Rock bolts, as a type of reinforcing element, are widely adopted in underground excavations\nand civil engineering structures. Given the importance of rock bolts, the research outlined in this\npaper attempts to develop a portable non-destructive evaluation method for assessing the length of\ninstalled rock bolts for inspection purposes. Traditionally, piezoelectric elements or hammer impacts\nwere used to perform non-destructive evaluation of rock bolts. However, such methods suffered\nfrom many major issues, such as the weak energy generated and the requirement for permanent\ninstallation for piezoelectric elements, and the inconsistency of wave generation for hammer impact.\nIn this paper, we proposed a portable device for the non-destructive evaluation of rock bolt conditions\nbased on a giant magnetostrictive (GMS) actuator. The GMS actuator generates enough energy to\nensure multiple reflections of the stress waves along the rock bolt and a lead zirconate titantate (PZT)\nsensor is used to detect the reflected waves. A new integrated procedure that involves correlation\nanalysis, wavelet denoising, and Hilbert transform was proposed to process the multiple reflection\nsignals to determine the length of an installed rock bolt. The experimental results from a lab test and\nfield tests showed that, by analyzing the instant phase of the periodic reflections of the stress wave\ngenerated by the GMS transducer, the length of an embedded rock bolt can be accurately determined
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