Development of Structural Health Monitoring System and Damage Assessment Technique with Piezoelectric Sensor/Actuator

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The objective of this research is to develop a practical and quantitative technique for damage assessment and structural health monitoring system by embedded in a pair of piezoelectric patches. Since the piezoelectric actuator can be driven at very high frequency and the piezoelectric sensor is very sensitive to the rate of strain changes, the piezoceramic patches can be utilized as both oscillators and receivers. A pair of piezoelectric patches is droved in a way so that a harmonic longitudinal elastic wave is propagated in the structure and the patches' electric impedance is measured simultaneously. Therefore the minute mechanical impedance change due to infancy damage can be estimated measuring the change of the electric impedance. At the first step of this research, a simple beam, which is usually used as a basic element of a structure, is dealt with for fundamental study. The longitudinal wave generation and impedance-based measurement techniques were proposed and the damage assessment based on the longitudinal strain mode was investigated. Further, a simple impedance measuring system for piezoelectric patches were developed. A plenty of experimental results were validated that impedance-based technique with longitudinal strain mode is efficient for assessment of damaged condition and position in the structure.Considering for practice use, a truss structure embedded with piezoelectric patches was constructed to establish the damage indices. As an example, the evaluation of the fastening bolts at the joint was investigated. Three damage indices, Peak Frequency Shift ΔF, Peak Amplitude Ratio δ and Quality Factor Ratio γ, which are calculated from the electric impedance response, are proposed. It is validated experimentally that the condition of a loosening bolts at the joint in a truss structure can be estimated accurately by using the three damage indices.