A Study on the Time Reversal Method for Focusing Ultrasonic Guided Waves Using Array Transducers
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For the long range inspection of ultrasonic guided wave using array transducer, phase tuning methods are quite often adopted to generate ultrasonic guided waves with particular modes. The phase tuning method is able to control wave modes but not to focus waves on the defects under interrogation. Thus, a robust tool that can control not only generated wave modes but also focus the guided waves on the flaw is strongly desired. To address such a need, in this study, we adopted the time reversal technique for focusing ultrasonic guided waves generated by array transducers on the defects since the time reversal technique can provide proper time delays for focusing on the defect and tuning wave modes. For investigation and verification of the time reversal method, we performed focusing of ultrasonic guided waves on the three types of flaws generated from an array transducer with time delays calculated by the D.O.R.T method using numerical simulation software. Then, we compared results obtained by adopting the time reversal technique to those by a conventional method. In this paper, numerical simulation results and comparison results were presented.Keywords:
Guided wave testing
The electromagnetic ultrasonic testing presents great potential in structural health monitoring due to its non-contact advantage. The electromagnetic acoustic transducers generating torsional and longitudinal guided wave have been extensively studied for pipeline structure. However, the transducer for helical wave and corresponding analysis still have not been adequately investigated. This paper proposes the novel transducer to generate helical Lamb wave in pipeline. Based on the Lorentz mechanism, the developed transducer contains the fan-shaped magnetic poles and a two-layer coil. The finite element model is built to simulate the wave generation and propagation from the transducer. The displacement distribution verifies the capacity of generating helical wave. The excitation and reception of helical wave are also carried out in the experiment. The actual frequency of the transducer measured from the frequency-sweeping coincides with the designed value. In addition, the defect detection experiment is conducted. The axial and circumferential location can be obtained according to the amplitude and time information. The further comparison and discussion indicate that the prior transducer with meander coil is not appropriate to generate helical wave. The developed transducer can generate wave with diverse incident angle which is superior than the transducer for torsional and longitudinal modes.
Electromagnetic acoustic transducer
Guided wave testing
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Within Philips Research, ultrasonic transducers are integrated into the minimally invasive needles and catheters for cardiac imaging. The existing ultrasonic transducers have a low signal to noise ratio (SNR) and a low bandwidth of the received echo and, therefore, the transducer does not perform well for the cardiac imaging application. A high SNR is needed to increase penetration depth into heart tissue and an increased bandwidth is required to improve resolution. Therefore, there is a need for a model that can predict the behavior of the ultrasonic transducer. In this report, a model is developed and implemented for the complete ultrasonic measurement system. This model is used for the improvement of the design of a transducer and can also be used for design of ultrasonic transducer for various applications. Various parameters needed for the simulation are estimated experimentally. The comparison between the response of an ultrasonic transducer and obtained model indicates the high accuracy of the model to predict the behavior of an ultrasonic transducer.
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In order to improve the performance of ultrasonic transducer, a new kind of complex ultrasonic transducer was advanced. Two kinds of piezoelectric material are used in this transducer, in which PZT is used as transmitter and PVDF (Polyvinylidene Difluoride) is used as receiver. Using KLM (Krimholtz, Leedom and Mattaei) equivalent circuit model, this kind of transducer was analyzed theoretically. Based on the theoretical analysis and calculation, the complex ultrasonic transducer was designed and fabricated. The theoretical and experimental results show that this kind of transducer has low input power loss and relatively short pulse echo.
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In view of the defect that the magnetostrictive guided wave transducer with meander coil structure can only excite and receive the strong guided wave signal at a fixed frequency, a broadband magnetostrictive Sheared Horizontal (SH) guided wave transducer is designed for structural health monitoring of metal pipes and plates. This paper demonstrates the energy transfer mechanism of magnetostrictive SH guided wave transducer and proposes a type of transducer structure of small-width coil and magnetostrictive patch. Comparative experiments are conducted to study how magnetostrictive patch width and coil width influence the relationship between amplitude and frequency of guided wave signal of the transducer, and the transducer designed in this paper is used for the defect detection experiment. According to the experimental results, when the coil and the magnetostrictive patch are of the same width, the change of their width can notably influence the center frequency of the transducer, and the transducer can excite and receive SH guided waves with higher amplitude in a larger frequency range. The transducer designed in this paper can detect the defects with a smaller section loss rate by increasing the excitation frequency without changing the coil, which is more suitable for the structural health monitoring of industrial equipment.
