Long range guided wave natural focusing pipe inspection

Long range ultrasonic guided wave nondestructive evaluation can be used to inspect pipelines over fairly long distances. Partial loading of transducers around the circumference leads to a non-axisymmetric energy distribution. At particular axial distances and frequencies, the ultrasonic energy is naturally focused at some spots around the circumference via constructive wave interference. This so called "natural focusing" phenomenon can be used to improve guided wave sensitivity for a defect since more energy is sent to the defect. However, defects located in other places could possibly be missed, unless we can move the natural focusing points throughout the pipe. We have done this by frequency and circumferential angle tuning for specific circumferential loading lengths. In order to utilize the natural focusing phenomenon to enhance detection sensitivity, a frequency and angle tuning (FAT) technique is employed to extend the area that is scanned by focal energy. It is observed that the natural focal points at a fixed axial distance move with frequency variation and circumferential excitation length change. In this paper, the natural focusing phenomenon with FAT is theoretically calculated and experimentally investigated. The results show that the natural focusing inspection technique can sufficiently inspect an entire pipe with FAT. Some sample inspection data is compared by applying axisymmetric excitations, FAT, and phased array focusing.