Graduation Semester and Year

2017

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering

Department

Mechanical and Aerospace Engineering

First Advisor

Haiying Huang

Abstract

Piezoelectric wafer active sensors (PWAS) are the most attractive transducers for guided-wave or ultrasound based structural health monitoring (SHM) because of their light weight and compact size. In order to excite guided waves in a structure, the PWASs are usually bonded on the surface of the structure by adhesive materials. The electrical signal applied to the surface bonded PWAS is converted to mechanical deformation of the PWAS which in turn is transferred to the structure through the adhesive layer. Thus, understanding the effects of the adhesive layer on the electrical response of the PWAS enables us to optimize the mechanical coupling between the PWAS and the structure. The preliminary study of the adhesive layer effects using experiments revealed that the electrical signal of the PWAS is significantly influenced by the adhesive layer thickness. However, comprehensive understanding of the adhesive layer effects on the PWAS signals is not feasible through experiments because conducting parametric studies experimentally are impractical. Therefore, studying the adhesive layer effects on the PWAS signals using Finite Element (FE) method has brought immense attentions to the researchers. However, FE based modeling is computationally very expensive as well as inaccurate especially at high frequency. Thus, FE based adhesive layer study is not feasible as well because PWAS based SHM techniques involve high frequency analysis. Even though researchers have developed analytical models for adhesive layer studies, detailed investigations of adhesive layer effects on the PWAS response is still not available in the literature due to the lack of an appropriate model that can completely capture the coupled electromechanical physics associated with the PWAS and the structure. Moreover, most of the available analytical models ignored the adhesive layer; a few of them that included a physics-based model to represent adhesive layer failed to demonstrate how the adhesive layer parameters, e.g. shear modulus, thickness etc. affect the PWAS signals; the majority of them considered either longitudinal mode or flexural mode of the structure. Therefore, an analytical model is sought that will incorporate a physics based adhesive layer model as well as all the possible vibration modes of the structure. We have established an analytical model by coupling the deformation of the PWAS with the structure through the distributed shear stress acting at the PWAS-adhesive and the adhesive-structure interfaces. To account for the loss in shear stress transfer from the PWAS-adhesive interface to the adhesive-structure interface, we have also introduced a new parameter named the shear transfer parameter. As a result, the physics-based adhesive layer model is a function of the shear transfer parameter, adhesive layer thickness, and shear modulus. The deformation of the structure includes both the longitudinal and the flexural vibration modes whereas only the longitudinal vibration of the PWAS is taken into account. We have established two distinct analytical models intended for the two most popular SHM techniques: I. electromechanical impedance (EMI) based SHM (the associated model is called admittance model where a PWAS acts as both actuator and sensor); and II. guided wave (GW) based SHM (the associated model is called the pitch-catch model where a PWAS acts as an actuator and another PWAS away from the actuator acts as a sensor). In the EMI technique, the simulated admittance was matched with that of the measurement by manually adjusting the unknown adhesive layer shear modulus and thus, the unknown shear modulus of the adhesive layer was estimated through this inverse technique. On the other hand, based on the pitch-catch simulation model, an optimized adhesive thickness was predicted for a thin slender structure, and validated by measurements. We have also studied the effects of adhesive layer degradation of lap joint structure, i.e. lap joint damage propagation on the electrical signal of PWAS bonded on the lap joint structure. Unlike other researchers who adopted amplitude change as an indication of damage, we implemented time-of-flight change as an indication of damage because the amplitude may change locally and do not show a monotonous trend with the damage propagation of lap joint structure. In this study, we combined the pulse-echo and pitch catch methods to predict, locate, and estimate the damage in lap joint structure. We used the pulse-echo signal to predict the damage propagation of the lap joint by comparing the time-of-flights between undamaged and damaged structures. Based on the pitch-catch signal, we also predicted the damage severity as well as estimate the damage length in the lap joint.

Keywords

Structural health monitoring, Electromechanical impedance, Pulse-echo, Pitch-catch, Piezoelectric Wafer Active Sensor (PWAS), S-parameters, Digital signal processing, Lamb wave, Longitudinal mode, Flexural mode, Lap joint, Delamination

Disciplines

Aerospace Engineering | Engineering | Mechanical Engineering

Comments

Degree granted by The University of Texas at Arlington

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