F. Song

Guided wave propagation in honeycomb sandwich structures using a piezoelectric actuator/sensor system

Due to the complex nature of such composite structures, an understanding of the guided wave propagation mechanism in honeycomb composite panels with different frequencies inherently imposes many challenges. In this paper, a numerical simulation is first conducted to investigate the wave propagation mechanism in honeycomb sandwich structures using piezoelectric actuators/sensors. In contrast to most of the previous work, elastic wave responses based on the real geometry of the honeycomb core are obtained by using the finite element method (FEM). Based on the simulation, the global guided waves in the composite can be observed when the loading frequency is low and the leaky guided waves in the skin panel are found when the loading frequency is sufficiently high. The applicability of the homogenization technique for a celled core is discussed. The effects of cell geometry on the wave propagation are also demonstrated. Experimental testing is finally conducted to validate the results of numerical simulation and very good agreement is observed. Specifically, some guided wave propagation characteristics such as group velocity dispersion and mode tuning capabilities with the presence of a honeycomb core are discussed.

On the study of surface wave propagation in concrete structures using a piezoelectric actuator/sensor system

Concrete structures have been used widely in civil infrastructural systems. Due to the complex nature of the microstructure, nondestructive testing (NDT) of concrete inherently imposes many challenges, which can cause severe limitations on the resolution and the sensitivity of the signals observed. In this study, a numerical simulation based on the finite element (FE) model is first performed to investigate surface wave generation and reception using piezoelectric actuators/sensors, especially in relatively higher frequency cases. The results provide a basic understanding of some features of the microstructure effects on the surface wave propagation. Experimental testing is then conducted to validate the numerical simulation. The group velocity dispersion curves of the surface waves, which are very useful for future damage detection in concrete materials, are obtained from both numerical and experimental results. The good agreement shows the great potential and feasibility of using piezoelectric actuators/sensors to generate and receive surface waves for quantitative damage detection in concrete structures.

Coupled piezo-elastodynamic modeling of guided wave excitation and propagation in plates with applied prestresses

Plate-like aerospace engineering structures are prone to mechanical/residual preloads during flight operation. This article focuses on the quantitative characterization of applied prestress effects on the piezoelectrically-induced guided wave propagation, which has been widely used in structural health monitoring systems. An analytical model considering coupled piezo-elastodynamics is developed to study dynamic load transfer between a surface-bonded thin piezoelectric actuator and a prestressed plate. The accuracy of the analytical prediction is evaluated by the comparison with the finite element analysis. Based on the developed model, the load-dependent guided wave signal variation in both time-of-flight and amplitude is determined, and its dependence on loading frequency and host material properties is also discussed. It is found that the guided wave signal variation due to the prestress could be significant under some circumstances. A signal difference coefficient is finally proposed to quantitatively assess the signal variation caused by different prestresses. This study can serve as a theoretical foundation for the development of the real-time piezo-guided wave–based structural health monitoring system in a realistic loading environment.

Multiple Debondings' Detection in Honeycomb Sandwich Structures Using Multi-Frequency Elastic Guided Waves

Due to the complex nature of sandwich structures, damage detection in honeycomb sandwich structures inherently imposes many challenges. In this study, leaky guided wave properties generated by piezoelectric wafer actuators/sensors in honeycomb sandwich structures are first simulated by the finite element method. In the numerical model, the detailed honeycomb core geometry is considered. Differential features due to presence of debonding are determined through an appropriate damage index analysis of the signals at the normal and debonded conditions. The image of the debonding is formed by using a probability analysis of the leaky guided wave at each frequency. The final image of the structure can be fused from multi-frequency leaky guided waves. A new method for multi-debonding detection is proposed. Based on the analysis, information about the debondings in the honeycomb sandwich structures can be quantitatively characterized.

Characterization of guided wave propagation with piezoelectric wafer actuators in prestressed plates

Plate-like aerospace engineering structures are prone to mechanical/residual load during flight operation. The mechanical/residual prestresses can cause significant changes in guided-wave (GW) propagation for structural health monitoring (SHM) systems. The paper focuses on the characterization of the GW propagation using surfacebonded piezoelectric wafer actuators in metallic spacecraft plates under prestresses. First, a new in-plane analytical model with coupled piezo-elastodynamics is proposed to quantitatively capture the dynamic load transfer between a thin piezoelectric actuator bonded onto an isotropic plate that is subject to prestresses. Based on the developed model, effects of prestresses on the GW propagation generated by piezoelectric actuators are then analyzed and demonstrated. It can be found that the both time-of-flight and amplitude of wave responses can be affected by the presence of prestresses in plates. The results hopefully provide useful information for the real-time SHM.

