Experimental study on wave velocity variation and energy dissipation law of red sandstone under different initial unloading levels

In order to study the law of wave velocity variation and the energy dissipation of red sandstone under different initial unloading levels(initial differential stress level and the corresponding ratio of the peak intensity under the condition of confining pressure of rock),using Rock Top several coupling tester and JSR-DPR300 ul-trasonic testing system, at the same confining pressure conditions, the compression and shear wave velocities in the stress-strain process of red sandstone under the three initial unloading levels were tested, which were in the triaxial loading path and unloading path of stress difference constant release confining pressure and stress difference increase release confining pressure.The results show that in the process of confining pressure loading, the compression and shear wave velocities increase with the increase of confining pressure, showing a power function relationship.With the increase of confining pressure, the sensitivity of porosity and density to confining pressure and the range of variation decrease.The stress threshold for the closure of different types of pores in the rock is different, and the wave velocity is controlled by the degree of compaction of the rock and the nature of the rock skeleton.In the process of confining pressure unloading, under different unloading paths, with the increase of the initial unloading, the ultimate strain increases, uniform confining pressure drop parameters linearly reduces, strain confining pressure increment ratio increases in power function, circumferential deformation of confining pressure is more sensitive, compared with unloading paths, the influence of the initial unloading level plays an control role in rock unloading deformation process.Under different initial unloading levels, the curves of compression and shear wave velocities with axial strain are corresponded to the stress-strain curves, reflecting the different degradation levels of rock samples, the overall growth and attenuation trends of compression and shear wave velocities are consistent.The unloading path has a weak influence on the wave velocity and there are obvious stage characteristics in the loading and unloading regions.The wave velocity has a large deflection at the initial point of unloading, and the compression wave can be divided into growth stage and attenuation stage accordingly, while the shear wave can be divided into stable stage and attenuation stage.The compression wave is more sensitive to axial pressure and has strong oscillation.At the beginning of confining pressure unloading, the rock rapidly changes from compression to expansion or expansion intensifies, and the strain energy density has the first and second deflection at the initial point of unloading and the stress drop point, the latter deflection amplitude is far less than the former.When the stress difference is constant and the confining pressure is relieved, the elastic strain energy is approximately stable, and its growth trend is consistent with the axial stress-strain curve, that is, the work of axial stress is the main energy supplement mode.The dissipation of energy is concentrated in the unloading zone, which increases stably in the early stage of unloading and rapidly in the later stage of unloading.The release of elastic energy is concentrated after the stress drop point, which is the inherent power of rock instability and failure.The triaxial loading process determines the overall failure pattern of the rock sample, and the prominent dilatancy of confining pressure unloading will lead to more compression cracks in the rock sample, leading to the diversity of rock failure patterns.