Fanping P. Sun

Automated real-time structure health monitoring via signature pattern recognition

Described in this paper are the details of an automated real-time structure health monitoring system. The system is based on structural signature pattern recognition. It uses an array of piezoceramic patches bonded to the structure as integrated sensor-actuators, an electric impedance analyzer for structural frequency response function acquisition and a PC for control and graphic display. An assembled 3-bay truss structure is employed as a test bed. Two issues, the localization of sensing area and the sensor temperature drift, which are critical for the success of this technique are addressed and a novel approach of providing temperature compensation using probability correlation function is presented. Due to the negligible weight and size of the solid-state sensor array and its ability to sense incipient-type damage, the system can eventually be implemented on many types of structures such as aircraft, spacecraft, large-span dome roof and steel bridges requiring multilocation and real-time health monitoring.

Electromechanical impedance modeling of active material systems

An active material system may be generalized as an electro-mechanical network because of the incorporation of the actuators (electrically driven) and sensors (that convert mechanical energy into electrical energy). This paper summarizes most of our research in the area of the electro- mechanical impedance (EMI) modeling of active material systems. In this paper, a generic electro-mechanical impedance model to describe the electro-mechanical network behavior (time domain and frequency domain) of active material systems is discussed. The focus of the discussion is on the methodology and basic components of the EMI modeling technique and its application to assist in the design of efficient active control structures. This paper first introduces the basic concept of the electro-mechanical impedance modeling and its general utilities in the area of active material systems. The methodology of the EMI modeling technique is illustrated using an example of PZT actuator-driven mechanical systems. The basic components in the EMI modeling are discussed. Finally, some applications of the EMI modeling approach are presented.

Structural modal analysis using collocated piezoelectric actuator/sensors: an electromechanical approach

This paper presents a modal analysis technique based on the measurement of electric admittance of collocated actuator/sensors. The technique utilizes thin piezoelectric patches bonded on structures as both sensor and actuator. A commercial electrical impedance analyzer is used to measure the electrical admittance of the PZT patch. An SDOF model governing the electromechanical interaction is derived and then used to extract the mechanical impedance of the structures from the measured electrical admittance. Two corresponding algorithms, revised inverse Nyquist plane curve fitting and admittance matching-half power bandwidth approaches are presented for the extraction of modal parameters. Both approaches exclude the stiffening effect of PZT on structure yielding better estimations of extracted structure natural frequencies. The placement of PZT on structure is also studied. An experimental example is given on a small flexible beam. The results show the advantages of this technique in modal test of lightweight and flexible structures whose modal parameters are extremely sensitive to the stiffening of the transducers and shaker.

Coupled electromechanical analysis of piezoelectric ceramic actuator-driven systems: determination of the actuator power consumption and system energy transfer

This paper presents a coupled electro-mechanical analysis of piezoelectric ceramic (PZT) actuators integrated in mechanical systems to determine the actuator power consumption and energy transfer in the electro-mechanical systems. For a material system with integrated PZT actuators, the power consumed by the PZT actuators consists of two parts: the energy used to drive the system which is dissipated in terms of heat as a result of the structural damping, and energy dissipated by the PZT actuators themselves because of their dielectric loss and internal damping. The coupled analysis presented herein uses a simple model, a PZT actuator-driven one-degree-freedom spring-mass-damper system, to illustrate the methodology used to determine the actuator power consumption energy flow in the coupled electro-mechanical systems. This method can be applied to more complicated mechanical structures or systems, such as a fluid-loaded shell for active structural acoustic control. The determination of the actuator power consumption can be very important in the design and application of intelligent material systems and structures and of particular relevance to designs that must be optimized to reduce mass and energy.

Magnetic activation of embedded sensory particles in active tagging interrogation of adhesive bonding

This paper presents an experimental technique of magnetically exciting embedded ferromagnetic particles in epoxy to facilitate active inspection and interrogation of bond integrity. The proposed concepts involve biasing an alternating magnetic field and optimizing the activating location to maximize and linearize the exciting force on the particles. The methods are analytically and experimentally proven to be effective in amplifying the force and reducing the harmonic distortion resulting from the nonlinearity of the system. The effects of material properties and the geometry of the embedded particles on the activation force are also extensively studied. The particles geometry is found to affect the activation significantly. The active tagging technique is thus pushed one more step forward on the way to practical application.

