The butt fusion process is a widely used method for connecting Polyethylene (PE) pipes. Conventional ultrasonic test is unable to detect defects due to heavy ultrasonic attenuation, disturbance by the structure of the joint, and the weak acoustic response caused by lack of fusion. A new technique named coupling focusing ultrasonic technique used for testing PE butt fusion joints is developed in this paper. The key of the technique is to get a special couplant, which is a mixture of glycerinum, sodium silicate, water, and defoamer, with almost the same sound velocity and acoustic impedance of PE. Through the couplant, sound beam is converged by spherical lens, entering the PE material with focusing zone overlapping the fusion area of the joint. It is proved that the technique has a good testing ability for defects of typical lack of fusion, gas holes with 1mm in diameter and crack with 1mm in height, based on the results of detection experiments of test sample accompanied with typical defect. It is also proved that its sensitivity is better than TOFD technique and traditional contact angle wave ultrasonic technique as well.

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