Development of Single-sided Non-collinear Frequency Mixing Ultrasonic Transducer (FMUT) for Detecting Micro-Defects in Oil and Gas Pipeline Welds

During the welding and long-term service of oil and gas pipelines, stress concentration in the weld area is prone to induce the formation and growth of micro-defects, thereby seriously threatening the structural integrity of the energy delivery systems. Although the nonlinear ultrasonic testing has significant advantages of high sensitivity and high resolution for micro-defects, almost all are focused on the identification of micro-defects in flat plate structures. At present, for the micro-defects in the welds of oil and gas pipelines, there is still a lack of systematic nonlinear ultrasonic detection and positioning methods as well as dedicated transducer probes. Herein, a single-sided non-collinear frequency mixing ultrasonic transducer (FMUT) is developed and specialized for detecting the micro-defects in the pipeline welds. By mathematically modelling and systematically analyzing the optimal resonance conditions of two different incident ultrasounds, the single-sided non-collinear frequency mixing mechanism of dual-frequency ultrasounds is established to promote the design of the FMUT probe. Both the simulation and experimental results demonstrate that the proposed FMUT can effectively detect common micro-defects in the oil and gas pipeline welds, such as cracks, gas pores, incomplete penetration, incomplete fusion, and slag inclusion. In addition, a micro-defect location technology based on the sliding rectangular window and time-frequency analysis has been proposed, achieving high-precision positioning of micro-cracks in the oil and gas pipeline welds with a relative error of only 0.8%. This work presents an easy-to-operate and effective single-probe ultrasonic mixing methodology to integrate micro-defect detection capability with precise positioning, remarkably expanding the adaptability of existing nonlinear ultrasonic testing technologies in complex engineering structures.