Author: Liao, Yaozhong
Title: A nanocomposite-inspired broadband sensing network coating for acousto-ultrasonics-based in situ structural health monitoring
Advisors: Su, Zhongqing (ME)
Zhou, Li-min (ME)
Degree: Ph.D.
Year: 2020
Subject: Nanotechnology
Nanocomposites (Materials)
Hong Kong Polytechnic University -- Dissertations
Department: Department of Mechanical Engineering
Pages: xxvi, 171 pages : color illustrations
Language: English
Abstract: With recent advances in quantum techniques and bourgeoning nanotechnology, multifunctional nanocomposites have been developed, to accommodate ever-increasing requirements of the rapidly progressing industries. In this PhD study, new breeds of nanoengineered piezoresistive sensors are explored and developed using additive manufacturing. By exploiting the excellent mechanical and chemical properties of graphene- and carbon black (CB)-filled nanocomposites including the good flexibility, customizable properties, and low density, the developed sensors have proven effectiveness and performance in facilitating acousto-ultrasonics-based in situ structural health monitoring (SHM) and human health monitoring. By virtue of the quantum tunneling effect in nanoparticle-formed electrical networks in the fabricated nanocomposites, these genres of innovative sensors, which are coatable, sprayable or printable on a medium surface, show great talent in in situ acquisition of broadband dynamic signals from quasi-static strains, through structural vibration signals of several hundred hertz, to high-frequency ultrasound in a regime of megahertz - a trait of nanocomposite-based piezoresistive sensing devices that have until now not been discovered and explored. With polyvinylidene fluoride (PVDF) as the matrix, fabrication of the nanocomposite sensors is attempted in a comparative manner, using multiscale nanofillers ranging from zero-dimensional CB, through one-dimensional multiwalled carbon nanotubes (MWCNTs), to two-dimensional graphene nanoparticles. Compared with conventional strain gauges that usually feature a gauge factor of ~2, much higher gauge factors of the nanocomposite sensors (ranging from ~5 to ~55) have been achieved in measuring quasi-static dynamic disturbances and low-frequency vibrations. With a morphologically optimal nano-architecture, the quantum tunneling effect can be triggered in the percolating nanofiller network when ultrasound signals traverse a sensor to induce dynamic alteration in the piezoresistivity manifested by the sensor. To understand the sensing mechanism of the developed nanocomposite-based sensors, a three-dimensional molecular model is established to simulate the micro-dynamics of the movement of nanoparticles under ultraweak disturbances induced by ultrasound. Results of in situ morphological analysis and experiment reveal high fidelity, ultrafast responses, and high sensitivity of the nanocomposite sensors to dynamic disturbances, from static strains to ultrasound in a regime of megahertz yet with an ultralow magnitude (of the order of microstrain or nanostrain). These findings are remarkable, as no other investigation has probed the responses of nanocomposite piezoresistive sensors over such a broad frequency spectrum.
To further quantitively manipulate the physical properties of the nanocomposite sensors, a nanoengineered ink delicately made from CB and polyvinyl pyrrolidone (PVP), is prepared, on which basis the thus-fabricated sensors are reproducible with high reliability via additive manufacturing approaches (inkjet printing). This ink can be deposited directly on the surfaces of various substrates or engineering structures such as polyimide (PI) films to configure nanocomposite sensor arrays or dense sensor networks via the computer-aided design. Strong structural adaptability and high flexibility make the printable sensor a highly promising candidate to substitute traditional piezoresistive and piezoelectric sensors in the acquisition of dynamic signals from engineering structures with arbitrary surfaces. Lightweight and without the need to use externally connected wires or cables, a printed sensor network significantly reduces the weight and volume penalties imposed on the host structure, even when the sensor network is installed in a large quantity. It also minimizes the possibility of exfoliation of the sensors from the host structure under cyclic loads. The printed pattern warrants superior performance in the perception of broadband acousto-ultrasonic signals. The fabrication process of the sensor network does not entail expensive printing facilities, and the CB/PVP hybrid can easily be injected into an inkjet cartridge for printing. To take a step further, several inkjet-printed sensors are arrayed to configure the printed sensor networks along with the printed electric circuits, to implement in situ SHM. The printed sensor networks, compared with conventional piezoelectric lead zirconate-titanate ceramics (PZT)-based sensor networks, exhibit higher compatibility with the host structures, sensors and electric wires, due to the good adhesion and lightweight of the printed sensors and electric circuits. Therefore, these printed sensing networks can maintain stable performance under heavy loads without concern of exfoliation of sensors and circuits. An all-in-one SHM system, consisting of a waveform generator, a power amplifier, a printed sensor network and an oscilloscope, is configured and manufactured to interrogate the area covered by the printed sensing network. This SHM system is used to monitor and evaluate the health status of an aircraft structure (i.e., an aircraft radome), in which two application paradigms of damage localization are fulfilled - passive localization for impact damage and active identification for undersized defects, respectively. Being sensitive and broadband, the impact and ultrasonic signals are faithfully captured by the printed sensing network in both active and passive manners. The location of impact damage is pinpointed in real time with high precision and resolution. In addition, the locations of undersized defects are identified via the time-of-flight-based triangulation location algorithm. The feasibility of using the GUWs-sensitive nanocomposites to monitor health indexes of human (e.g., pulse and viscosity of human blood) is preliminarily investigated, highlighting that this sensor also has great potential for developing human wearable healthcare devices. Being flexible, lightweight, tailorable and printable, the nanocomposite sensors can be densely deposited on curve structures, unlike the brittle and heavy PZT wafers which can only be installed on relatively flat structures in sparse fashions. The printed sensing networks, with proven sensitivity and precision in responding to GUWs up to megahertz (i.e., 1.4 MHz), are configured to render rich information for depicting structural health status, highlighting that this new kind of sensors has paved a new path for implementing in situ SHM, by striking a compromise between 'sensing effectiveness' and 'sensing cost'.
Rights: All rights reserved
Access: open access

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