The deployment cost of the structural health monitoring (SHM) system is the major argument against the more widespread use of the structural health monitoring techniques. Optimization of sensor pla...

Optimization of sensor placement for structural health monitoring: a review:

Electroactive polymers, generally used as actuators, offer very promising levels of performance in sensor mode, particularly in scavenging ambient mechanical energy applications. New, innovative, high performance, flexible generators can be developed to supply low consumption systems. This technology has the potential to be an alternative to traditional solutions based on electromagnetism, electrostatic or piezoelectricity, which are rigid solutions, mostly used in the high frequency range. This paper reports an extensive investigation of electroactive polymers for energy scavenging applications. The operating principle, properties and analytic models of six polymers are presented and compared in terms of scavenging energy density, operating area, advantages and inconveniences.

https://iopscience.iop.org/article/10.1088/0964-1726/19/8/085012/pdf

Comparison of electroactive polymers for energy scavenging applications

The coupling of waves between the piezoelectric generators, detectors, and propagating media is challenging due to mismatch in the acoustic properties. The mismatch leads to the reverberation of waves within the transducer, heating, low signal-to-noise ratio, and signal distortion. Acoustic impedance matching increases the coupling largely. This article presents standard methods to match the acoustic impedance of the piezoelectric sensors, actuators, and transducers with the surrounding wave propagation media. Acoustic matching methods utilizing active and passive materials have been discussed. Special materials such as nanocomposites, metamaterials, and metasurfaces as emerging materials have been presented. Emphasis is placed throughout the article to differentiate the difference between electric and acoustic impedance matching and the relation between the two. Comparison of various techniques is made with the discussion on capabilities, advantages, and disadvantages. Acoustic impedance matching for specific and uncommon applications has also been covered.

A Review of Acoustic Impedance Matching Techniques for Piezoelectric Sensors and Transducers.

Vibration-based spatial damage identification in honeycomb-core sandwich composite structures using wavelet analysis

Any electric transmission lines involving the transfer of power or electric signal requires the matching of electric parameters with the driver, source, cable, or the receiver electronics. Proceeding with the design of electric impedance matching circuit for piezoelectric sensors, actuators, and transducers require careful consideration of the frequencies of operation, transmitter or receiver impedance, power supply or driver impedance and the impedance of the receiver electronics. This paper reviews the techniques available for matching the electric impedance of piezoelectric sensors, actuators, and transducers with their accessories like amplifiers, cables, power supply, receiver electronics and power storage. The techniques related to the design of power supply, preamplifier, cable, matching circuits for electric impedance matching with sensors, actuators, and transducers have been presented. The paper begins with the common tools, models, and material properties used for the design of electric impedance matching. Common analytical and numerical methods used to develop electric impedance matching networks have been reviewed. The role and importance of electrical impedance matching on the overall performance of the transducer system have been emphasized throughout. The paper reviews the common methods and new methods reported for electrical impedance matching for specific applications. The paper concludes with special applications and future perspectives considering the recent advancements in materials and electronics.

A Review of Electric Impedance Matching Techniques for Piezoelectric Sensors, Actuators and Transducers

We report on the Lamb wave type guided wave propagation in honeycomb core sandwich structures. An experimental study supported by theoretical evaluation of the guided wave characteristics is presented that proves the potential of Lamb wave type guided wave for detection of damage in sandwich structures. A sandwich panel is fabricated with planar dimension of 600 mm x 600 mm, having a core thickness of 7 mm, cell size of 5 mm and 0.1 mm thick aluminum face sheets. Thin piezoelectric patch actuators and sensors are used to excite and sense a frequency band limited guided wave with a central frequency. A linear phased array of piezoelectric patch actuators is used to achieve higher signal strength and directivity. Group velocity dispersion curves and corresponding frequency response of sensed signal are obtained experimentally. Linearity between the excitation signal amplitude and the corresponding sensed signal amplitude is found for certain range of parameters. The nature of damping present in the sandwich panel is monitored by measuring the sensor signal amplitude at various different distances measured from the center of the linear phased array. Indentation and low velocity impact induced damages of increasing diameter covering several honeycomb cells are created. Crushing of honeycomb core with rupture of face sheet is observed while introducing the damage. The damages are then detected experimentally by pitch-catch interrogation with guided waves and wavelet transform of the sensed signal. Signal amplitudes are analyzed for various different sizes of damages to differentiate the damage size/severity. Monotonic changes in the sensor signal amplitude due to increase in the damage size has been established successfully. With this approach it is possible to locate and monitor the damages with the help of phased array and by tracking the wave packets scattered from the damages. (C) 2012 Elsevier Ltd. All rights reserved.

