Novel Design of Magnetostrictive Patch Transducers Based on Demagnetizing Theory to Efficiently Emit and Receive Guided Waves for Detecting Defects Occurring in Cylindroid Structures


Student thesis: Doctoral Thesis

View graph of relations



Awarding Institution
Award date2 Dec 2019


Pipes are widely used to transport water, oil, and gas. Severe corrosion of pipes can cause leakage of natural gas, and result in an explosion. The large attenuation and mode conversion caused by concrete makes it difficult to test wall-covered pipes using guided wave (GWs). Another difficult problem is determination of the circumferential position of a defect within a small diameter pipe. This thesis reports the design of an axially magnetized magnetostrictive patch transducer (AM-MPT) and segmented axially magnetized magnetostrictive patch transducer (SAM-MPT) array for efficient transduction of longitudinal GWs. These transducers can be used to detect defects in concrete-covered pipe, and to determine the circumferential position of defects in pipes with a small diameter.

A theoretical background is presented that includes application of the theory of GWs to the inspection of cylindroid structures, the principles of magnetostriction and demagnetization and criteria for the design of magnetostrictive patch transducers.

Next, the effects of varying the length-to-width ratio of the magnetized rectangular patch on the demagnetizing factors and on the patch’s magnetic field intensity are investigated, and these effects are used to improve the design of an AM-MPT. The analytical, simulation, and experimental results prove that the greater the length-to-width ratio of the magnetized iron-cobalt patch, the greater the magnetic field intensity and the amplitude of the signal triggered by the AM-MPT. Comparison experiments prove that the static magnetic field of an AM-MPT provided by an iron-cobalt patch leads to greater signal amplitudes than those provided by magnets. Moreover, the meander coil that provides the dynamic magnetic field is optimized to further improve the amplitude of the signal excited by the AM-MPT.

Various kinds of existing MPTs are compared with the designed AM-MPT to test pipes covered by concrete. These tests include three types of commonly used MPTs, two kinds of commonly used piezoelectric transducers (PZTs), and two types of designed PZTs. Unlike those of the commonly used 20 × 5 × 0.5-mm extension PZT, the signals triggered by other kinds of transducers could not be detected within concrete-covered pipes. In experiments of defect detection within concrete-covered pipe, the defect echo and pipe end echo could not be recognized within the signal produced by the 20 × 5 × 0.5-mm extension PZT; in contrast, the defect echo and the pipe end echo were remarkable within the signal excited by the developed AM-MPT. These experimental results prove that the designed AM-MPT works better than existing transducers.

The attenuation of L(0,2) in a concrete-covered pipe was studied experimentally with the AM-MPT according to the results of simulations with Disperse software. Based on these studies, the AM-MPT was applied to identify defects within the concrete-covered pipe sample in a laboratory environment. A defect within a concrete-covered pipe sample could be detected by the L(0,2) signal triggered by the AM-MPT. The designed AM-MPT was then applied in the concrete-covered pipe field test. The echoes from the pipe elbows were remarkable in the three field tests, which means that defects that occur within the pipe can be detected by the L(0,2) signal triggered by the AM-MPT. These experimental results show that the AM-MPT can potentially be applied to identify pipe defects.

Because axisymmetric GWs cannot determine the circumferential position of a defect within a pipe, and because propagation of the non-axisymmetric GWs in a small-diameter pipe is complicated, the circumferential position of a defect is difficult to determine. An SAM-MPT array for efficient transduction of non-axisymmetric L(M,2) modes to determine the defect’s axial and circumferential position in the small-diameter pipe was found. 1) A method of monitoring pipe health with the designed SAM-MPT array was determined. 2) The most suitable multi-belts of flexible printed coils (FPCs) were chosen to provide the dynamic magnetic field for the comparison experiments. 3) The signal amplitude of the SAM-MPT array was compared with magnetostrictive materials with various length-to-width ratios to further prove the principle of the demagnetizing field. 4) The two further MPT arrays (equipped with permanent magnets) were compared with that of the SAM-MPT array. 5) Pipe health monitoring experiments were carried out. The circumferential position of each defect as estimated by the SAM-MPT array correlated well with the defect’s true location by matching the angular profiles of the experimental results and the modulated numerical analysis for several axial distances. The experimental results for the SAM-MPT array demonstrate promise in the detection of the axial and circumferential positions of defects, even in pipes with a small diameter.

The benefits of these new GW-based transducers and their related nondestructive testing methods to the industry are as follows. 1) The labor costs and time required for on-site pipe inspection can be greatly reduced. 2) The flexibility in mounting the sensor can help minimize the inconvenience caused by variations in pipe sizes. 3) It is sensitive enough for inspection of corrosion in pipes, even where corroded areas are concealed by soil or walls. 4) It enables early detection of pipe defects before real leakage occurs.

    Research areas

  • Demagnetizing field, nondestructive testing, pipe defect detection, sensor design, non-axisymmetric guided wave, magnetostrictive patch transducer