Electromechanical impedance–based structural health monitoring method had attracted several researchers in the recent past for aerospace, civil, mechanical, timber and biological structures. Smart materials such as piezoelectric (lead zirconate titanate) and macro fibre composite transducers are either surface bonded or embedded inside the host structure to be monitored. These smart materials with an applied input sinusoidal voltage interact with the structure, to sense, measure, process and detect any change in the selected variables (stress, damage) at critical locations. These can be categorized as wire-based ‘advanced non-destructive testing’, wireless-based ‘battery-powered lead zirconate titanate/macro fibre composite’ and energy harvesting–based ‘self-powered lead zirconate titanate/macro fibre composite’ methods. Most importantly, the effectiveness of these electromechanical impedance–methods can be classified into active and passive based on the properties of the material, the component and the structure to be monitored. Furthermore, they also depend on variables to be monitored and interaction mechanism due to surface bonding or embedment. This article presents some of the important developments in monitoring and the path forward in wired, wireless and energy harvesting methods related to electromechanical impedance–based structural health monitoring for metals and non-metals.

Asia is the largest and most populous continent in the world with over 45 million square kilometers of land mass and 4.5 billion people. Asia is characterized by numerous densely populated cities. Structural health monitoring is a non-issue for the underdeveloped countries where basic amenities of survival are more important. However, structural health monitoring is crucial for the developing countries, especially those with densely populated cities like Singapore, Mumbai, and Hong Kong, where any infrastructural failure could be devastating to their society and economy. Structural health monitoring of mechanical and aerospace structures is mostly similar worldwide, but of civil infrastructures could vary due to socio-economic, cultural, geographical, and governmental reasons across countries, and even across states within the same country. This article, which is an enhancement to the keynote paper of the International Workshop on Structural Health Monitoring (IWSHM 2015, Stanford University, USA), presents some of the better known structural health monitoring studies of key civil infrastructures in a few Asian countries. In addition, the authors' research and applications of structural health monitoring technology carried out at the Nanyang Technological University for civil infrastructures in Singapore are presented. At the end, the authors also discuss recent work on energy harvesting using piezoelectric transducers as an alternative to wired structural health monitoring for automated and self-powered structural health monitoring.

Damage detection using electromechanical (EM) impedance in structural health monitoring (SHM) of engineering structures is rapidly emerging as a useful technique. In the EM impedance method, piezoceramic (PZT) transducers are either surface bonded to or embedded inside the host structure and are subjected to electric actuation. The EM admittance signatures of the PZT transducers, which consist of real and imaginary parts, serve as indicators to predict the health/integrity of the host structure. However, in real life, structural components such as slabs, beams and columns are constantly subjected to some form of external loading. The EM admittance signature obtained for such a constantly loaded structure is different from that obtained when damages are present in the structure. This paper presents an experimental and statistical investigation to show the influence of loading on EM admittance signatures. It is also observed that the susceptance signature is a better indicator than the conductance signature for detecting in situ stress in the host structure. This observation is further supported by a statistical analysis. This paper is expected to be useful for the non-destructive evaluation of engineering structures with external loading.

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Reliable structural health monitoring (SHM) including nondestructive evaluation (NDE) is essential for safe operation of infrastructure systems. Effective monitoring of the rock components of civil infrastructures such as tunnels and caverns remains challenging. The feasibility of employing smart optical fibre sensor (OFS) and piezoelectric impedance sensor made up of lead zirconate titanate (PZT) for comprehensive health monitoring of rocks, covering load history monitoring/retrieval as well as damage assessment is presented in this paper. The rock specimens are subjected to cyclic loading and their conditions are continuously monitored using OFS and PZT sensors. OFS based multiplexed fibre Bragg grating (FBG) sensors are surface bonded on the rock specimens. Their strain sensing performance is compared with the conventional electric strain gauges (ESGs). In addition, PZT patches are also bonded on the specimens to study the damage pattern during different loading cycles. Unlike the FBGs or ESGs, PZT patches are used as bi-functional sensors and actuators, enabling them to be efficient detectors of incipient damages using the principle of electromechanical impedance. The experimental study demonstrated superior performance of these smart FBG and PZT impedance sensors. This work is expected to be useful for SHM based NDE application of rock structures such as caverns and tunnels.

SUMMARY In the last decade, electromechanical impedance (EMI)‐based monitoring technique using piezoceramic (PZT) sensors have been successfully implemented in health monitoring of lab‐sized engineering structures. However, its implementation in real life application, such as monitoring underground support structures, has not been done before. In general, the EMI technique utilizes the unique EMI signature where any changes in the signature during the period of monitoring indicate possible damage in the host structure. This paper presents a part of monitoring results of the soil excavation carried out for the construction of new mass road transport (MRT) station in the southern part of Singapore using a PZT‐based EMI technique. The MRT site consists of typical clayed soil of varying properties along the depth of excavation. To prevent the soil collapse during excavation, temporary support structures were laid with suitable monitoring systems. The paper presents the results obtained from the PZT sensors and the comparisons with conventional measurement devices. However, there were no damages reported in the structure, and hence the PZT sensors, which were initially aimed to capture possible damages, were used later to capture load variations on the struts due to the surrounding soil. Copyright © 2010 John Wiley & Sons, Ltd.

In the past few decades, piezoceramic (PZT) transducers have been used extensively in the vibration and noise control of engineering structures. However, in the last decade, PZT transducers have also been used in electromechanical impedance (EMI) based methods of structural health monitoring (SHM). In the EMI methods, the PZT transducers are either surface bonded using adhesive or wrapped with a protective cover and then bonded or embedded inside the host structure. They are then subjected to excitation in the desired frequency ranges to predict the electromechanical (EM) admittance signatures. These EM signatures serve as an indicator of the health/integrity of the structure. The existing PZT-structure interaction methods consider both the PZT transducer and the adhesive layer to be negligible in mass and are thus ignored. However, for wrapped PZT, the presence of thick adhesive significantly reduces the magnitude of the EM signature. This paper presents the formulation of a three-dimensional (3D) interaction model of a PZT-structure which considers the mass of both the PZT transducers and the adhesive. The model is generic in nature compared to the existing interaction models. The model is verified experimentally and is expected to be applicable to the non-destructive evaluation (NDE) of most engineering structures.

The recent advent of highly durable engineering materials and the advancement of latest structural design theories have made possible the fabrication of more efficient engineering structures. However, the safety and reliability of these structures remains the primary challenge and concern for engineers. Especially, for those structures which involve human traffic and huge investments such as the aerospace structures and bridges. Therefore, there is a compelling need to have high-quality online structural health monitoring (SHM) of such structures. The development of a real-time, in-service, and smart material-based SHM method has recently attracted the interest of a large number of academic and industrial researchers. In the recent past, piezoceramic (PZT) transducer has evolved as an efficient smart material, which is usually employed in electro mechanical impedance (EMI) and guided ultrasonic wave propagation techniques. In EMI technique, a PZT transducer interact with the host structure to result in unique health signature, as an inverse function of structural impedance, when it is subjected to high-frequency structural excitations in the presence of electric field. Using the self-actuating and sensing capabilities of PZT transducers, the EMI models attempted to detect loadings on, and damages in, the structures to be monitored. This article reviews some of the advancements in the field of PZT-based SHM made over the past two decades in engineering structures. This article also provides an insight into the possible future work and improvements required for PZT-based EMI technique.