Ajay Raghavan

Bio: Ajay Raghavan is an academic researcher from PARC. The author has contributed to research in topics: Structural health monitoring & Guided wave testing. The author has an hindex of 22, co-authored 65 publications receiving 2329 citations. Previous affiliations of Ajay Raghavan include Massachusetts Institute of Technology & Panasonic.

Review of guided-wave structural health monitoring

Abstract: In this paper we present the state of the art in the field of guided-wave structural health monitoring (SHM). We begin with an overview of damage prognosis, and a description of the basic methodology of guided-wave SHM. We then review developments from the open literature in various aspects of this truly multidisciplinary field. First, we discuss different transducer technologies, including both piezoelectric and non-conventional popular and non-conventional piezoelectric transducers. Next, we examine guided-wave theory, tracing its early history down to modern developments. Following this, we detail the efforts into models for guided-wave excitation by SHM transducers. Then, we review several signal processing related works. The next topic in Section 6 is guided-wave SHM system development, and we explore various packaging ideas, integrated solutions and efforts to examine robustness to different service conditions. We also highlight the broad spectrum of applications in which this technology has been tested. We then present some investigations that have attempted to combine guided-wave approaches with other complementary SHM technologies for better system performance. Finally, we propose desirable developments for further advancement of this field.

Finite-dimensional piezoelectric transducer modeling for guided wave based structural health monitoring

Abstract: Among the various schemes being considered for structural health monitoring (SHM), guided wave (GW) testing in particular has shown great promise. While GW testing using hand-held transducers for non-destructive evaluation (NDE) is a well established technology, GW testing for SHM using surface-bonded/embedded piezoelectric wafer transducers (piezos) is relatively in its formative years. Little effort has been made towards a precise characterization of GW excitation using piezos and often the various parameters involved are chosen without mathematical foundation. In this work, a formulation for modeling the transient GW field excited using arbitrary shaped surface-bonded piezos in isotropic plates based on the 3D linear elasticity equations is presented. This is then used for the specific cases of rectangular and ring-shaped actuators, which are most commonly used in GW SHM. Equations for the output voltage response of surface-bonded piezo-sensors in GW fields are derived and optimization of the actuator/sensor dimensions is done based on these. Finally, numerical and experimental results establishing the validity of these models are discussed.

Guided-wave signal processing using chirplet matching pursuits and mode correlation for structural health monitoring

Abstract: Signal processing algorithms for guided wave pulse echo-based structural health monitoring (SHM) must be capable of isolating individual reflections from defects in the structure, if any, which could be overlapping and multimodal. In addition, they should be able to estimate the time–frequency centers, the modes and individual energies of the reflections, which would be used to locate and characterize defects. Finally, they should be computationally efficient and amenable to automated processing. This work addresses these issues with a new algorithm employing chirplet matching pursuits followed by a mode correlation check for single point sensors. Its theoretical advantages over conventional time–frequency representations for SHM are elaborated. Results from numerical simulations and experiments in isotropic plate structures are presented, which show the capability of the proposed algorithm. Finally, the issue of in-plane triangulation is discussed and experimental work done to explore this issue is presented.

Effects of Elevated Temperature on Guided-wave Structural Health Monitoring

Abstract: Elevated temperatures can cause significant changes in guided-wave (GW) propagation and transduction for structural health monitoring (SHM). This work focuses on GW SHM using surface-bonded piezoelectric wafer transducers in metallic plates for the temperature range encountered in internal spacecraft structures (20—150°C). First, studies done to determine a suitable bonding agent are documented. This is then used in controlled experiments to examine changes in GW propagation and transduction using PZT-5A piezoelectric wafers under quasi-statically varying temperature (also from 20 to 150°C). Modeling efforts to explain the experimentally observed increase in time-of-flight and change in sensor response peak-to-peak magnitude with increasing temperature are detailed. Finally, these results are used in detection and location of mild and moderate damage using the pulse-echo GW testing approach within the temperature range.