Quantitative modeling of dynamic behavior of a piezoelectric wafer actuator bonded to an elastic plate for guided wave propagation

Piezoelectric wafer actuators (piezo-actuators) have been extensively used in integrated structural health monitoring systems to generate ultrasonic guided waves (GWs) for structural damage interrogation. The big issue surrounding precise characterization of piezoelectrically excited GWs is addressing the dynamic interfacial stress between the piezo-actuator and the host structure. In this paper, an analytical actuator model is developed to quantitatively describe the dynamic load transfer between a bonded thin piezo-actuator and an isotropic plate under in-plane mechanical and electrical loading. The piezoelectrically induced GW responses are studied by coupling the actuator dynamics with the Rayleigh-Lamb equations and solving the resulting integral equations in terms of the interfacial shear stress. Typical examples are provided to show the capability of the current actuator model to capture the effects of the geometry and the loading frequency upon the load transfer. The analytical prediction of transient GW mode signals from the proposed model is compared with finite element simulation results for various excitation frequencies, and excellent agreement can be observed especially for high-frequency cases, which is a useful extension to the ultrasonic frequencies of most of existing analytical solutions for low frequency approximations.

Wave Propagation in Elastic Media with Micro/Nano-Structures

Microand nano-scale materials and structures such as plateor beam-like structures with submicron or nano thicknesses have attracted considerable interest from the scientific community due to the increasingly strong demands of miniaturization in the fields of microelectronics and nanotechnology. More and more nano-structures, e.g. ultra-thin films, nanowires and nanotubes, have been fabricated and served as the basic building blocks for microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) (Jin et al., 1998; Craighead, 2000; Husain et al., 2003; Feng et al., 2007). For long-term stability and reliability of various devices at nanoscale, researchers should possess a deep understanding and knowledge of mechanical properties of nano materials and structures, especially for dynamic properties. Among many techniques, high-frequency acoustic wave technique has been regarded as one of very efficient nondestructive methods to characterize elastic media with microor nanostructures. (Hernaandez et al., 2002) used high-frequency laser-excited guided acoustic waves to estimate the in-plane mechanical properties of silicon nitride membranes. Mechanical properties and residual stresses in the membranes were evaluated from measured acoustic dispersion curves. The mean values of the Young’s modulus and density of three nanocrystalline diamond films and a free standing diamond plate were determined by analyzing the dispersion of laser-generated surface waves (Philip et al., 2003). Recently, growing interest of using terahertz (THz) waves in nanoscale materials and nano-photonic or nano-phononic devices has opened a new topic on the wave characteristics of nanomaterials (Schneider et al., 2000, Vollmannn et al., 2004; Ramprasad & Shi, 2005; Sampathkumar et al., 2006). As dimensions of the material become smaller, however, their resistance to deformation is increasingly determined by internal or external discontinuities (such as surfaces, grain boundary, strain gradient, and dislocation). Although many sophisticated approaches for predicting the mechanical properties of nanomaterials have been reported, few addressed the challenges posed by interior nanostructures such as the surfaces, interfaces, structural discontinuities and deformation gradient of the nanomaterials under extreme loading conditions. The use of atomistic simulation may be a potential solution in the long run. However, it is well known that the capability of this approach is

Online debonding detection in honeycomb sandwich structures using multi-frequency guided waves

Due to the complex nature of sandwich structures, development of the online structural health monitoring system to detect damages in honeycomb sandwich panels inherently imposes many challenges. In this study, the leaky guided wave propagation in the honeycomb sandwich structures generated by piezoelectric wafer actuators/sensors is first simulated numerically based on the finite element method (FEM). In the numerical model, the real geometry of the honeycomb core is considered. To accurately detect debonding in the honeycomb sandwich structures, signal processing based on continuous wavelet transform is adopted to filter out the unwanted noise in the leaky Lamb wave signals collected from the experimental testing. A correlation analysis between the benchmark signals at the normal condition and those recorded at the debonded condition is then performed to determine the differential features due to the presence of debonding. Finally, the image of the debonding is formed by using a probability analysis. Specifically, fusing images acquired from multi-frequency leaky Lamb waves are obtained to enhance the quality of the final image of the structure. The location and size of the debonding in the honeycomb sandwich structures are estimated quantitatively.

Wireless integration, design, modeling, and analysis of nanosensors, networks, and systems: a systems engineering approach

The design of nanosensor networks and systems encompass multiple areas of research, which include: Design of nanosensors and modeling; Design of wireless interfaces; Design of reliable sensor networks that sense and collect data reliably; Design of backbone networks capable of reliably transporting collected data to remote servers; Design of secure servers for data transfer. This paper provides a systems engineering framework and provides insights into the above design issues.

Elastic wave propagation in hexagonal honeycomb sandwich composite by using piezoelectric actuators/sensors

Honeycomb composite structures have been widely used in aerospace and aeronautic industries due to their unique characteristics. Due to the complex nature of honeycomb composite with the celled core, structural health monitoring (SHM) of honeycomb composite panels inherently imposes many challenges, which requires a detailed knowledge of dynamic elastic responses of such complex structures in a broad frequency domain. This paper gives numerical and experimental analyses of elastic wave propagation phenomena in sandwich panels with a honeycomb core, especially when the frequency domain of interest is relative high. Numerical simulation based on the Finite Element (FE) method is first performed to investigate wave generation and reception using piezoelectric actuators/sensors. The effectiveness of homogenized core model is discussed, compared with the dynamic responses based on honeycomb celled core model. The reliability of the simulated wave will be verified with the experimental results. Specific attention will be paid on core effects on group wave velocity. This research will establish a solid theoretical foundation for the future study of the structural health monitoring in the composites.