Guided wave testing
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In this study, a 55.5 kHz frequency ultrasonic transducer is designed for an electrosurgical electrode to satisfy the requirement of minimally invasive surgery. The structure of the ultrasonic transducer is modified to induce longitudinal vibration at 55.5 kHz. Simulation results demonstrate that the ultrasonic transducer would longitudinally vibrate at frequencies of 48.01 kHz and 56.035 kHz. An ultrasonic transducer is fabricated to experimentally test the simulation results; the results demonstrate that the resonant frequency is approximately 55.5 kHz, and the output amplitude is satisfactory for cutting soft tissues.
Electromagnetic acoustic transducer
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In order to overcome the conventional guided wave transducer's unsuitability for piled pipe inspection, a non-axisymmetric end loading (NEL) transducer is designed and studied in this paper. Propagation of guided waves in a hollow cylinder and the circumferential distribution of echoes from notches are studied by using the finite element (FE) method under different end loading conditions. The simulated results show that when the radial angle between the first notch on a pipe and the NEL transducer is zero, the energy distribution from notch echoes is a maximum, even if there are different direction notches on the pipe. A specially designed clamp is developed to fix the NEL transducer to the pipe. Finally, the simulated and experimental results show that the circumferential and axial location of the notch on pipe can be located by rotating the NEL transducer during the process of inspection.
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In this paper, an integrated loading model of ultrasonic transducer is established to explain the phenomenon that different tissue loads have different effect on resonance frequency of ultrasonic transducer. Mechanical loss of ultrasonic transducer is taken into consideration in the model and mass-spring-damping tissue model and contact impedance between ultrasonic transducer and tissues are given. Experiments show that this model is accurate enough to analyze loading characteristic of ultrasonic transducer. Effect of tissue elastic module on resonance frequency of ultrasonic transducer is studied. Results show that soft tissue with low elastic module make resonance frequency of ultrasonic transducer lower than unloaded condition, while tissue with large elastic module make resonance frequency larger.
Electromagnetic acoustic transducer
Acoustic impedance
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Guided wave testing
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In designing process of the transducer,fixed-side which is used fixing transducer ususlly be ignored.In this paper,based on equivalent circuit of half wavelength transducer,the conductance and resonance equation of ultrasonic transducer with fixed-side were given using electromechanical analogy method.And the influence of ultrasonic transducer with fixed-side on the resonance performance was discussed.The results show the resonace frequency of ultrasonic transducer decreases with the increases of the length and the increase of the diameter of the fix-side;it should change the longth of the front-side base on the size of the fixed-side and make ultrasonic transducer in best vibration state.A multiple longitudinal transducer viberation device is designed by the principle and requirements of power ultrasonic vibrations system.According to the result of research,ultrasonic transducer was designed and measured.The result show that the transducer work normally on the designing frequensy and the correct design.The study can provide theoretical basis for optimized designing and using ultrasonic transducer.
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Structural health monitoring (SHM) technology has attracted more and more attention in aerospace and energy industries as it may detect and monitor early degradation and damage in the structures and thus avoid life and financial loss. Guided wave (GW) based inspection is very useful for the SHM of thin-walled structures due to its capability for long-distance and wide-range inspection with high sensitivity. However, the lack of small and high-performance transducers seriously limits the development of GW-based SHM technology. In this paper, a thin-film magnetostrictive patch transducer (MPT) that can generate and receive single-mode and almost nondispersive GW is proposed. The new transducer is composed of an alloy patch and a double-layer flexible printed circuit (FPC). Firstly, the basic structure and principle of the proposed transducer are presented and analyzed. Compared with current single-mode GW transducers, the main advantages of the proposed transducer are lightweight, flexibility, and high transduction efficiency. Both the numerical simulation and experimental results validate that the new transducer has good performance in generating and receiving single and almost nondispersive S0 mode GW in an aluminum plate. It is very promising to be applied for the SHM in curved thin-walled structures.
Structural Health Monitoring
Guided wave testing
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