 

 quickest at first, and latterly to be very slow. In Curves 3, 4, and 5 the load was 

 not allowed to exceed 27 tons per square inch. Curve No. 6 shows the recovery to 

 have been nearly, though not quite, perfect after the material had been allowed to 

 rest for 6 days 3 hours. In this test the load was gradually increased beyond the 

 27 tons, and a new yield-point was not obtained till rather over 30 tons to the square 

 inch was reached. 



Curve No. 7 shows the plastic nature of the material immediately after this second 



overstrain, and No. 8 the condition after 4 days' rest. Thus after 4 days the recovery 





 * See page 139, &c., of his paper " On Measurements of Small Strains, &c." 



VOL. CXCIII. A. C 



