Experimental Study On Changes In Ultrasonic Coda Wave And Acoustic Emission During The Deformation Of Rocks Under Loading

The coda wave propagation path has received extensive attention since it is more sensitive to small changes in the medium than the direct wave.During the process of loading,the wave velocity,medium or source changes may cause the coda wave to change.The physical mechanism of change in the ultrasonic coda wave varies during different deformation stages.Meanwhile,there exist local damages in the rock sample during the deformation,and it will be accompanied by acoustic emission.Combining the ultrasonic coda wave and acoustic emission is beneficial to characterize the coda wave characteristics and damage degree of the sample at different deformation stages.In this paper,three kinds of rocks,including granodiorite,marble and sandstone with the sizes of 50 mm × 50 mm × 150 mm,are used to carry out observations of ultrasonic coda wave and acoustic emission during the whole process of loading,so as to study characteristics of the coda wave at different deformation stages.The major results are given below:(1)The physical mechanisms of change in coda wave are different at different stages of deformation.Firstly,in the low-stress level,change in the coda wave is mainly dominated by scatterers,and the wave velocity varies greatly and does not change regularly.Secondly,in the medium stress level,especially the linear elastic deformation stage,change in the coda wave is dominated by the change of wave velocity,and the increase amplitude of wave velocity decreases linearly with the loads.Finally,in the high stress level,changes in scatterers and wave velocity both exist.The rock will locally be damaged before the rupture,and the role of the scatterer will be enhanced.(2)The coda wave variation is accordant with the evolution of the acoustic emission.When the acoustic emission frequency increases,the coda wave changes accordingly.In particular,in the early stages,with acoustic emission frequency increasing,the coda wave varies.This implies that the early damage information of the rock can be obtained by analysis of the coda wave.(3)The results of both simple and cyclic loading experiments indicate that the increment amplitude of the wave velocity gradually decreases with the increase of the load,which is consistent with the understanding obtained through the direct wave.(4)Under the condition of cyclic loading,the interference of acoustic emission can be eliminated because of the Kaiser effect when analyzing the coda wave.The consistency of the wave velocity change and stress loading and unloading is further verified.In short,the micro-fracture generated during rock deformation will change the physical mechanism of change in the coda wave,and the scatterer effect will be significantly enhanced.Meanwhile,the acoustic emission waveform will cause interference to the ultrasonic coda wave.This means that attention needs to be paid when analyzing rock damage only using coda wave data.Ultrasonic coda wave changes and acoustic emission can reflect the damage in the rock,and the change mechanism of the coda wave in different deformation stages is different.The joint observation of the two methods can improve reliability of the observation results.