Date of Award

May 2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

Anoop Dhingra

Second Advisor

Nathan Salowitz

Committee Members

Sudhir Kaul, Habib Tabatabai, Ilya Avdeev

Keywords

Damage Detection, Joint Defects, Laminate Structures, Shear-mode Piezoelectric Transducers, Structural Health Monitoring, Ultrasonic Inspection

Abstract

Structural health monitoring (SHM) employing embedded piezoelectric transducers has shown potential as a promising solution for inspection of different engineering structures such as aircraft, bridges, and renewable energy structures. Despite advancements in the field of ultrasonic SHM, inspection of laminate structures is still a major challenge due to their susceptibility to various joint defects. This thesis presents a novel approach to tackle the challenge of inspecting laminate structures using shear-mode (d35) piezoelectric transducers that are made of lead zirconate titanate (PZT).

This study begins with the characterization of d35 piezoelectric transducers using analytical, numerical, and experimental approaches. The results were found to match well. A finite element (FE) simulation of a laminate structure was developed based on multiphysics analysis to identify the propagating waves generated by d35 PZT actuators embedded within the bondline of the laminate structure. The group velocities of voltage signals as well as the distributions of normal displacements and stresses induced by the propagating waves showed that the elastic waves generated by the d35 PZT actuator exhibit the characteristics of antisymmetric (flexural) waves coupled with strong transverse shear stress across the thickness of the adhesive layer. The FE results were validated by testing laminate specimens with bondline-embedded d35 PZTs in a pitch-catch arrangement.

A parametric study was performed to provide design guidelines for d35 PZT sensors and actuators. The thickness and length of d35 PZT transducers were varied while monitoring the actuation strength and the sensed voltage signal. It was found that thicker and shorter d35 PZT sensors can produce stronger signals compared to thinner and longer d35 PZT sensors. On the contrary, d35 PZT actuators were noticed to exhibit the opposite response to d35 PZT sensors. The selectivity of d35 PZT sensors was also investigated in multiphysics simulations by comparing voltage signals obtained from a bondline-embedded d35 PZT sensor and a surface-mounted conventional (d31) PZT sensor. It was found that d35 PZTs offer a selective hardware filter that primarily captures antisymmetric wave modes in the laminate structure while suppressing symmetric wave modes. Filtering symmetric modes significantly reduced the complexity of signal processing and this could potentially enhance the process of SHM as well.

Various joint defects including disbonds, cracks, and voids were introduced in the bondline of laminate structures to investigate the feasibility of embedding d35 PZT transducers in the bondline of laminate structures for detection of joint defects. It was observed that antisymmetric waves generated by d35 PZT actuators exhibited strong interaction with joint defects especially nonlinear defects such as cracks and disbonds. By placing the transducers within the bondline and at the neutral axis of the laminate structure, it provided a direct strong coupling between the bondline and the d35 PZT transducers resulting in high transmission and sensitivity of flexural waves to joint defects. Several specimens were prepared and tested. The results obtained from experiments and simulations were found in good agreement.

The proposed approach was also evaluated experimentally for health monitoring of bondline integrity. A laminate specimen with bondline-embedded d35 PZT and surface-mounted d31 PZT piezoelectric transducers was subjected to a three-point bending test to create joint defects. Damage indices were implemented to detect the presence of damage and its severity. The experimental results demonstrate the ability of bondline-embedded d35 PZTs to be used as sensors and actuators for ultrasonic SHM of bondline integrity. The proposed approach successfully produced promising results for detection of joint defects that often impose a significant challenge to detect using conventional nondestructive evaluation techniques. The results presented in this thesis provided fundamental work towards creating embedded, automated damage detection systems for laminate structures using bondline-embedded d35 piezoelectric transducers.

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