Ultrasound Phased Array Imaging on Curved Surface for Weld Inspection of Elbow Pipe as a Replacement for Radiographic Inspection

Ultrasound phased array (PA) imaging has been regarded as a safe replacement for radiographic inspection, even though they have different detectability depending on flaw types. There are innumerable demands for non-destructive testing on weld joints of various pipes which are indispensable to most of industrial structures. Diverse ultrasound testing has been used to detect flaws in welding joints, but some technical deficiencies still remain. For example, ultrasound PA imaging on weld of elbow pipe has many challenging issues due to its geometrically varying surface along the circumferential direction. Conventional PA imaging for weld inspection has particularly focused on ultrasonic wave propagation based on ray theory. This confines the incident angle to be narrowly fixed and the position of array transducers as well. Ultrasound PA imaging by total focusing method (TFM), however, can provide not only high resolution images but also flexibility that makes particular use of a transmitted wave with a specific direction from a single element in the array transducer. This leads us to develop a method to get TFM images of weld zone from an elbow part that curves. It is inevitable of each ultrasonic wave from the array transducer to transmit through different media and to be reflected from the boundary in the elbow part with an angle along the curved surface. To form a correct PA image, careful calculation is made to ensure that time delay of receive-after-transmit is correctly shifted and summed even under non-planar boundary condition. Here, a method to calculate wave paths for the zone of interest at weld joint of an elbow pipe is presented. A numerical simulation of wave propagation on an elbow pipe is made to verify the proposed method, and it clearly shows that the proposed method precisely localizes true source position. In addition, it is experimentally demonstrated that the proposed method is well applied to various actual pipes that contains artificial flaws in them. More info about this article: http://ndt.net/?id=19760