Craig A. Rogers

One-Dimensional Thermomechanical Constitutive Relations for Shape Memory Materials:

The use of the thermoelastic martensitic transformation and its reverse transformation has recently been proposed and demonstrated for several active control ap plications. However, the present constitutive models have lacked several important funda mental concepts that are essential for many of the proposed intelligent material system ap plications such as shape memory hybrid composites. A complete, unified, one-dimensional constitutive model of shape memory materials is developed and presented in this paper. The thermomechanical model formulation herein will investigate important material characteristics involved with the internal phase transformation of shape memory alloys. These characteristics include energy dissipation in the material that governs the damping behavior, stress-strain-temperature relations for pseudoelasticity, and the shape memory effect. Some numerical examples using the model are also presented.

Coupled Electro-Mechanical Analysis of Adaptive Material Systems-Determination of the Actuator Power Consumption and System Energy Transfer

This article 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 actua tors, 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 dissi pated 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-of- freedom spring-mass-damper system, to illustrate the methodology used to determine the actuator power consumption and 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 im portant 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 consumption.

Truss Structure Integrity Identification Using PZT Sensor-Actuator

This paper presents a frequency domain impedance-signature-based technique for health monitoring of an assembled truss structure. Unlike conventional modal analysis approaches, the technique uses piezoceramic (PZT) elements as integrated sensor-actuators for acquisition of signature pattern of the truss. The concept of the localization of sensing/actuation area for damage detection of an assembled structure is presented for the first time. Through a PZT patch bonded to a truss node and the measurement of its electric admittance, which is coupled with the mechanical impedance of the truss, the signature pattern of a truss is monitored. The admittance of a truss in question is compared with that of the original healthy truss. Statistic algorithm is then applied to extract a damage index of the truss based on the signature pattern difference. Experimental proof that over a selected band, the detection range of a bonded PZT sensor on a truss is highly constrained to its immediate neighborhood is presented. This characteristic allows accurate determination of the damage location in a complex real-world structure with a minimum mathematical modeling and numerical computation.

Laminate Plate Theory for Spatially Distributed Induced Strain Actuators

Classical laminated plate theory (CLPT) is applied to a laminate plate with induced strain actuators, such as piezoceramic patch, bonded to its surface or embedded within the laminate to develop an induced strain actuation theory that allows for the actuator patch to be spatially distributed. When piezoceramic patches are subjected to voltage fields, the equivalent external forces induced by piezoceramic patches can be determined upon the assumption of free constraint for the expansion or contraction of piezoceramic patches. This assumption is generally done in thermal expansion problem. Several examples, including pure bending and pure extension, are illustrated. For the case of pure bending, a comparison between the current work and that of Dimitriadis et al. (1989) is given. In addition, an orthotropic angle-ply laminate with an embedded piezoceramic patch is presented to show the coupling of bending and extension.

Qualitative impedance-based health monitoring of civil infrastructures

This paper presents a qualitative health monitoring technique to be used in real-time damage evaluation of civil infrastructures such as bridge joints. The basic principle of the technique is to monitor the structural mechanical impedance which will be changed by the presence of structural damage. The mechanical impedance variations are monitored by measuring the electrical impedance of a bonded piezoelectric actuator/sensor patch. This mechanical-electrical impedance relation is due to the electromechanical coupling property of piezoelectric materials. This health monitoring technique can be easily adapted to existing structures, since only a small PZT patch is needed, giving the structure the ability to constantly monitor its own structural integrity. This impedance-based method operates at high frequencies (above 50 kHz), which enables it to detect incipient-type damage and is not confused by normal operating conditions, vibrations, changes in the structure or changes in the host external body. This health monitoring technique has been applied successfully to a variety of light structures. However, the usefulness of the technique for massive structures needs to be verified experimentally. For this purpose, a 500 lb quarter-scale deck truss bridge joint was built and used in this experimental investigation. The localized sensing area is still observed, but the impedance variations due to incipient damage are slightly different. Nevertheless, by converting the impedance measurements into a scalar damage index, the real-time implementation of the impedance-based technique has been proven feasible.

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.

