16-08-2012, 11:22 AM
Structural Health Monitoring by Piezo–Impedance Transducers. II: Applications
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Abstract:
This paper, the second in a two-part series, presents a new methodology for structural identification and nondestructive
evaluation by piezo–impedance transducers. The theoretical development and experimental validation of the underlying lead–zirconium–
titanate (PZT)–structure interaction model was presented in the first part. In our newly proposed method, the damage in evaluated on the
basis of the equivalent system parameters “identified” by the surface-bonded piezo–impedance transducer. As proof of concept, the
proposed method is applied to perform structural identification and damage diagnosis on a representative lab-sized aerospace structural
component. It is then extended to identify and monitor a prototype reinforced concrete bridge during a destructive load test.
Introduction
The theoretical background of the electro-mechanical impedance
(EMI) technique, a new nondestructive evaluation (NDE) method,
was presented in the first part of this two-part series (Part I). Part
I (Bhalla and Soh 2004) also introduced the concept of effective
mechanical impedance’ and presented a new simplified electro–
mechanical approach to model the interaction between the piezoimpedance
transducers and their host structures in the smart systems.
This paper presents the necessary analysis to extract the
effective drive point (EDP) impedance of the host structure from
the experimentally measured conductance and susceptance signatures.
This is followed by a simplified methodology to derive
structural parameters from the extracted EDP impedance and use
them in identifying structural damages. Proof-of-concept applications
on aerospace and civil structures are finally presented.
Extraction of Structural Mechanical Impedance
Bhalla and Soh (2003) outlined a computational procedure for
extracting 1D drive point mechanical impedance of skeletal structures
from the active admittance signatures of surface-bonded
piezo-impedance transducers. The computational procedure described
in this section is not only more concise but at the same
time applicable to all structure types since it is based on effective
impedance model formulated in the first paper on modeling.
System Parameter Identification
The structural EDP impedance, extracted by means of the procedure
outlined above, carries information about the dynamic characteristics
of the host structure. Bhalla and Soh (2003) idealized
the host structure (1D skeletal structure) as a parallel combination
of a resistive element (damper) and a reactive element (stiffness–
mass factor). The section presents a more general approach to
predict the equivalent structural system much more accurately.
Before analyzing any complicated structural system, it would be a
worthwhile exercise to study the impedance behavior of some
simple systems.
Extension to Diagnosis of Civil-Structural Systems
In order to test the feasibility of the proposed methodology for
monitoring large civil structures, the data recorded during the
destructive load test on a prototype reinforced concrete (RC)
bridge (Soh et al. 2000) was utilized. The test bridge consisted of
two spans of about 5 m, instrumented with several PZT patches,
1031030.2 mm in size, manufactured by PI Ceramic (2003).
Key mechanical and electrical properties of the PZT patch are
listed in Table 2. The bridge was subjected to three load cycles so
as to induce progressively severe damages. Details of the instrumentation
as well as loading can be found in Soh et al. (2000).
Root mean square deviation index was used to evaluate damages
in the previous study. In the present investigation, the evaluation
of damage is carried out more realistically using the newly developed
approach.
Conclusions
This paper has presented a new simplified diagnostic approach for
identification and NDE of structures based on the equivalent system
“identified” by means of the EMI technique. It makes use of
real as well as imaginary components of the admittance signature
for determining damage sensitive equivalent structural parameters.
As proof of concept, the method has been successfully applied
to diagnose damages on a representative aerospace structure
and a prototype civil structure. The piezo-impedance transducers
can be installed on the inaccessible parts of crucial machine components,
aircraft main landing gear fitting, or turbo-engine blades
and civil structures to perform continuous real-time SHM.