Ultrasound nondestructive Testing (NDT) of materials, processes, and structures has been broadly applied to space, defense, aerospace, civil and energy industries. Especially, industrial facilities such as nuclear power plants or chemical plants contain numerous high-temperature (< 700°C) pipes and pressure vessels that require continuous health monitoring for instant detection of structural flaws.
In this work, a new ultrasound NDT method based on photoacoustic Lamb wave detection was proposed for high temperature NDT. AlN single crystal is chosen as the sensor material primarily due to its temperature-robust piezoelectric properties and high Curie temperature (> 2000 °C). The fundamental symmetric (S0) and antisymmetric (A0) mode Lamb waves generated by the pulsed laser were successfully detected by the AlN sensor on a stainless steel plate. The time-of-flights (ToFs) of the S0 and A0 mode waves proportionally increased with the distance (D) between the laser source and the sensor, and almost no attenuation of the amplitude was observed as the distance increases. At the HT NDT experiment, the combination of non-contact, portable laser source as a Lamb wave generator and temperature-robust NDT sensor made of AIN has shown its great capability to detect the Lamb waves at elevated temperatures up to 800 °C. Based on the ToFs analysis of S0 and A0 mode Lamb waves propagated in a specimen with defects, the capability of this NDT method to localize the defect even at high temperature (~800 °C) has been verified.
Piezo-composite transducers have been widely used for medical applications such as the medical imaging, brain stimulator, blood flowmeter and humidity sensor due to its merits of high sensitivity and broad bandwidth. Conventional ultrasonic transducers and arrays were mostly developed with a rigid flat or curved front with a fixed curvature. However, most parts of the human body would have curved or irregular shapes such as a human skull or chest. Thus, a flexible ultrasound transducer may be preferred in the medical diagnosis devices. Recently, the flexible sensors or transducers are of great interest and the associated progress has been made for the medical imaging. However, published works have not provided an appropriate solution to overcome the limitations of metal-type electrodes such as cracking or delamination at the presence of transducer surface bending.
In this work, we have developed a flexible piezo-composite transducer composed of the active piezoelectric material (PZT-5H) and passive polymer matrix (PDMS) to achieve sufficient flexibility, sensitivity, and bandwidth for the medical applications. In addition, the flexible electrodes composed of silver nanowires (AgNWs) and PDMS were deposited on the transducer using the spray coating method. AgNW/PDMS electrode is a promising alternative to metal-type electrodes such as Au to possess a reliable durability to the cracks from the strained fatigue while providing a sufficient conductivity as an electrode.
The prototyped transducers can be applicable to the curved or irregular surface of the target structure for detecting any acoustic variation with high sensitivity and good matching contact.
In this work, a new ultrasound nondestructive testing (NDT) method based on laser-generated Lamb wave detection was proposed for high temperature (HT) NDT. Lamb waves were introduced to a stainless steel plate by the Nd:YAG pulsed laser at one point and detected by aluminum nitride (AlN) transducer at a distant position. The fundamental symmetric (S0) and antisymmetric (A0) mode Lamb waves were successfully propagated in the thin stainless steel plate. The time-of- flight (TOF) of the S0 and A0 mode waves proportionally increased with the distance (D) between the laser source and the sensor, and almost no attenuation of the amplitude was observed. For the HT NDT experiment, AlN single crystal was adopted as the ultrasonic sensor material due to its high thermal resistance of the dielectric and piezoelectric constants at the elevated temperature up to 800 °C. The combination of non-contact, portable laser source as a Lamb wave generator and temperature-robust NDT sensor made of AIN has shown its great capability to detect the Lamb waves at elevated temperatures.
In this paper, a piezoelectric sensor with a floating element was developed for shear stress measurement. The piezoelectric
sensor was designed to detect the pure shear stress, suppressing effects of normal stress components, by applying opposite
poling vectors to the piezoelectric elements. The sensor was first calibrated in the lab by applying shear forces where it
demonstrated high sensitivity to shear stress (91.3 ± 2.1 pC/Pa) due to the high piezoelectric coefficients of
0.67Pb(Mg1∕3Nb2∕3)O3-0.33PbTiO3 (PMN-33%PT, d31=-1330 pC/N). The sensor also exhibited negligible sensitivity to
normal stress (less than 1.2 pC/Pa) because of the electromechanical symmetry of the device. The usable frequency range
of the sensor is up to 800 Hz.