In a paper a “The Crystalline Structure of Metals,” Prof. J. A. Ewing, F. R. S., and one of the present authors described observations which led to the conclusion that the internal structure of metals is truly crystalline and that plastic deformation takes place either by mechanical twinning or by means of a series of minutes slips which occur on the gliding or cleavage planes of the metallic crystals. This conclusion, with slight modifications, has since been supported by one of the present authors in paper on “The Plastic Yielding of Iron and Steel,” and on Deformation and Fracture in Iron and Steel.” In another direction the generalisation quoted above has received further support from the explanation which it has afforded for the failure of metals under alternating stresses and fatigue. In these investigations, however, strain was applied to the metal at the ordinary temperature; the present authors desired to extended the experiments to the study of the modes of deformation of metals at such temperature changes of shape are applied to heated metals, as for instance, in such familiar operations as forgoing and hot-rolling. Further, iron and steel are known to undergo marked changes of constitution when heated above certain temperatures known as their “critical” points, and the study of the behaviour of these metals under strain at high temperatures promised to throw fresh light upon their true constitution. Finally, special interest attaches to the manner in which iron behaves under strain at high temperatures, owing to the peculiar character of the slip-bands which appear on a previously polished specimen of iron or very mild steel when subjected to plastic strain at the ordinary temperature; while the slip-bands seen in such circumstances in copper, lead, and most ductile metals are rectilinear and regular in character, in iron they are usually more or less curved and irregular. It is thus a matter of interest to determine whether, if deformation by slip occurs at high temperatures, the slip-bands then formed would retain the irregular nature of those, formed in the cold, or whether the heated iron would behave in a manner similar to other metals.
ANALYSIS OF ANNUAL VARIATIONS IN GROUND TEMPERATURE AT AN OTTAWA SITE
