Mi:. -T. MUIB ON THE RECOVERY OF li;o\ n;. >\| ( .Yi.KM I:.\1N 



37 



( >n inn-easing the load, a well-defined yield-point was now got at a load of 26j 

 per square inch. Comparison of Curves Nos. 9, 10, and 1 1 shows the remarkable hasten- 

 ing in the recovery from this overstrain, produced by a temperature of 50 ('. ; \\liil.- 

 Curve No. I 1 -' shows the material once more in the perfectly elastic condition. On 

 now carefully increasing the load a fracture was obtained close to the upper machine 

 grips, at a stress of 29 tons per square inch. The specimen was gripped and loaded 

 again, with the result that fracture occurred close to the lower grips at 29 j tons per 

 squan- inch. This was repeated a third and a fourth time, so that the occurrence of the 

 fracture close to the grips was not due to a primary effect of the gripping, that is, it 

 was not due to the gripping having prevented the material from becoming hardened 

 by overstrain. The breaking load, of over 29 J tons per square inch, was equivalent to 

 a stress of fully 27 tons per square inch of the original area of the specimen. Another 

 specimen of the same rod was found to give a yield-point at 14 tons per square inch, 

 and on steadily increasing the load fracture occurred, near the centre of the specimen, 

 at slightly under 23 tons per square inch of original area. The elongation was alxnit 

 21 percent, on an 8-inch length. Common iron thus. exhibits the same features as 

 steel in respect of recovery from overstrain and the effect of temperature on it; but 

 in the case of common iron recovery is comparatively rapid. 



The Effect of Mechanical Vibration on Recovery from Overstrain. 



Diagram No. XII. illustrates the effect of mechanical vibration on recently over- 

 strained iron, and shows that such treatment has an opposite effect to that of increase 

 of temperature instead of the recovery process being hastened, the material is made 

 distinctly less elastic. The following table gives most of the figures from which 

 various curves of this diagram have been plotted. The material employed is t In- 

 flame as that used for Diagram No. IX., but the specimen in this case was not turned 

 down, and so was of the full diameter of 1 inch throughout its length. 