Modeling of piezoelectric-based Lamb wave generation and sensing for structural health monitoring

Abstract: Among the various schemes being considered for Structural Health Monitoring (SHM), Lamb-wave testing has shown great promise. While Lamb-wave testing using hand-held transducers for Non Destructive Evaluation (NDE) is a well-established technology, Lamb-wave testing for SHM using surface-bonded/embedded piezos is a relatively new field. Little effort has been made towards a precise characterization of Lamb-wave excitation using piezos and often the various parameters involved are chosen without mathematical foundation. In this work, modeling of transient plane and circular-crested Lamb-wave generation and sensing using surface-bonded piezos in isotropic plates based on the 3-D linear elasticity equations is explored. Equations for the output voltage response of surface-bonded piezo-sensors in Lamb-wave fields are presented and optimization of the actuator/sensor geometry and materials is done based on those. Finally, numerical and experimental results to examine the validity of these models are discussed.

Structural Health Monitoring: A Machine Learning Perspective

Abstract: This book focuses on structural health monitoring in the context of machine learning. The authors review the technical literature and include case studies. Chapters include: operational evaluation, sensing and data acquisition, introduction to probability and statistics, machine learning and statistical pattern recognition, and data prognosis.

Tuned Lamb-Wave Excitation and Detection with Piezoelectric Wafer Active Sensors for Structural Health Monitoring

Abstract: The capability of embedded piezoelectric wafer active sensors (PWAS) to excite and detect tuned Lamb waves for structural health monitoring is explored. First, a brief review of Lamb waves theory is presented. Second, the PWAS operating principles and their structural coupling through a thin adhesive layer are analyzed. Then, a model of the Lamb waves tuning mechanism with PWAS transducers is described. The model uses the space domain Fourier transform. The analysis is performed in the wavenumber space. The inverse Fourier transform is used to return into the physical space. The integrals are evaluated with the residues theorem. A general solution is obtained for a generic expression of the interface shear stress distribution. The general solution is reduced to a closed-form expression for the case of ideal bonding which admits a closed-form Fourier transform of the interfacial shear stress. It is shown that the strain wave response varies like sin a, whereas the displacement response varies like sinc a. ...

Sustainability and in situ monitoring in battery development

Abstract: The development of improved rechargeable batteries represents a major technological challenge for this new century, as batteries constitute the limiting components in the shift from petrol (gasoline) powered to electric vehicles, while also enabling the use of more renewable energy on the grid. To minimize the ecological implications associated with their wider use, we must integrate sustainability of battery materials into our research endeavours, choosing chemistries that have a minimum footprint in nature and that are more readily recycled or integrated into a full circular economy. Sustainability and cost concerns require that we greatly increase the battery lifetime and consider second lives for batteries. As part of this, we must monitor the state of health of batteries continuously during operation to minimize their degradation. It is thus important to push the frontiers of operando techniques to monitor increasingly complex processes. In this Review, we will describe key advances in both more sustainable chemistries and operando techniques, along with some of the remaining challenges and possible solutions, as we personally perceive them.

Guided wave based structural health monitoring: A review

Abstract: The paper provides a state of the art review of guided wave based structural health monitoring (SHM). First, the fundamental concepts of guided wave propagation and its implementation for SHM is explained. Following sections present the different modeling schemes adopted, developments in the area of transducers for generation, and sensing of wave, signal processing and imaging technique, statistical and machine learning schemes for feature extraction. Next, a section is presented on the recent advancements in nonlinear guided wave for SHM. This is followed by section on Rayleigh and SH waves. Next is a section on real-life implementation of guided wave for industrial problems. The paper, though briefly talks about the early development for completeness,. is primarily focussed on the recent progress made in the last decade. The paper ends by discussing and highlighting the future directions and open areas of research in guided wave based SHM.