中文摘要：

为研究岩石在卸荷作用下的细观结构损伤破坏规律,对大理岩岩样分别进行不同初始围压、不同卸荷围压比的常规三轴加–卸载试验,并对卸荷后的岩样进行核磁共振（NMR）测量,得到不同卸荷围压比作用后大理岩的弛豫时间T2谱分布、孔隙度和核磁共振成像图像。结果表明：（1）随着卸荷围压比的增大,T2谱曲线不断右移,T2谱面积不断增大,孔隙度不断增大,反映出岩样内部损伤不断加剧;（2）小孔隙的谱峰不断右移,但谱峰面积较小,反映出岩样内部小孔隙的孔径不断增大,但数量变化具有随机性,对岩样孔隙度变化影响微弱;卸荷围压比75%附近存在一临界值,小于临界值时,岩样内部不断产生新的细小裂纹,大于临界值时,产生新裂纹的趋势则受到遏制;（3）大孔隙的谱峰不断右移,谱峰面积值较大且不断增大,占总谱面积的96%以上,反映出岩样内大孔隙孔径不断增大,数量不断增多,对岩样的孔隙度和损伤程度起主导作用;（4）核磁共振图像动态地显示了不同卸荷围压比作用后岩样内部孔隙和裂隙的扩展、演化过程。

英文摘要：

To study the damage characteristics of rock microstructure under unloading condition,the loading and unloading confining pressure tests on the marble specimens under different initial confining pressures and different unloading confining pressure ratios are conducted. The crosswise relaxation time T2 distribution, rock porosity and nuclear magnetic resonance imaging（MRI） are obtained by the nuclear magnetic resonance（NMR）technique. The results show that：（1） As the unloading confining pressure ratio increases,the T2 spectrum curves move towards right with the spectrum areas keeping increasing, and the porosity of the specimens increases,which indicates that the damage of specimens increases.（2） As the unloading confining pressure ratio increases,the spectral peaks of small microcracks move towards right, but the spectrum areas of small microcracks are relatively small,which indicates that the size of small microcracks increases and the number of small microcracks changes randomly. The influence of small microcracks is weak to the porosity of the specimens. There is a criticalvalue near the unloading confining pressure ratio of 75%. When the unloading confining pressure ratio is below the critical value,new microcracks appear constantly in the specimens. When the unloading confining pressure ratio is above the critical value, the generation of new microcracks is limited.（3） As the unloading confining pressure ratio increases, the spectral peaks of big microcracks move towards right. The spectrum areas of big microcracks keep increasing and account for over 96% of all microcracks in the rock specimens,which indicates that the size and number of big microcracks increase and the change of big microcracks determines the porosity and damage of rock.（4） The development process of microcracks and fracture in the rock specimens is shown dynamically.