Study On Rock Mechanical Properties Under Coupled Static-Cyclic Impact Loadings

During blasting excavation in many rock engineerings, the surrounding rock mass is subjected to repeated impact loads coupled with vertical and horizontal static stresses. The static or quasi-static fatigue mechanical behaviors of rock have been studied sufficiently, and perfect theory system has been established. The studies on dynamic strength, deformation characteristics, energy dissipation, failure modes and mechanism of rock under cyclic impact loadings have been generally neglected with the exception of a few rather limited studies, and the existing theory cannot meet the needs of practical engineering. The dynamic mechanical characteristics of rock under coupled static and cyclic impact loadings were studied by using modified split-Hopkinson pressure bar (SHPB) in this paper. The main contents and conclusions are as follows:(1) The theoretical analysis and experimental results show that the incident energy varies with change of axial static load, when the striker impacts against input bar with the same velocity. The bigger the axial pressure is, the smaller the incident energy is. So when the same level cyclic impact tests on rock samples with different axial stress are conducted, it is necessary that the incident stress waves are equal, instead of the same velocity of striker.(2) The single typical stress-strain curve of rock specimen under cyclic impacts can be divided into five stages:compacted stage, elastic stage, loading stage of internal crack propagation, the first unloading stage and the second unloading stage. With the increase of impact number, the amplitude of the transmitted wave decreases and the corresponding peak value occur increasingly earlier; the amplitude of the reflected wave increases, and the corresponding peak value appears later, respectively.(3) With the increasing impact number, the rock average strain rate, peak strain and energy absorption per unit volume gradually increase, whereas its peak stress, deformation modulus of loading stage, the deformation modulus absolute value of the first unloading stage, restorable strain and stress decrease under the coupled cyclic impacts and axial stress. It can be concluded that the rock strength and deformation capacity resisting external impact loading degrade gradually. The value of axial stress has great influence on resistance capacity of rock with the same cyclic impacts, and the capacity of rock resistance is the strongest when22%uniaxial compressive strength is pre-compressed.(4) When rock is under coupled three-dimensional static loading and cyclic impacts, the total cyclic number will decreases with axial stress increased when the confining pressure is a constant; The total cyclic number has an increasing trend with the increasing confining pressure when the axial compression is a constant; the dynamic strength and deformation capacity degrade gradually when the number of impacts is increasing. Rock dynamic strength has a good negative linear correlation with average strain rate.(5) The theoretical analysis and experimental results show that energy absorption efficiency increases with the increasing number of cyclic impacts, when the cyclic impact loadings are a constant. Meanwhile reflected energy increases while the transmitted energy decrease, which shown the relationship with an exponential function. Average strain rate has a good positive correlation with energy absorption per unit volume. The linear-fitted slope K of average strain rate and energy absorption per unit volume indicates a trend of "increase, constant, then decrease" with the increasing axial stress. When the axial stress is smaller, K increases and then become lower with the increasing of confining pressure. The smaller axial stress is, the bigger the confining pressure corresponding to slope's turning point is. After the axial stress reaches to125MPa, K continually reduces with the increase of confining pressure.(6) Rock density and P-wave velocity have a good positive correlation on the basis of theoretical and statistical analysis, the relative variation range of wave impedance is nearly equal to P-wave velocity on the same damage process, and the damage variables defined by velocity and wave impedance shown the same trend. Therefore the damage variable of rock under cyclic impacts can be defined by wave impedance.(7) Inverted S-shaped nonlinear model for cumulative damage evolution of rock was established under coupled static and cyclic impact loadings. The physical meanings and range of parameters were investigated. The advantage of the proposed model is that it can express effects of the axial stress and confining pressure on rock damage cumulative evolution (RDCE), and may describe damage evolution of rock under multilevel cyclic impact loadings. When confining pressure is increased to a certain value, the value of axial stress has weak effect on RDCE, and confining pressure always influences on RDCE regardless of the value of axial stress.(8) The rock sample gradually fractures into several fragments when it is just subjected to cyclic impact loadings, and their fracture planes are parallel to longitudinal section of sample. The lateral tensile strain is a main factor of failure, and the failure mode is tension crack failure. When rock sample is under coupled axial stress and cyclic impacts, the shape of the sample became to be conjugate hyperbolas after the first fracture due to the interfacial friction constrain, the following failures occur at around of incident interface, and the failure mode is tension-shear failure. Rock specimens rapture into circular cone frustum or circular cone under three dimensional static loading and cyclic impact loadings, and the failure mode is tension-shear failure which is caused by reflected longitudinal wave and reflected transversal wave. The value of axial stress has great effect on the sample final failure shape which is circular cone frustum or circular cone.