There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Guided Lamb waves for identification of damage in composite structures: A review

In this paper we summarize the hardware and software issues of impedance-based structural health moni- toring based on piezoelectric materials. The basic concept of the method is to use high-frequency structural excitations to monitor the local area of a structure for changes in structural impedance that would indicate imminent damage. A brief overview of research work on experimental and theoretical stud- ies on various structures is considered and several research papers on these topics are cited. This paper concludes with a discussion of future research areas and path forward. Piezoelectric materials acting in the "direct" manner pro- duce an electrical charge when stressed mechanically. Con- versely, a mechanical strain is produced when an electrical field is applied. The direct piezoelectric effect has often been used in sensors such as piezoelectric accelerometers. With the converse effect, piezoelectric materials apply local- ized strains and directly influence the dynamic response of the structural elements when either embedded or surface bonded into a structure. Piezoelectric materials have been widely used in structural dynamics applications because they are lightweight, robust, inexpensive, and come in a variety of forms ranging from thin rectangular patches to complex shapes being used in microelectromechanical systems (MEMS) fabrications. The applications of piezoelectric mate- rials in structural dynamics are too numerous to mention and are detailed in the literature (Niezrecki et al., 2001; Chopra, 2002). The purpose of this paper is to explore the importance and effectiveness of impedance-based structural health mon- itoring from both hardware and software standpoints. Imped- ance-based structural health monitoring techniques have been developed as a promising tool for real-time structural dam- age assessment, and are considered as a new non-destructive evaluation (NDE) method. A key aspect of impedance-based structural health monitoring is the use of piezoceramic (PZT) materials as collocated sensors and actuators. The basis of this active sensing technology is the energy transfer between the actuator and its host mechanical system. It has been shown that the electrical impedance of the PZT material can be directly related to the mechanical impedance of a host structural component where the PZT patch is attached. Uti- lizing the same material for both actuation and sensing not only reduces the number of sensors and actuators, but also reduces the electrical wiring and associated hardware. Fur- thermore, the size and weight of the PZT patch are negligible compared to those of the host structures so that its attach- ment to the structure introduces no impact on dynamic char- acteristics of the structure. A typical deployment of a PZT on a structure being monitored is shown in Figure 1. The first part of this paper (Sections 2 and 3) deals with the theoretical background and design considerations of the impedance-based structural health monitoring. The signal processing of the impedance method is outlined in Section 4. In Section 5, experimental studies using the impedance approaches are summarized and related previous works are listed. Section 6 presents a brief comparison of the imped- ance method with other NDE approaches and, finally, sev- eral future issues are outlined in Section 7. 2. Theoretical Background

Overview of Piezoelectric Impedance-Based Health Monitoring and Path Forward

Vibration-based model-dependent damage (delamination) identification and health monitoring for composite structures — a review

This paper presents a state-of-the-art review on a hot topic in the literature, i.e., vibration based energy harvesting techniques, including theory, modelling methods and the realizations of the piezoelectric, electromagnetic and electrostatic approaches. To minimize the requirement of external power source and maintenance for electric devices such as wireless sensor networks, the energy harvesting technique based on vibrations has been a dynamic field of studying interest over past years. One important limitation of existing energy harvesting techniques is that the power output performance is seriously subject to the resonant frequencies of ambient vibrations, which are often random and broadband. To solve this problem, researchers have concentrated on developing efficient energy harvesters by adopting new materials and optimising the harvesting devices. Particularly, among these approaches, different types of energy harvesters have been designed with consideration of nonlinear characteristics so that the frequency bandwidth for effective energy harvesting of energy harvesters can be broadened. This paper reviews three main and important vibration-to-electricity conversion mechanisms, their design theory or methods and potential applications in the literature. As one of important factors to estimate the power output performance, the energy conversion efficiency of different conversion mechanisms is also summarised. Finally, the challenging issues based on the existing methods and future requirement of energy harvesting are discussed.

