Taking the marble as the specimens

conventional triaxial compression tests under 20 MPa confining pressure and different axial compressions are first conducted. Then the nuclear magnetic resonance (NMR) tests are conducted on the loaded specimens. The stress-strain curves

crosswise relaxation time T2 distribution and rock porosity are obtained. The function about damage degree and porosity is established. The results show that: (1) The fitted function about porosity and axial compression ratio indicates that porosity of marble increases exponentially with the increase of axial compression. When the axial compression ratio is below 70%

specimens perform elastic deformation and the increase of porosity is small. When the axial compression ratio is above 70% and below 90%

the specimens perform plastic deformation and the porosity of specimens increases significantly. When the axial compression ratio is above 90%

the dilatancy of specimens increases sharply and the porosity of specimens multiplies. (2) The number and size of microcracks in specimens increase with the increase of axial compression. (3) The function about damage degree and axial compression ratio indicates that as the axial compression ratio increases

the damage degree of marble grows

and the effective stress of external loads also grows.