On the overstraining of iron by tension and compression

Abstract

The experiments about to be described were undertaken with the view of throwing some light on the uncertainty which seems to exist as to the effect produced by tensile overstrain on the behaviour of iron when afterwards subjected to compression. As long ago as 1848, Professor James Thomson called attention to this question, but although much experimental work has been done since then, the following quotations should serve to justify the proposed line of research. In Thurston’s “Iron and Steel” (1891), it is stated that “it has been shown that the exaltation of the elastic limit in iron is not confined to the direction of the strain produced, but that it affects the metal in such a manner as to give it an exalted elastic limit with respect to subsequent strains however applied. Thus the engineer may . . . . strain his bars in tension to secure stiffness in either tension or compression, or transversely, or he may give his bars a transverse set to obtain a higher elasticity in all other directions.” Ewing, in his “Strength of Materials” (1899), writes: “It may be concluded that when a piece of iron or steel (and probably the remark applies to most other metals) has been overstrained in any way—that is to say, when it has received a permanent set by the application of stress exceeding its limits of elasticity—it is hardened in the sense of being rendered less capable of plastic deformation.” On the other hand, in Johnson’s “Materials of Construction” (1900), the statement may be found that, “Both wrought iron and rolled steel in their normal state have ‘apparent elastic limits’ in tension and compression numerically about equal. If this material be stressed much beyond these limits, however, in either direction, its elastic limit in this direction is numerically raised to about the limit of the greatest stress, while the elastic limit in the opposite direction is greatly lower or even reduced to zero.” Practically the same view is taken in Unwin’s “Testing of Materials of Construction” (1899), with the addition that “the elastic limits of a material are variable limits, restricted only by this, that the range of perfect elasticity seems to be a fixed range.” In an account of Bauschinger’s work, given in the ‘Proceedings of the Institution of Civil Engineers’ (Vol. 87), the statement as to the effect of tensile overstrain in lowering the elastic limit in compression to zero is also to be found, with the curious addition that “time in these cases has little effect.” It is possible that some or all of these statements may be reconciled, but it is thought that the experiments about to be described show, at least, that further research is desirable. The experiments, although mainly performed more than three years ago, are merely of a preliminary character; but as the present writer does not see his way at present to continue the research, it is hoped that the publication of the results so far obtained may lead to the work being taken up by some other experimenter.