A Waveguide Ultrasonic Phased Array Radar for High Temperature Equipment Monitoring

Ultrasonic guided waves have been widely investigated as a powerful tool for structural health monitoring (SHM) and nondestructive evaluation (NDE) due to their ability to propagate long distances with little energy loss and due to their sensitivity to incipient structural changes. For different engineering applications, various types of sensors have been developed, such as Piezoelectric Wafer Active Sensors (PWAS), electro-magnetic sensors, Fiber optic sensors, and air-coupled transducers.

Permanent sensor installation will allow measurements to be real-time and to remove errors introduced by repetitive sensor installations. Many investigations have pointed out that monitoring the health status of critical components with permanently installed transducers is an effective approach to maintaining structural safety. On the other hand, high temperature structures find their wide application in aerospace, process, and nuclear industries. Thus, there has been a strong interest in developing ultrasonic transducers that can operate at high temperature environments for a long time. However, conventional piezoelectric transducers cannot sustain such harsh conditions beyond their Curie temperature. Waveguide transducers have been recently studied as a good candidate for solving this difficulty. However, up till now, waveguide transducers are only used in generating torsional wave or shear type of waves for detecting thickness loss of pipes. Furthermore, the active sensing setups are generally conventional pulse-echo or pitch-catch modes. These facts intensively restricted their application for a more accurate damage assessment purpose.

To overcome these drawbacks, this research proposes a design for monitoring high temperature structures utilizing phased array waveguide transducers, where the active sensing elements work synthetically together to achieve better damage detection quality. The waveguide transducer is comprised of a wave-generation piezo element and a waveguide bar conducting the wave energy into the host structure. Absorbing materials are implemented on the far end of the waveguide to eliminate the boundary reflections. A coupled-field local finite element model (FEM) is constructed to grasp an in-depth understanding of the wave generation behavior. Via the harmonic analysis, optimum wave generation frequency and mode type can be analyzed. Thereafter, an array of waveguide transducer elements are attached to the host structure to study their wave manipulating ability. The system works based on the principle of phased array theory; the excitation instant of each element is controlled to form an equiphasic surface. In this way, the wave propagation phenomenon such as focusing and directional steering can be realized. An ultrasonic radar for high temperature working condition can thus be realized. Transient dynamic finite element simulations are conducted to demonstrate the wave focusing and deviation control capability of the phased array waveguide radar. Damage detection and imaging are further illustrated through case studies. The proposed system possesses great application potential to enhance the performance of Lamb wave SHM and NDE systems for high temperature structures. This paper finishes with summary, concluding remarks, and suggestions for future work.

Keywords: waveguide bar, phased array, wave manipulation, guided waves, high temperature damage detection, ultrasonics