Characteristics of ultrasonic guided waves in rock bolts considering rib influence and an energy ratio index based damage detection method

Ultrasonic guided waves (UGWs) have demonstrated utility in rock bolt testing, yet the influence of ribs on UGW propagation characteristics within these structures remains largely unexplored. This investigation seeks to address this gap by conducting numerical and experimental analyses of UGW behaviour in rock bolts, with a focus on rib effects. Furthermore, a simplified energy ratio index is developed to enhance defect detection capabilities. The semi-analytical finite element (SAFE) method is employed to generate dispersion curves for rock bolts, revealing that rib presence increases UGW mode quantity and decreases the group velocity of the L(0,1) mode. Numerical and experimental studies of UGW propagation in rock bolts indicate that the amplitude of rib-reflected L(0,1) mode UGWs correlates positively with centre excitation frequency. The significance of these rib-reflected waveforms is underscored, as they may obscure damage-reflected UGWs, complicating defect identification. To address this challenge, a novel energy ratio index is proposed and validated. Results demonstrate that this index, when accounting for rib influence, effectively detects defects in rock bolts.