Research On Nonlinear Variation Of Elastic Wave Velocity Dispersion Characteristic In Limestone Dynamic Fracture Process

The elastic wave propagating in rock has velocity dispersion characteristic that holds an imperative significance in rock engineering. This paper aims at study elastic wave velocity dispersion characteristic in limestone dynamic fracture process. For this purpose, the experiment of elastic wave propagates in the limestone when cyclic loading and unloading process is conducted, and elastic wave velocity dispersion characteristic is analyzed. Based on multifractal theory and Raiga model, the influence of fracture degree on velocity dispersion characteristic is discussed qualitatively and quantitatively with evolution characteristic of the AE event. The research results indicate that there is velocity dispersion characteristic when elastic wave propagates in the limestone sample, which presents positive correlation between velocity and frequency. The variation of wave velocity dispersion characteristic is consistent with that of AE activities and both of them show a nonlinear increase trend with an increase of loading. The reason for velocity dispersion characteristic is that the retarding effect of microcracks on elastic wave velocity with different frequencies is different, and the effect is enhanced with the microcrack scale increasing. According to the variation of multifractal parameter Δα of the AE event energy series, the limestone sample under loading experiences a transformation process from small-scale microcrack to large-scale microcrack and small-scale microcrack dominance to multi-scale microcrack coexistence, and this complex transformation process results in nonlinear variation of velocity dispersion characteristic. Further, the multifractal parameter Δα is introduced into Raiga model to establish the velocity dispersion quantitative model. The model can describe well elastic wave velocity dispersion characteristic, and indicates quantitatively that there is an obvious nonlinear positive correlation between the dispersion characteristic and the fracture degree. The research results have important theoretical and practical significance for understanding elastic wave velocity dispersion characteristic in limestone dynamic fracture process. It can provide important support for characterizing rock fracture process using elastic wave velocity dispersion characteristic.