Study on the Fatigue Failure and Fractal Characteristics of Coal Subjected to Triaxial Cyclic Loading
Author:Duan Hui Qiang
Supervisor:yang yong jie
The similar periodic stresses induced by mining and excavation in underground coal mines have an unfavorable effect on the stability of all types of engineering coal masses.Under these stresses,the deformation and damage of coal masses increase,and finally subjected to fatigue failure.So,it is important to study the coal fatigue failure and mechanics under cyclic loading with different confining pressures because such findings can be used to evaluate the stability of engineering coal masses,and prevent mine from disasters and accidents.(1)Study the fatigue strength and deformation characteristics of No.16 upper coal from Yangcun colliery.When the coal sample is subjected to fatigue failure,there is a minimum discreteness of axial deformation,and the deformation is slightly less than the axial strain corresponding to the post peak control point in conventional compression test.(2)Analyze the failure modes of coal samples in conventional compression and cyclic loading tests.The former is beneficial to the rapid development of the large cracks,and broken into several big blocks;while the latter helps the production of small cracks for the long test time,the coal samples are eventually reduced to a mound of small fragments.(3)Research the dissipated energy evolution of coal sample and influencing factors in cyclic loading test.When the valley stress level of cyclic loading is constant,the dissipated energy increases with the peak stress level.Under the same type of test,the dissipated energy increases with the confining pressure.The dissipated energy is exponentially related to the stress level of cyclic loading.(4)Put forward a new damage formula considering zero cycle damage arming at obvious compaction characteristics of coal sample.Compared with the dalna9e formula defined by other strain methods,the new formula can well describe the zero cycle damage value of coal samp e subjected to cyclic loading.(5)Study the AE’s characteristics of coal sample subjected to cyclic loading using PFC numerical simulation software.At the failure stress level of cyclic loading,the AEs exhibit three-phase evolution law,namely:initial,relatively quiet,and active phases.Besides,the AE ratio was proposed,which can better reflect the different phases of coal fatigue failure.(6)Research the macro and meso-scale fractal characteristics of coal samples in conventional compression and cyclic loading tests.The fractal value of coal surface crack in conventional compression test is less than that in cyclic loading test.The size versus quantity fractal value and size versus quality fractal value decrease with the confining pressure.The closer the meso-scale dimension,the smaller the fractal difference.