A comprehensive review on vibration energy harvesting: Modelling and realization

Health monitoring of composite repairs and joints using optical fibres

Optical fibre sensors for health monitoring of bonded repair systems

This paper reports on an investigation into the use of a vibro-impact approach to construct a relatively broadband kinetic energy harvester. Potentially, the vibro-impacting process may be exploited as an autotuning mechanism for energy harvesting in an environment where the source vibration spectrum varies in time, such as an aircraft in flight. The energy harvester examined in this paper is based on a vibro-impacting oscillator with double-sided, symmetrical, piezoelectric bimorph-stops. The energy harvester operates in the frequency range of 100–113 Hz and has a (non-optimized) maximum energy of 5.3 mW from an rms host vibration of 450 mG.

A low profile vibro-impacting energy harvester with symmetrical stops

This article reports on a vibration energy harvesting approach that uses a magnetoelectric (ME) transducer to harvest energy from bi-axial vibrations. A bi-axial oscillator is created using a permanent-magnet/ball-bearing arrangement, which has the added benefit of permitting a relatively compact design. The magnet produces a bi-axial restoring force on the bearing, and as the bearing oscillates, it steers magnetic field through the transducer thereby producing an oscillating charge that can be harvested. A simple laboratory demonstrator of a bi-axial ME harvester was created using a magnetostrictive/piezoelectric laminate transducer, and was shown to produce a peak rms power of 121 μW from an rms acceleration of 61 mG at 9.8 Hz.

A bi-axial magnetoelectric vibration energy harvester

A smart structure is one that senses its internal state and external environment, and based on the information gained responds in a manner that fulfils its functional requirements. The primary advantage of moving towards smart structures technology is the potential cost benefit of condition-based maintenance strategies and the prospective life extension that may be achieved through in situ health monitoring. The monitoring of operational health and performance, and diagnosis of any faults as they occur, is a relatively new concept that is being developed globally to provide advantages of safer , more reliable and affordable structures. Health monitoring can be achieved by positioning (embedded or surface mounted) sensor systems on a structure to measure those physical parameters that are informative with respect to the state of the structural health. Information relating to the severity and location of damage, as well as to the nature of the loading is of obvious importance to this endeavour. In the aerospace industry bonded composite patches are increasingly being used to extend the operational life of ageing aircraft. The application of bonded composite patches to repair or reinforce defective metallic structures is widely acknowledged as an effective and versatile procedure. Such patches have been successfully applied to the repair of cracked structures, to the reinforcement of components subject to material loss due to corrosion damage and as a general means of stress reduction through the provision of a supplementary load path. However, certification requirements mandate the need for a methodology for monitoring the damage state of both the defective underlying structure and of the repair. In this case the concept of smart structures can be used to detect damage in the repair itself as well as monitor damage growth in the parent structure. This paper reports on the development of a `perceptive repair' or `smart' system which will provide information on the in-service performance of the repair and the associated structure. In this respect, this paper focuses on the detection of disbond in the adhesive layer between adhered and the metallic parent structure. One of the criteria of this smart system is that it must be economical, reliable and, preferably, self-powered. To this end, it was proposed that piezoceramic of piezoelectric material be utilized because of their ease of application. These materials were chosen because they can be used both as a actuator and as a sensor. This paper presents a set of numerical investigations performed to highlight the viability of using this material system, and the associated signal analysis that can be employed to detect the presence or the development of disbonds in the adhesive in a bonded repair situation.

http://iopscience.iop.org/article/10.1088/0964-1726/9/4/309/pdf

Steve C Galea

Papers

Health monitoring of composite repairs and joints using optical fibres

Optical fibre sensors for health monitoring of bonded repair systems

A low profile vibro-impacting energy harvester with symmetrical stops

A bi-axial magnetoelectric vibration energy harvester

Damage Detection in Bonded Repairs using Piezoceramics