Langasite (LGS) has attracted considerable attention from researchers as a potential substrate material for piezoelectric device applications due to its very good frequency stability and moderately large electromechanical coupling factors for acoustic wave devices. This paper presents a new determination of the second order elastic constants of LGS using acoustic wave velocities measured by a time-of-flight cross-correlation method. Two types of ultrasonic transducers have been used to generate longitudinal and shear bulk waves that propagate through cube-shaped specimens. The specimens and attached transducers are placed inside a temperature chamber to maintain a constant environmental temperature during the measurements. We present a full comparison between the new values determined here and those previously reported.

In the United States, many civil, aerospace, and military aircraft are nearing the end of their service life. Many of these service life predictions were determined by models that were created at the time of the design of the structure, possibly decades ago. As a precaution, these structures are inspected on a regular basis with techniques that tend to be expensive and laborious, such as tear-down inspections of aircraft. To complicate matters, new complex materials have been incorporated in recent structures to take advantage of their desirable properties, but these materials sustain damage in a manner that is different from that of past monolithic materials. One example is fiber-reinforced polymer (FRP) composites, which are heterogeneous, direction-dependent, and tend to manifest damage internal to their laminate structure, thus making the detection of this damage nearly impossible. For these reasons, numerous groups have focused on developing sensors that can be applied to or embedded within these structures to detect this damage. Some of the most promising of these approaches include using piezoelectric materials as passive or active ultrasonic sensors and actuators, fiber optic-based sensors to measure strain and detect cracking, and carbon nanotube-based sensors that can detect strain and cracking. These are mostly point-based sensors that are accurate at the location of application but require interpolative methods to ascertain the structural health elsewhere on the structure. To conduct direct damage detection across a structure, we have coupled the ability to deposit a carbon nanotube thin film across large substrates with a spatially distributed electrical conductivity measurement methodology called electrical impedance tomography. As indicated by previous research on carbon nanotube thin films, the electrical conductivity of these films changes when subjected to strain or become damaged. Our structural health monitoring strategy involves monitoring for changes in electrical conductivity across an applied CNT thin film, which would indicate damage. In this work, we demonstrate the ability of the Electrical Impedance Tomography (EIT) methodology to detect, locate, size, and determine severity of damage from impact events subjected to glass fiber-reinforced polymer composites. This will demonstrate the value and effectiveness of this next-generation structural health monitoring approach.

Monitoring of fatigue cracking in steel bridge structures using a combined passive and active scheme has been approached by the authors. Passive acoustic emission (AE) monitoring is able to detect crack growth behavior by picking up the stress waves resulting from the breathing of cracks while active ultrasonic pulsing can quantitatively assess structural defect by sensing out an interrogating pulse and receiving the structural reflections. The dual-mode sensing functionality is pursued by using the R15I ultrasonic transducers. In the paper, we presented the subject dual-mode sensing on steel compact tension (CT) specimens in a laboratory setup. Passive AE sensing was performed during fatigue loading and showed its capability to detect crack growth and location. At selected intervals of loading cycles, the test was paused to allow for active sensing by pulsing the transducers in a round-robin pattern. Plate waves were excited, propagated and interacted within the structure. Several approaches were proposed to analyze the interrogation data and to correlate the data features with crack growth. Root means square deviation (RMSD) damage index (DI) was found as a good indicator for indicating the overall crack development. Short time Fourier transform (STFT) provided both time and frequency information at the same time. Moreover, wave velocity analysis showed interesting results when crack developed across the transmitter-receiver path.

Piezoelectric materials are the active elements in e.g. ultrasound transducers and thus widely used in sensor and actuator systems. The rather recent addition to ultrasonic transducer materials are the so-called ferroelectrets, voided piezoelectric-active polymer films. One type among the most studied ferroelectrets is cellular polypropylene. The properties of ferroelectrets, such as low elastic stiffness, high piezoelectric activity, good matching with air makes them suitable as ultrasonic transducer material. Here, we demonstrate the controlled adjustment of the piezoelectric properties, especially of the resonance frequency of cellular polypropylene films by means of transducer-structure modifications.

Increasing complexity of aerospace structures facilitates a growing need for structural health monitoring (SHM) systems capable of real-time active damage detection. A variety of sensing approaches have been demonstrated using embedded ultrasonic sensors such as piezoelectric wafer active sensors (PWAS) and magneto-elastic active sensors (MEAS). Common methodologies consider wave propagation (pitch-catch or pulse-echo) and standing wave (vibration or impedance) techniques with damage detection capabilities dependent upon structural geometry, material characteristics, distance to damage and damage size/orientation. While recent studies have employed damage detection and classification approaches that are dependent on cumulative statistics, this study explores the contribution of sensor parameters and experimental setup variability on the damage detection scheme. The impact of variability in PWAS and MEAS are considered on sensor use in ultrasonic and magneto-mechanical impedance damage detection. In order to isolate sensor parameters, measurements were conducted with PWAS in free-free boundary conditions. Variability of PWAS parameters was evaluated by measuring the sensors impedance response. An analytical model of PWAS was used to estimate sensor parameters and to determine their variability. Additionally, experiments using MEAS were performed that demonstrate variation of magneto-mechanical impedance during structural dynamic tests. From these experiments the importance of sensor setup is discussed and its contribution into the overall detection scheme is explored.

