Abstract
The magnetostrictive guided wave sensor with a single induced winding cannot distinguish axially symmetric from non-axially symmetric features in a pipe, because it is impossible for the sensor to detect the non-axially symmetric mode waves. When we study the effect of the change of the magnetic field in the air zone for receiving the longitudinal guided wave mode, we find that the change of the magnetic flux in the air zone is almost equivalent to the change of the flux in the pipe wall, but in opposite directions. Based on this phenomenon, we present a sensor that can detect the flexural-mode waves in pipes based on the inverse magnetostrictive effect. The sensor is composed of several coils that are arranged evenly on the outside of pipes. The coils induce a change in magnetic flux in the air to detect the flexural-mode waves. The waves can be determined by adding a phase delay to the induced signals. The symmetric and asymmetric features of a pipe can be distinguished using the sensor. A prototype sensor that can detect F(1,3) and F(2,3) mode waves is presented. The function of the sensor is verified by experiments.
Highlights
The guided wave technique has become successful in nondestructive testing (NDT) over the last decade because guided elastic waves can propagate over a long distance and may be excited and received using transducers positioned at a given location [1,2,3,4,5]
When we recently studied the effect of the change in the magnetic field in the air zone for receiving guided waves based on the inverse magnetostrictive effect, we observed that the change in the magnetic flux in the air zone was nearly equivalent to the change in the flux in the pipe wall, but along the opposite direction [22]
The coils induce a change in the magnetic flux in the air zone to detect the guided waves in pipes
Summary
The guided wave technique has become successful in nondestructive testing (NDT) over the last decade because guided elastic waves can propagate over a long distance and may be excited and received using transducers positioned at a given location [1,2,3,4,5]. The L(0,2) mode is practically non-dispersive over typical frequency ranges, and particle motion is roughly uniform throughout the pipe wall. The reflection signal from flanges and square ends is generally bigger than the defect signal. All of the energy in the signal is reflected from flanges and square ends. The reflection from a butt weld is small, because the weld cap and weld bead present only a small change to the geometry This phenomenon introduces the possibility of a weld being incorrectly identified as a defect. It is difficult to identify these features and defects from axially symmetric reflections. The mode conversion induced by non-axially symmetric features can be measured to overcome this problem [9,10,11,12,13,14,15]. If the T(0,1) mode is incident, the mode conversion is primarily to the
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