Effect of random corrosion pit dimensions on ultrasonic guided wave propagation characteristics in rock bolts

Rock bolts in complex underground environments are prone to randomly distributed localized corrosion damage, posing a severe threat to the stability of anchorage structures. Ultrasonic guided waves represent a core technology for nondestructive testing of bolt corrosion; however, existing studies predominantly focus on the influence of total corrosion extent on guided wave propagation characteristics, neglecting the mechanism of defect size discretization effects under a fixed total corrosion amount. To this end, a numerical model of a free rock bolt containing random pitting defects with different individual pit areas is established. Under strictly controlled total corrosion area and corrosion depth, with individual defect area as the sole variable, the propagation behavior of the L(0,1) mode ultrasonic guided wave at a center frequency of 50 kHz is systematically analyzed. Through time-domain waveform analysis, Hilbert transform, energy statistics, and peak-to-peak velocity calculation, the mechanisms by which individual defect area influences waveform distortion, energy scattering, and peak-to-peak velocity are revealed. The results indicate that: under a fixed total corrosion amount, increasing the individual pit area progressively exacerbates waveform distortion and scattered clutter; the cumulative energy generally exhibits a declining trend, yet extremely small-sized dense pitting causes weak scattered energy to be temporally dispersed, resulting in abnormally higher values than the control group; the amplitude attenuation rate shows no obvious monotonic pattern; the reduction in peak-to-peak velocity, within a certain range, exhibits a nonlinear characteristic that first recovers rapidly and then stabilizes as the individual pit area increases, which can be described by a logarithmic relationship. The findings can provide a theoretical basis for ultrasonic guided wave detection of random corrosion in rock bolts.