Magnetorheological fluids : Materials, characterization, and devices

Magnetorheological (MR) fluids consist of stable suspensions of magnetic particles in a carrying fluid. Magnetorheological effect is one of the direct influences on the mechanical properties of a fluid. It represents a reversible increase, due to an external magnetic field of effective viscosity. MR fluids and devices have the potential to revolutionize the design of hydraulic systems, actuators, valves, active shock and vibration dampers, and other components used in mechanical systems. At present, there is a compelling need to develop new and improved MR fluids, to lower their production cost through improved manufacturing processes, and to develop MR fluid-based application devices that will demonstrate the engineering feasibility of the MR fluids concept and will highlight the implementation challenges. To this end, the present study is undertaken. A unique high-speed bead mill machine, especially designed to the manufacture of MR fluids, is used to fabricate MR fluids in a laboratory-scale MR fluid fabrication facility at the Center for Intelligent Material Systems and Structures (CIMSS) at VA Tech. Characterization studies are conducted to optimize the quality and the properties of MR fluids, and different ingredients and formulations are tested to produce MR fluids that meet most appropriately the design specifications. A modified HAAKE cone-plate viscometer is used to measure the basic properties of the manufactured MR fluids. As a demonstration of MR fluid-based devices, a cross-stepper exercise machine is modified to incorporate an MR throttle valve, which is the most important element of any MR fluid system.

Electro-mechanical impedance modeling of active material systems

An active material system may be generalized as an electro-mechanical network because of the incorporation of actuators (electrically driven) and sensors (that convert mechanical energy into electrical energy). An investigation of the coupled electrical and mechanical aspects of an active material system will help reveal some of its most important characteristics, in particular regarding energy conversion and consumption issues. The research performed in the area of the electro-mechanical impedance (EMI) modeling of active material systems is herein summarized. In this paper, a generic EMI model to describe the electro-mechanical network behavior (time domain and frequency domain) of active material systems will be discussed. The focus of the discussion will be 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 will first introduce the basic concept of the EMI modeling and its general utility in the area of active material systems. The methodology of the EMI modeling technique will be illustrated using an example of PZT actuator-driven mechanical systems. The basic components of the EMI modeling, including the electro-mechanics of induced strain actuators, the dynamic analysis of active material systems, and the electrical power consumption and requirements, will be discussed. Finally, some applications of the EMI modeling approach, including the determination of the optimal actuator locations, modal analysis using collocated PZT actuator - sensors, and the prediction of radiated acoustic power, will be presented.

Experimental Investigation of E/M Impedance Health Monitoring for Spot-Welded Structural Joints:

Health monitoring results obtained during fatigue testing of a spot-welded lap-shear structural-joint specimen using the electromechanical (E/M) impedance technique are presented. The test specimens were instrumented with piezoelectric wafer transducers, and the base E/M impedance signature was recorded in the 200-1,100 kHz frequency range. Fatigue testing was applied to initiate and propagate crack damage of controlled magnitude. Calibration tests using plain specimens were first performed to correlate stiffness decrease with damage progression and remaining life. During the subsequent health-monitoring tests, the decrease in structural stiffness was used to assess damage progression in the specimen. As damage progressed, the E/M impedance signatures were recorded at predetermined intervals. Signature data were processed, and the RMS impedance change was calculated. Damage index values were observed to increase as crack damage increases. The initiation and propagation of damage was successfully correlated with the E/M impedance measurements. Sensing and the localization principles of E/M impedance method were confirmed, and the rejection of spurious information was verified. These experiments demonstrated that the E/M impedance technique is a potentially powerful tool for damage detection, health monitoring, and NDE of spot-welded structural joints.

An impedance-based system modeling approach for induced strain actuator-driven structures

This paper presents the theoretical development and experimental verification of a system model of piezoelectric (PZT) patch actuators for induced strain actuation of two-dimensional active structures. The model includes the dynamic interaction between PZT actuators and their host structures. Analytical solutions of the output behavior of the PZT actuators have been developed based upon the actuator input impedance and the mechanical impedance of the host structures. The impedancebased model was then applied to thin plates and thin shells, and to beams. The case studies demonstrate the generality and utility of the impedance modeling approach. A simply-supported thin plate with surface-bonded PZT patches was built and tested so that the ability of the impedance model to accurately predict the dynamic performance of the actuator and the host structure has been verified. When compared with conventional static models, the impedance modeling method offers insight into the dynamic coupling of the integrated PZT/substrate systems.