58 



Prof, H. Nagaoka on the Elastic Constants of 



Hoohe's Law and Elastic A/ter-Effect. — Preliminary expe* 

 riments with granite showed that Hooke's law does not hold 

 even for very small flexure and torsion, and that the after- 

 effect is very considerable when the prism is sufficiently 

 loaded or twisted ; the deviation . from the direct propor- 

 tionality between the strain and stress was incomparably 

 great compared with that observed in common metals. This 

 must be chiefly due to the low limit of elasticity, so that it 

 is necessary to experiment only within narrow limits of loading 

 or twisting. These limits are widely different for different 

 specimens of rocks, and the modulus of elasticity as well as 

 that of rigidity was always determined with such stresses as 

 will approximately produce strains proportional to them. 



The deviation from Hooke's law was prominent in certain 

 specimens of sandstones, and it was more marked in torsion 

 than in flexure experiments. In certain rocks it is indeed 

 doubtful if anything like a proportionality between stress and 

 strain can be found even for extremely small change of shape. 

 On releasing these rocks from stress the return towards the 

 former state is extremely small, showing that the elasticity of 

 rocks is of very inferior order. The elastic yielding of rocks 

 under continuous action of stress is very remarkable, as the 

 following readings of the deflexion in the experiment on 

 torsion will show. 



Specimen : Izumi Sandstein. 

 a= 100-0 mm., b = K)-V2 mm., c— 10*09 mm. 

 Torsional loading, 400 grms. 

 Zero reading before loading, 24 - 2. 

 Loaded 2 h 18 m -0 Sept, 10, 1898. 



Time. 





Time. 





h. m. 



Reading. 



h. hi. 



Reading 



2 18-1 



72-0 



2 24-0 



82-4 



18-5 



75-3 



25-0 



83-6 



19-0 



77-1 



27-0 



84-1 



19-5 



78-1 



28-0 



84-5 



20-0 



78-9 



29-0 



84-9 



20-5 



79-6 



30-0 



85-2 



21-0 



80-1 



31-0 



85-5 



21-5 



80-6 



32-0 



85-9 



22-0 



81-0 



33-0 



86-2 



22-5 



81-4 



34-0 



86-5 



23-0 



81-8 



35-0 



86-8 



23-5 



82-1 