Guided wave based detection of damage in honeycomb core sandwich structures

Simulation has become a prerequisite in engineering and science today for visualization of ideas and concepts. In non-destructive evaluation, simulation is increasingly used to show how an inspection method functions with regard to the component to be inspected and is even used for determining the probability of detection of a respective flaw with regard to the inspection method applied. Probability of detection in non-destructive evaluation is optimized in a way that the best sensor positions, as well as sensor tracking paths, can be found through simulation. In classical non-destructive evaluation, a transducer or transducer array can be virtually moved over the surface of a component to be inspected until a full capture of the component's surface and hopefully volume is achieved in terms of the inspection process. However, with structural health monitoring, no movement of the transducers is possible in case those become an integral and hence fixed part of the component considered. Determining the optimum position of a respective structural health monitoring transducer network can therefore only be achieved through optimization procedures, where numerical simulation is possibly the only viable solution to get this done. Establishing a numerical simulation platform for structural health monitoring purposes has been the major objective of the recently completed INDEUS (Integration of Non-Destructive Evaluation-based Ultrasonic Simulation) project, which is described in this article. The open simulation platform includes different simulation tools, where the requirements and options for further extension of those tools and different test cases applied for validation so far are described. The target is to even simulate real complex structures such as applied in civil, aeronautical, and other engineering disciplines made of metallic and polymer-based monolithic and composite materials where the digital models are inherited from traditional computer-aided design and finite element-based designs. This lays the ground for determining the probability of damage for a given loading condition and structure, and the propagation of guided waves in the structure considered for an undamaged and a damage tolerant condition. From those simulation results, the determination of an optimum configuration of sensing transducers for a given set of actuating transducers is then shown for a guided wave-based structural health monitoring system solution to be designed allowing the tolerable damage to be detected reliably.

Integration of Non-Destructive Evaluation-based Ultrasonic Simulation: A means for simulation in structural health monitoring

Applications of Linear Phased array concept have been extended from electromagnetic antennae to many other areas due
to their capability to direct, magnify and pick up energy in and from desired directions. Apart from radar, optics and
medical imaging, one such growing area is in the non-destructive testing of structures. The extensive use of linear array
can be attributed to the attenuation of the waves generated in the structure due to inherent damping and loss in the
materials and discontinuities. Linear phased arrays are used as actuator in ultrasonic imaging and diagnostics to magnify
the energy at a given direction or point in the structure. In the present work the property of amplifying the wave
generated in a particular direction is exploited and is studied on a carbon composite structure. Almost all of the existing
imaging methods in context of phased array are based on through thickness and bulk wave modes. In the present research
we employ Lamb wave which propagates in a doubly bounded media like structural panels. The spreading of energy in a
composite laminate is studied in the form of lobe patterns obtained using amplitude of symmetric Lamb wave mode (S0)
with a particular orientation of the linear array with fiber direction. The effect of damage in the form of a delamination in
a CFRP composite plate on the lobe pattern is analyzed.

Linear phased array of piezoelectric transducers for delamination monitoring in a composite laminate using Lamb waves

Rapid localization of multiple damages and non-intrusive imaging of these damages can greatly help in reducing the inspection time for guided wave based NDE and in integrated SHM. A circular array of sensor-actuator network was employed for scanning a panel type structure for quantitative damage detection besides nonintrusive imaging with the help of the available reduced number of measurements and a mathematical model. This method employs Lamb waves with a narrow frequency band of excitation. What is advantageous in the proposed method is that, a circular array of sensor-actuator network with a very small physical area of coverage is sufficient to construct the virtual image of an entire panel without physically scanning using the actuators or the sensors. By employing this technique, surface breaking cracks on either inaccessible or accessible face and of size as small as 3 mm in a typical aircraft structural panel are detected with a relatively low frequency band of 100-250 kHz. The coordinates of the damages are estimated from the experimentally obtained signals using triangulation technique. Change in the wavelet coefficient with respect to a baseline signal is used to construct a quantitative relationship with damage size parameters. A damage index is then estimated by integrating the corresponding wavelet coefficient over the frequency scales of interest and over the time window of interest according to the group velocity dispersion. The magnitude of the damage index shows the severity of damage. Further correlation of these quantities with failure mechanisms and prognosis remain an open area of research.

Rapid Localization and Ultrasonic Imaging of Multiple Damages in Structural Panel with Piezoelectric Sensor-Actuator Network

Structural Health Monitoring (SHM) systems require integration of non-destructive technologies into structural design and operational processes. Modeling and simulation of complex NDE inspection processes are important aspects in the development and deployment of SHM technologies. Ray tracing techniques are vital simulation tools to visualize the wave path inside a material. These techniques also help in optimizing the location of transducers and their orientation with respect to the zone of interrogation. It helps in increasing the chances of detection and identification of a flaw in that zone. While current state-of-the-art techniques such as ray tracing based on geometric principle help in such visualization, other information such as signal losses due to spherical or cylindrical shape of wave front are rarely taken into consideration. The problem becomes a little more complicated in the case of dispersive guided wave propagation and near-field defect scattering. We review the existing models and tools to perform ultrasonic NDE simulation in structural components. As an initial step, we develop a ray-tracing approach, where phase and spectral information are preserved. This enables one to study wave scattering beyond simple time of flight calculation of rays. Challenges in terms of theory and modelling of defects of various kinds are discussed. Various additional considerations such as signal decay and physics of scattering are reviewed and challenges involved in realistic computational implementation are discussed. Potential application of this approach to SHM system design is highlighted and by applying this to complex structural components such as airframe structures, SHM is demonstrated to provide additional value in terms of lighter weight and/or longevity enhancement resulting from an extension of the damage tolerance design principle not compromising safety and reliability.

Nibir Chakraborty

Papers

Guided wave based detection of damage in honeycomb core sandwich structures

Integration of Non-Destructive Evaluation-based Ultrasonic Simulation: A means for simulation in structural health monitoring

Linear phased array of piezoelectric transducers for delamination monitoring in a composite laminate using Lamb waves

Rapid Localization and Ultrasonic Imaging of Multiple Damages in Structural Panel with Piezoelectric Sensor-Actuator Network

Modeling ultrasonic NDE and guided wave based structural health monitoring