Butt thermal-fusion joint is one of the main joints for polyolefin pipes. It could lead to different failure modes caused by factors such as different environments and operations, which will makes the joints become one of the most dangerous potential risks in polyolefin pipe system. As different defects could lead to different failure modes, defect recognition and classification are considered to be the critical issue for ultrasonic inspection. Based on the previous researches about defect recognition, simulation and experiments on artificial specimens in ultrasonic inspection, the imaging mechanism and identifying method were investigated for different defects, and the corresponding relations between joint defect characteristics and ultrasonic images were proposed. Furthermore, a defect recognition method was established. Many defect inspection, identification and dissection experiments were conducted on unknown defects, and the proposed method of automatic defect recognition was verified.

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