THE BEHAVIOUR OF OVER-STRAINED 



MATERIALS 



By A. O. RANKINE, B.Sc. 



Assistant in the Department of Physics, University College, London. 



In 1835 Weber 1 discovered a phenomenon which threw an 

 entirely new light on the question of the elasticity of bodies. 

 He found that when a silk thread was stretched by a weight, 

 the immediate stretch which resulted was not the only effect 

 observable, but that, if the weight were allowed to remain, the 

 length continued slowly to increase for many hours. On the 

 removal of the weight, although an immediate shortening took 

 place, the thread did not at once regain its original length, but 

 continued to contract perceptibly for twenty days longer. This 

 effect, which Weber called " Elastische Nachwirkung," has been 

 the subject of continued investigation from that time to this, and 

 much important information has been gathered ; but, curiously 

 enough, very little reference to it can be found in the standard 

 treatises on the properties of matter. This lack is particularly 

 noticeable in English text-books. They contain complete records 

 of the known properties of solids subjected to stresses less than 

 the elastic limit, and of the viscous properties of fluids, but little 

 is said about the behaviour of solid bodies in a state of over- 

 strain, when they appear to partake both of the properties of 

 a truly elastic solid and of those of a viscous liquid. Perhaps 

 the results of investigations in this field are regarded as too 

 indefinite, or possibly even somewhat inconsistent. Neverthe- 

 less, they are of sufficient interest and importance to justify 

 consideration ; for an effect which, since its discovery in 1835, 

 has been shown to exist in very many other substances besides 

 silk (so that, indeed, it may be regarded as a property common 

 to all solid bodies), must be capable of throwing much light on 

 the internal structure of matter. 



1 W. Weber, Pogg. Ann. 34 (1835) ; 54 (1841). 



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