Fractal Characteristics And Law Of Strain Energy Conversion For Marble Fracturing Under High Stress And Unloading

With the development of the western, some hot and difficult problems, includingstability evaluation and disaster prediction and prevention due to large-scale excavationof rock mass under high geostress conditions, in areas of rock mechanics andengineering geology are being more and more concerned. The mechanical propertiesand the evolution mechanism during injury and rupture of rock itself are directlyaffected by the environment and load ways. Generally speaking, in high confiningpressure and conventional triaxial compression conditions, rock mass will notimmediately lose its’ original shape and be destroyed but experience a rather longperiod of time’ plastic flow deformation around the peak intensity. While the case inhigh stress and unloading is quite different, as the binding reduced in particulardirection or multiple directions, rock mass will quickly lose strength before the peakstrength point and lose instability easily after the peak strength point,even causemomentum effect,such as rock burst. Energy is the fundamental factor to drive materialphysical changes. Aiming to the basic problem of energy transformation and rupture ofrock in high stress and strong unloading conditions, this paper relied on the YouthScience Fund (40902078) and the National Natural Science Foundation (41172243) ofNational Natural Science Foundation of my tutor, based on unloading confiningpressure test in triaxial and high stress conditions of the marble from underground plantin Jinping, and combined with the energy principle and fractal theory, has carried outthe study of energy conversion law and process mechanisms of injury and rupture andfractal and has obtained the following major findings:①Based on the classical laws of thermodynamics, the thesis summarized theconversion law for the whole process under high stress and unloading confiningpressure conditions, analyzed the strain characteristics of marble in peak intensity pointsbetween conventional triaxial compression tests and unloading confining pressure tests,and explored the strain energy state in strength point and the rate of Ud, U3and Uebefore peak strength points and after strength points and compared the aboveconclusions with the situation under conventional triaxial compression conditions.Accumulation and dissipation of strain energy in the process of unloading tests are moreintense than in conventional triaxial compression tests. The absorb strain energy U1andthe elastic strain storage energy Ueincrease rapidly, the accumulation of dissipation strain energy Udbecause of rupture increases obviously, and more obviously, the strainenergy U3, which is result of negative power done by σ3, increases rapidly. The periodof stress dropping lasts very short after the peak strength points, and the faster the rateof unloading, the shorter of the period. Compared with the rock specimen afterconventional triaxial compression tests, we found that the share of release of elasticstain energy Ueand the dissipation of strain energy U3because of σ3becomes greater, inaddition, the faster the rate of unloading, the more greater of the share.②Based on conventional triaxial tests in high stress, unloading confining pressuretests in high stress, and the high-precision laser scanning of main fracture surface underthree-dimension, then combined with the fractal geometry principles, we calculated outthe fractal dimensions stands for the roughness of main rupture surface and distributioncharacteristics of fragments. The fractal characteristic which reflected the distribution offragments in high stress and unloading conditions is quite obvious in a local scale. Inother words, the feature size which reflected the fractal has threshold. If the initialconfining pressure is the same, the fractal dimension D monotonically reduces with theincrease of the unloading rate, and generally speaking the higher the initial confiningpressure, the faster the rate of the reduction. The higher the initial confining pressureand the faster the unloading rate, the main fracture surface is more rough and the greaterthe fractal dimension.If the condition is the opposite, the result is contrary.③It revealed the correlation between the strain energy dissipation and release andfragmentation fractal dimension, along with the fractal dimension of the roughness ofthe marble in different initial confining pressure and unloading rate. Before the peakstrength points: the fragmentation fractal dimension has a negative correlation with Udand Ue, and so does the fractal dimension with U3, but the amplitude of the latter issmaller than the former; The fractal dimension of the main fracture surface has betterlinear positive relationship with ud, ue, and u3, which are correspond to the rate of Ud,Ue, and U3. After the peak strength point: according to fitting function, the relationshipbetween fractal dimension of the main fracture surface and ud, ue, and u3are linear andpositive proportional, but not a single function. There are two different linear functionseither the fractal dimension is smaller or greater than2.05. And the rate of one functionsis10~40times to the other ones’.④Based on the law of strain energy conversion, it revealed the mechanism whichreflected the process of strain energy transformation of injury, rupture and damageunder high stress and unloading confining pressure conditions. The main elastic strain energy is stored before the peak strength points and it release rapidly after the peakstrength points, which is the main cause to the rock failure during high-speed unloading.Energy dissipation is the theme throughout the whole process during high confiningpressure conventional triaxial compression tests. The tensional and brittle fracturecaused by unloading is the best way for energy dissipation and release. So the ruptureand damage under high stress and unload characterizes as shows rifting or splitting,while shear failure is the main characteristic under conventional triaxial compressiontests.