In answering this question, neither the vast topic of UT inspection set-ups for tubes and pipes, nor the details of what probe to choose for what type of inspection, will be explored. The aim is to simply point out some major considerations that make UT inspection of pipes and tubes different from that of flat plates.
If Snell's law is applied to infinitesimally small strips of the ultrasonic beam incident to the surface, the beam in the specimen diverges due to the curvature of the surface. The effect becomes more severe as the diameter of the pipe decreases. The ultrasonic beam may then be collimated or focused, therefore minimising the spread of the refracted angle due to surface curvature. By making the circumferential dimension of the focus on the surface of the test material small compared with the diameter of the pipe being inspected, the incidence angles vary little from those that would occur if the surface were a flat plate.
Due to the curvature of the internal wall as well as the external wall, special care also needs to be taken when inspecting in transmit/receive mode. For the same inspection angle in the specimen, the full skip distance between the transmitting and the receiving probes will always be bigger in a pipe than in a flat plate and will increase with decreasing pipe diameter.
One last important aspect to always bear in mind when defect sizing in tubes and pipes is that the interaction between the probe scanning motion and the ultrasonic energy incident onto the defect is substantially different from that for flat plates. Because the inspection surface is curved, the scanning motion of the probe continuously changes the ultrasonic beam incidence angle onto the defect, which, in turn, modifies the way the energy is reflected back for detection. Defect sizing techniques must be applied to the inspection of tubes and pipes with extreme care. An accurate geometrical plot of specimen, ultrasonic beam and possible defects should always be constructed prior to, and consulted during, inspection and interpretation.
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