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XL VII. Tests of Metals in Reverse Torsion. By E. L. 

 Hancock, Assistant Professor of Applied Mechanics, Purdue 

 University, La Fayette, Ind.* 



- [Plates IX. & X.] 



IT is known generally that, in a cylindrical bar acted upon 

 by a torsional couple, the outer fibres are subjected to the 

 greatest stress, and the stress on the inner fibres varies as 

 the distance from the centre of the bar. If the torsional 

 couple be sufficiently increased the outer fibres are stressed 

 to the elastic limit of the material, and any further increase 

 causes an overstrain of these fibres, the depth of overstrain 

 depending upon the amount that the couple has been increased 

 beyond the elastic limit. If, now, the couple be released, the 

 bar does not return to its original condition, but shows a set, 

 and in its new condition possesses properties quite different 

 from those possessed originally. Its strength to resist torsion 

 is now greater in the direction of the original couple than in 

 the opposite direction. A simple analysis shows, that the bar 

 is twice as strong in one direction as in the other (see paper 

 by James Thomson, Cambridge & Dublin Math. Journ., 

 Nov. 1848, and Lord Kelvin, Elasticity, Pncyc. Britannica). 

 This means that in its new condition the bar has two limits 

 of elasticity — one higher than the elastic limit before over- 

 strain, and one lower. The absence of experimental data on 

 this point led the writer to undertake a series of tests (about 

 150 in all) to determine the truth of the theoretical deter- 

 minations as applied to iron and steel, and to determine 

 whether or not the known phenomena shown by metals 

 when overstrained in tension or compression, held also for 

 torsion. 



Materials. — The materials tested were: low nickel-steel, low 

 carbon-steel, common wrought-iron 5 and staybolt wrought- 

 iron. All specimens were about 20 inches long and tested on 

 a gauge-length of 10 inches. The diameters varied from 

 0'5 inch to 1*0 inch. None of the specimens were turned 

 down or annealed. 



Method of Testing. — The tests were made in the laboratory 

 for testing materials at Purdue University, on a Riehle torsion 

 machine of G0,000 inch-pounds capacity (see PI. IX. fig. 1). 

 The suspended head of this machine allowed the specimen to 

 shorten without introducing external tensile forces, and gave 

 more accurate results than could have been obtained by 

 using a machine where no such adaptation was possible. In 



* Communicated by the Author, having been read before the American 

 Society for Testing Materials, June 1900. 



