384 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Metallography, etc. 



Measurement of Stress by Thermal Methods.* — It has long been 

 known that a thermal effect is produced when bodies are subjected to 

 stress, and formulas expressing the relation between change of tem- 

 perature and stress have been worked out. E. G. Coker has experi- 

 mentally investigated this relationship for tension and compression 

 stresses on metals. As the coefficient of expansion enters into the 

 equation, it was considered necessary to determine if variation of stress 

 had any effect upon the coefficient. Steel and brass tubes were subjected 

 to definite tension stress, and their expansion on heating measured by 

 means of a Ewing extensometer, alteration of temperature being effected 

 by circulation of water through the tube. It was found that there was 

 no difference in the linear expansion of brass and steel within the range 

 of stress up to the yield point. In the determination of the relation 

 between thermal change, stress, and strain, bars were loaded in a test- 

 ing machine, elongation being measured by an extensometer, and tem- 

 perature variations by a thermopile connected with a galvanometer. 

 The cooling effect caused by tensile stress could thus be measured 

 with extreme delicacy. Similar determinations were made of the heat- 

 ing effect, stress, and strain in test pieces subjected to compression. 

 The author concludes that the thermal change is very nearly propor- 

 tional to the stress, in the same way as the strain. The directions in 

 which the methods employed are capable of practical application are 

 indicated. 



Overstraining of Iron by Tension and Compression.! — When iron 

 is overstrained in tension, its elastic limit in tension is raised, if re- 

 covery from the temporary effects of overstrain be allowed to take place. 

 The elastic limit in compression, however, is lowered, according to some 

 authorities, while others appear to take the view that it also is raised. 

 J. Muir gives an account of some tests bearing on this question. A 

 test piece was stressed in compression beyond its yield point. It was 

 then placed in boiling water for ten minutes. A second compression 

 test gave a yield point higher than the maximum stress applied in the 

 first loading. Test pieces were then cut from bars which had been 

 overstrained in tension ; they were warmed to effect recovery from 

 overstrain, and subjected to compression stress. It appeared that the 

 compression yield point had been raised by the overstrain in tension, 

 but not to the same extent as the tension yield point. The author 

 suggests that overstraining hardens the material, both as regards resist- 

 ance to tension and compression, but that the process of recovery from 

 tensile overstrain raises the tension yield point above the overstraining 

 stress, and lowers the compression yield point below the overstraining 

 stress, by approximately an equal amount. The author points out that 

 his experiments were not satisfactory in some respects, and that further 

 research is desirable. 



* Trans. Roy. Soc. Edinburgh, xli. (1905) pp. 229-50 (11 figs.). 

 t Proc. Roy. Soc, Series A, lxxvii. (1906) pp. 277-89 (5 figs.). 



