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

Efficient temperature compensation strategies for guided wave structural health monitoring

Structural health monitoring (SHM) is being widely evaluated by the aerospace industry as a method to improve the safety and reliability of aircraft structures and also reduce operational cost. Built-in sensor networks on an aircraft structure can provide crucial information regarding the condition, damage state and/or service environment of the structure. Among the various types of transducers used for SHM, piezoelectric materials are widely used because they can be employed as either actuators or sensors due to their piezoelectric effect and vice versa. This paper provides a brief overview of piezoelectric transducer-based SHM system technology developed for aircraft applications in the past two decades. The requirements for practical implementation and use of structural health monitoring systems in aircraft application are then introduced. State-of-the-art techniques for solving some practical issues, such as sensor network integration, scalability to large structures, reliability and effect of environmental conditions, robust damage detection and quantification are discussed. Development trend of SHM technology is also discussed.

Piezoelectric Transducer-Based Structural Health Monitoring for Aircraft Applications.

In this paper we review the state of the art in an emerging new technology: embedded ultrasonic non-destructive evaluation (NDE). Embedded ultrasonic NDE permits active structural health monitoring, i.e. the on-demand interrogation of the structure to determine its current state of structural health. The enabling element of embedded ultrasonic NDE is the piezoelectric wafer active sensor (PWAS). We begin by reviewing the guided wave theory in plate, tube, and shell structures, with special attention to Lamb waves. The mechanisms of Lamb wave excitation and detection with embeddable PWAS transducers is presented. It is shown analytically and verified experimentally that Lamb wave mode tuning can be achieved by the judicious combination of PWAS dimensions, frequency value, and Lamb mode characteristics. Subsequently, we address in turn the use of pitch-catch, pulse-echo, and phased array ultrasonic methods for Lambwave damage detection. In each case, the conventional ultrasonic NDE results are contrasted with embedded NDE results. Detection of cracks, disbonds, delaminations, and diffuse damage in metallic and composite structures are exemplified. Other techniques, such as the time reversal method and the migration technique, are also presented. The paper ends with conclusions and suggestions for further work.

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Embedded non-destructive evaluation for structural health monitoring, damage detection, and failure prevention

Effects of local stiffness changes and delamination on Lamb wave transmission using surface-mounted piezoelectric transducers

Smart structure application in bonded repairs

Integrity assessment of composite repair patch using propagating Lamb waves

Smart structure for composite repair

This paper reports on an experimental study in which an array of surface-mounted lead zirconate titanate elements (PZT) are used for the in situ detection of disbond growth in a bonded composite repair patch. Two techniques are used to track the evolution of disbond growth: the transfer function method and the electromechanical impedance method. Both techniques were found to provide a reliable and robust basis for the detection of disbond growth. The results also demonstrate the importance of transducer placement relative to the disbond location as a factor in the sensitivity to disbond growth.

Y L Koh

Papers

Effects of local stiffness changes and delamination on Lamb wave transmission using surface-mounted piezoelectric transducers

Smart structure application in bonded repairs

Integrity assessment of composite repair patch using propagating Lamb waves

Smart structure for composite repair

Detection of Disbond Growth in a Cyclically Loaded Bonded Composite Repair Patch Using Surface-mounted Piezoceramic Elements