Dynamic output characteristics of piezoelectric actuators

Like every dynamic output device, piezoceramic (PZT) actuators have their own output characteristics. The output characteristics of PZT actuators are functions of frequency and may be treated as frequency independent only in a certain frequency range. The dynamic excitation provided by a PZT actuator depends on its dynamic characteristics and structural mechanical impedance. The conventional method of using the statically determined induced moment or the force of PZT actuators as forcing functions is not correct. This paper uses a simple example, a PZT actuator-driven one-degree-of-freedom spring-damper-mass system, to illustrate the methodology used to determine the dynamic characteristics of PZT actuators and the structural response based on the concept of structural impedance.

Dynamic analysis of active structures under multiple actuators excitations using an impedance approach

Dynamic interaction between an integrated induced strain actuator and its host structure depends on the driving point mechanical impedance of the host structure and the mechanical impedance of the actuator. For the case of several actuators, the individual actuator/structure impedance will not only depend on the driving point host structural impedance, but also on the electromechanical coupling with the other actuators. In this paper, a general dynamic model of a structure driven by multiple actuators is presented, as based on an impedance modeling approach developed for a single actuator. The basic formulation is provided in terms of structural admittance of an arbitrary structure and dynamic transduction relations of transducers. It can be easily applied to any form of structure and actuators, even an electromagnetic shaker, provided their dynamic transduction relations can be developed. The developed theoretical model has been used to predict the dynamic response of a simply- supported beam with several bonded piezoelectric actuators activated simultaneously. Experiments have also been conducted which validate the theoretical model.

Disbond detection of composite repair patches on concrete structures using laser Doppler vibrometer

A technique is presented capable of detecting hidden voids underneath a composite patch adhered to concrete structures as reinforcement. The approach is based upon local membrane vibration of a thin panel when it becomes debonded from the host structure. A state-of-the-art laser Doppler vibrometer is used for high-sensitivity, high-speed and noncontact surface vibration detection. The composite repair patch is excited by piezoelectric patch actuators over a wide frequency range to ensure the presence of the local resonance modes in the scanning image. The results show that both boundary and inner disbonds are detectable by this technique, and the location as well as the size of a disbond as small as 0.75 by 1 in2 can be detected when the excitation frequencies are properly selected and the response image is post-processed. The technology has demonstrated its potential for applications such as quality assurance in concrete infrastructure reinforcement and metal structure repair by composite wrapping or patching. It is also appropriate for delamination detection of composite products during manufacturing.

Development of an actuator power factor meter for experimental determination of the optimal actuator location on complex structures

Actuator power factor, defined as the ratio of structural dissipative mechanical power to apparent supplied electrical power, describes the effectiveness of the integrated actuators to convert supplied electrical power to mechanical power which creates the intended structural response. A large actuator power factor in the frequency range of application indicates that the corresponding actuator (position) has a higher authority to excite its host structure in that frequency range than positions with a lower power factor. The use of acutator power factor provides an alternate tool in the determination of the optimal actuator locations and dimensions to theoretical calculations. More importantly, the power factor can be experimentally measured on large-scale complex structures, thereby eliminating the need for sophisticated theoretical modeling. The concept of an actuator power factor was first introduced by the authors (Liang et al., 1993a) and will be briefly described herein. The utility of using the actuator power factor in optimizing an actuator system will then be illustrated using a numerical case study. The hardware development of the actuator power factor meter will be presented. The application of actuator power factor meter for optimal experimental actuator placement will be discussed.

In-situ sensory technique for in-service quality monitoring: measurement of the complex Young's modulus of polymers

Applications of polymeric adhesives in joining different materials have necessitated quantitative health inspection of adhesive joints (coverage, state of cure, adhesive strength, location of voids, etc.). A new in-situ sensory method has been proposed in this paper to inspect the amount and distribution of the critical constituents of polymers and to measure the characteristic parameters (complex Youngs modulus and damping). In this technique, ferromagnetic particles have been embedded in a polymeric matrix, similar to a particle- reinforced composite. The dynamic signatures extracted from the tests as a result of magnetic excitation of the embedded ferromagnetic particles are used to evaluate the complex Youngs modulus of the host polymers. Moreover, the amplitude of the frequency response is utilized to identify the amount and distribution of embedded particles in polymeric materials or adhesive joints. The results predicted from the theoretical model agree well with the experimental results. The theoretical analyses and the experimental work conducted have demonstrated the utility of the sensory technique presented for in-service health interrogation.