Abstract: The investigation of low frequency ultrasonic guided waves propagation in anchored rock bolts is of great significance to selecting optimal guided wave frequencies for non-destructive testing (NDT). The propagation of 0-100 kHz guided waves through anchored rock bolts of varying grouting structures has been investigated by determining their frequency domains and time-domains using theoretical calculations and numerical simulations, respectively. The numerical simulation and theoretical calculation results of the anchored rock bolt models with finite grouting thicknesses agree well with the test results in related literature, which verifies the validity of the numerical and theoretical calculation methods presented in this paper. Afterward, the propagation of low frequency guided waves through anchored rock bolts with infinite grouting thicknesses was investigated using the validated theoretical calculations and numerical simulations. The results indicate that the grouting thickness and properties of the grouting material significantly influence the propagation characteristics of the guided wave. Though the optimal guided wave frequencies in anchored rock bolts with finite thicknesses exhibit sufficiently low attenuation and a great propagation distance, they are not suitable for field NDT testing due to the significant attenuation in anchored rock bolts with infinite thicknesses. The attenuation of low frequency guided waves increases as the elastic modulus of grout increases; therefore, accurate field NDT testing can only be conducted within eight hours after the initial pouring of the grout.

Key words: low-frequency ultrasonic guided wave, grouting rock bolt, finite anchored structure, curing time, non-destructive testing