I. On the recovery of iron from overstrain

Abstract

It has long been known that iron which has been overstrained in tension—that is to say strained beyond the yield-point so that it suffers a permanent stretch—possesses very different elastic properties from the same iron in its primitive condition. The material is said to be “ hardened ” by stretching,* since the ultimate effect of such treatment is to raise the elastic limit and reduce the ductility of the material. More recently, attention has been called to the fact that, primarily, the result of tensile overstrain is to make iron assume a semi-plastic state, so that the elastic limit, instead of being raised by stretching, is first of all lowered, it may be to zero. This plasticity may be shown by applying a comparatively small load to a bar of iron or steel which has just been overstrained by the application and removal of a large stretching load. When the small load is put on, the bar will be found to elongate further than it would had the material been in its primitive state ; and a slight continued elongation—a “ creeping ”—may occur after the small load has been applied. If this load be withdrawn, a quite appreciable permanent, or semi-permanent, set will be found to have been produced ; a set which diminishes slightly, and, if small, may vanish, provided time be allowed for backward creeping to take effect. It may also be shown that, if the re-applied load be increased, the elongation produced will increase in a greater proportion. Thus, if a stress-strain curve be obtained from a recently overstrained bar of iron or steel, it will show, even for small loads, a marked falling away from the straight line which would indicate obedience to Hooke’s law.