Cycle loading tests of rock samples under direct tension and compression and bi-modular constitutive model

Cycle loading tests under direct tension and compression were conducted for five rock types with a new testing frame that could carry out both of these tests on a same rock sample.The results showed that for three rock types of which the average Young’s moduli in tension,ETA,were much smaller than those in compression,ECA,(ECA/ETA ratio equals 2,2.5 and 6),the average deformation modulus of unloading in tension,ETAUN,was much greater than ETA,and the values of ETAUN were approximately equal to ECA.Therefore,for these rocks,the relationship between stress and strain was no longer elastic even though their deformation in compression was approximately elastic.Some researchers had developed a constitutive model named “double elasticity” and applied it in the rocks with ET ≠EC.The present test results showed that a rock with ET ≠ EC did not mean that the double elasticity model was applicable to this rock,because its deformation in tension might be plastic.For the rocks having plastic behaviour in tension as those presented in this paper,a more complicated constitutive model had to be developed.