This paper presents a wireless ultrasound sensor that uses frequency conversion to convert the ultrasound signal to a microwave signal and transmit it directly without digitization. Constructed from a few passive microwave components, the sensor is able to sense, modulate, and transmit the full waveform of ultrasound signals wirelessly without requiring any local power source. The principle of operation of the unpowered wireless ultrasound sensor is described first. Implementation of the sensor and the sensor interrogation unit using commercially-available antennas and microwave components is described in detail. Validation of the sensing system using an ultrasound pitch-catch system and the power analysis model of the system are also presented.

Research interests in structural health monitoring have increased due to in-situ monitoring of structural components to detect damage. This can secure personal safety and reduce maintenance effort for mechanical systems. Conventional damage detection techniques known as nondestructive evaluation (NDE) have been conducted to detect and locate damaged area in structures. Ultrasonic testing, using ultrasonic transducers or electromagnetic acoustic transducers, is one of the most widespread NDE techniques, based on monitoring changes in acoustic impedance. Although the ultrasonic testing has advantages such as high sensitivity to discontinuities and evaluation accuracy, it requires testing surface accessibility, close location to the damaged area, and decent skill and training of technicians. In recent years, modal analysis techniques to capture changes of mode shapes and natural frequency of structures have been investigated. However, the technique is relatively insensitive to small amount of damage such as an initial crack which can rapidly grow in structures under cyclic loadings. In addition, structural health monitoring based on guided waves has become a preferred damage detection approach due to its quick examination of large area and simple inspection mechanisms. There are many techniques used to analyze sensor signals to bring out features related to damage. A phased array coupled with the guided wave approach has been introduced to effectively analyze complicated guided wave signals. Phased array theory as a directional filtering technique is usually used in antenna applications. By using phased array signal processing, virtually steering the array to find the largest response of source, the desired signal component can be enhanced while unwanted information is eliminated.

A novel active squeeze film journal bearing actuated by high power piezoelectric transducers is developed aiming for non-contact suspension of axial rotating member with active error compensation and active axis positioning. A mathematical model based on acoustic radiation pressure theory is developed to predict the levitation force of the proposed bearing system. The levitation force model is then integrated into the model of the electro-mechanical system to describe the total dynamic behavior of the bearing system. Experimental results are carried out using a prototype system, which show good agreement with the calculation.

Cellulose Electro-Active Paper (EAPap) has been discovered as a smart material that can be used as a sensor and actuator [1]. It has many advantages in terms of low voltage operation, light weight, low power consumption, low cost, biocompatibility and biodegradability. EAPap is made with cellulose paper coated with thin electrodes. EAPap shows a reversible and reproducible bending movement as well as longitudinal displacement under electric field. The out-of-plane bending deformation is useful for achieving flapping wings, micro-insect robots, and smart wall papers. On the other hand, in-plane strains, such as extension and contraction of EAPap materials are also promising for artificial muscle applications. The actuation principle of cellulose EAPap bending actuator is known to be a combination of piezoelectric effect and ion migration effect. This paper presents further investigation of cellulose EAPap for actuator, sensor and MEMS devices. Piezoelectricity is one of major actuating mechanism of cellulose EAPap. Cellulose is a complex anisotropic material. Aligning cellulose fibers in the fabrication process is a critical parameter to improve mechanical and electromechanical properties of EAPap such as stiffness, strength, piezoelectricity and so on. Cotton cellulose fibers are dissolved into a solution using NaOH/urea and DMAc/LiCl methods. In the later method, the dissolution and shaping of cellulose can be carried out by DMAc/LiCl. Cellulose pulp was mixed with lithium chloride (LiCl) and dehydrated by heating. After adding DMAc (N, N-dimethylacetamide) to the mixture, swell it in room temperature. By heating it a solution formation can be obtained. There are some issues on eliminating solvent and ions and regenerating a pure cellulose films. The material processing all about EAPap has been introduced [2, 3]. Wet drawn stretching method is used in the fabrication process of cellulose film to increase its mechanical and electromechanical properties. This wet-drawn cellulose EAPap is termed as Piezo-Paper. Cellulose EAPap material can be customized to satisfy the material requirement for specific applications. Piezo-Paper can be used for strain sensors, vibration sensors, ultrasonic transducers, SAW devices, speakers, microphones, stack actuators, bending actuators and MEMS devices. Figure 1 shows some applications. Piezoelectric charge constant of Piezo-Paper is 70 pC/N. Details of piezoelectric characteristics of Piezo-Paper and its applications are presented in this paper. Micro-fabrication on cellulose EAPap has many applications, for example, MEMS sensors, e-Paper, thin film transistor (TFT), and even microwave-driven EAPap actuator. To develop microwave-driven EAPap actuator, rectenna (rectifying antenna) has been developed [4]. Rectenna can rectify microwaves and feed dc power without wire. Thus, this technology has many applications. To fabricate the rectenna array on cellulose EAPap, micro patterning of metallic layer and Schottky diode fabrication were studied. The Schottky diode fabrication gives the possibility of TFT on cellulose sheet. Advancing from this technology, SAW (Surface Acoustic Wave) device fabrication for humidity sensor is possible. The devices fabrication along with the characterization and their demonstration will be shown. Cellulose EAPap technology will bring the dream of flying magic paper into real world in the near future.