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CONTENTS
Volume 3, Number 2, April 2007
 

Abstract
This paper addresses a new type of broad and stage-based hybrid carbon fiber reinforced polymer (HCFRP) sensor that is suitable for the sensing of infrastructures. The HCFRP sensors, a type of composite sensor, are fabricated with three types of carbon tows of different strength and moduli. For all of the specimens, the active materials are carbon tows by virtue of their electrical conductivity and piezoresistivity. The measurement principles are based on the micro- and macro-fractures of different types of carbon tows. A series of experiments are carried out to investigate the sensing performances of the HCFRP sensors. The main variables include the stack order and volume fractions of different types of carbon tows. It is shown that the change in electrical resistance is in direct proportion to the strain/load in low strain ranges. However, the fractional change in electrical resistance (DR/R0) is smaller than 2% prior to the macro-fractures of carbon tows. In order to improve the resistance changes, measures are taken that can enhance the values of DR/R0 by more than 2 times during low strain ranges. In high strain ranges, the electrical resistance changes markedly with strain/load in a step-wise manner due to the gradual ruptures of different types of carbon tows at different strain amplitudes. The values of DR/R0 due to the fracture of high modulus carbon tows are larger than 36%. Thus, it is demonstrated that the HCFRP sensors have a broad and stage-based sensing capability.

Key Words
structural health monitoring; HCFRP sensors; broad and stage-based sensing.

Address
Z. S. Wu,1,2 and C. Q. Yang, 1
1Department of Urban & Civil Engineering, Ibaraki University, Nakanarusawa-cho 4-12-1, Hitachi, 316-8511, Japan
2International Institute for Urban System Engineering, Southeast University, Nanjing, 210096, China

Abstract
The present paper is concerned with the design of distributed sensors and actuators. Strain type sensors and actuators are considered with their intensity continuously distributed throughout a continuous structure. The sensors measure a weighted average of the strain tensor. As a starting point for their design we introduce the concept of collocated sensors and actuators as well as the so-called natural output. Then we utilize the principle of virtual work for an auxiliary quasi-static problem to assign a mechanical interpretation to the natural output of the sensors to be designed. Therefore, we take the virtual displacements in the principle of virtual work as that part of the displacement in the original problem, which characterizes the deviation from a desired one. We introduce different kinds of distributed sensors, each of them with a mechanical interpretation other than a weighted average of the strain tensor. Additionally, we assign a mechanical interpretation to the collocated actuators as well; for that purpose we use an extended body force analogy. The sensors and actuators are applied to solve the displacement tracking problem for continuous structures; i.e., the problem of enforcing a desired displacement field. We discuss feed forward and feed back control. In the case of feed back control we show that a PD controller can stabilize the continuous system. Finally, a numerical example is presented. A desired deflection of a clamped-clamped beam is tracked by means of feed forward control, feed back control and a combination of the two.

Key Words
continuous systems; distributed sensing/actuation; natural output; collocated control; dynamic displacement tracking.

Address
Institute for Technical Mechanics, Johannes Kepler University Linz, Altenbergerstr. 69, A - 4040 Linz, Austria

Abstract
The general solution for two-dimensional magneto-electro-elastic media in terms of four harmonic displacement functions is proposed analytically. The expressions of specific solutions of magneto-electro-elastic plane problems with specific body forces are derived. Finally, based on the general solution in the case of distinct eigenvalues and the specific solution for density functionally gradient media, two kinds of beam problems with body forces depending only on the z or x coordinate are solved by the trial-and-error method.

Key Words
general solution; magneto-electro-elastic plane; density functionally gradient media; analytical solutions.

Address
Aimin Jiang; Department of Civil Engineering, Zhejiang University, Hangzhou, 310027, P. R. China
Department of Civil Engineering, Quzhou College, Zhejiang, 324000, P. R. China
Haojiang Ding; Department of Civil Engineering, Zhejiang University, Hangzhou, 310027, P. R. China

Abstract
As a functional material, shape memory alloy (SMA) has attracted much attention and research effort to explore its unique properties and its applications in the past few decades. Some of its properties, in particular the electrical resistance (ER) based self-sensing property of SMA, have not been fully studied. Electrical resistance of an SMA wire varies during its phase transformation. This variation is an inherent property of the SMA wire, although it is highly nonlinear with hysteresis. The relationship between the displacement and the electrical resistance of an SMA wire is deterministic and repeatable to some degree, therefore enabling the self-sensing ability of the SMA. The potential of this self-sensing ability has not received sufficient exploration so far, and even the previous studies in literature lack generality. This paper concerns the utilization of the self-sensing property of a spring-biased Nickel-Titanium (Nitinol) SMA actuator for two applications: ER feedback position control of an SMA actuator without a position sensor, and estimation of the opening of a SMA actuated valve. The use of the self-sensing property eliminates the need for a position sensor, therefore reducing the cost and size of an SMA actuator assembly. Two experimental apparatuses are fabricated to facilitate the two proposed applications, respectively. Based on open-loop testing results, the curve fitting technique is used to represent the nonlinear relationships between the displacement and the electrical resistance of the two SMA wire actuators. Using the mathematical models of the two SMA actuators, respectively, a proportional plus derivative controller is designed for control of the SMA wire actuator using only electrical resistance feedback. Consequently, the opening of the SMA actuated valve can be estimated without using an extra sensor.

Key Words
shape memory alloy; electric resistance; feedback control; sensorless feedback control.

Address
Gangbing Song; Department of Mechanical Engineering at University of Houston, Houston, Texas, USA
Ning Ma; Cyrus Solutions Corporation, Conroe, Texas, USA
Ho-Jun Lee; NASA Johnson Space Center, Houston, Texas, USA

Abstract
An active damage detection technique is introduced to locate damage in an isotropic plate using Lamb waves. This technique uses a time-domain energy model of Lamb waves in plates that the wave amplitude inversely decays with the propagation distance along a ray direction. Accordingly the damage localization is formulated as a least-squares problem to minimize an error function between the model and the measured data. An active sensing system with integrated actuators/sensors is controlled to excite/receive A0 mode of Lamb waves in the plate. Scattered wave signals from the damage can be obtained by subtracting the baseline signal of the undamaged plate from the recorded signal of the damaged plate. In the experimental study, after collecting the scattered wave signals, a discrete wavelet transform (DWT) is employed to extract the first scattered wave pack from the damage, then an iterative method is derived to solve the least-squares problem for locating the damage. Since this method does not rely on time-of-flight but wave energy measurement, it is more robust, reliable, and noise-tolerant. Both numerical and experimental examples are performed to verify the efficiency and accuracy of the method, and the results demonstrate that the estimated damage position stably converges to the targeted damage.

Key Words
damage localization; least-squares method; structural health monitoring; Lamb waves; wavelet transform; wave energy.

Address
Department of Mechanical and Aerospace Engineering,
North Carolina State University, Campus box 7921, Raleigh, NC 27695-7921, USA

Abstract
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Key Words
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Address
Steven D. Glaser; Department of Civil and Environmental Engineering, University of California, Berkeley, CA, 94708, USA
Hui Li; School of Civil Engineering, Harbin University of Technology, P.O. Box 2546, Campus 2,
Harbin, 150090 P.R. China
Ming L. Wang; University of Illinois at Chicago, Department of Civil and Materials engineering, 842 West Taylor st., Chicago, IL 60607-7023, USA
Jinping Ou; Dalian University of Technology, No.2 Linggong Road, Dalian City, 116024 P.R. China
Jerome Lynch; Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, MI 48109-2125, USA

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