This paper reports on ultrasonic measurement of the axial preload of anchor rods loaded by a pulling machine. The axial preload of ultrasonic measurement was as high as 90 kN, limited by manual operation and the anchor protection structure. Ultrasonic response characterisation, high accuracy and effective ultrasonic measurement of axial preload in anchor rods are reported here and are fully demonstrated. By bonding lead zirconate titanate (PZT) piezoelectric ceramic sensors and self-designed ultrasonic probes on the anchor rods, A-scan ultrasonic signals were successfully measured and originated from the bottom of the hollow anchor rod. The temporal A-scan signal featured a pure and broad frequency spectrum that peaked at around 16 MHz, but no original frequency signal component of the sensors was found. This phenomenon is attributed to the reflective characteristics of the thread-like structure of the hollow anchor rod, potentially inducing a non-linear ultrasound effect, independent of the test instrumentation and coupling agent. When the loading tension of the calibrated pulling machine is lower than 30 kN, the axial preload of the anchor rods obtained from the ultrasonic measurement has an error as high as ±16%. By comparison, the error is less than ±5% for the tension increasing from 60 kN to 90 kN. The experimental results indicate that the proposed ultrasonic measurement method is a simple, accurate and effective way to obtain the axial preload of anchor rods.
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