Author: Su, Yiyin
Title: Nanocomposite-functionalized fibre-reinforced polymer composites with integrated self-sensing and monitoring capabilities
Advisors: Su, Zhongqing (ME)
Degree: Ph.D.
Year: 2022
Subject: Nanocomposites (Materials)
Hong Kong Polytechnic University -- Dissertations
Department: Department of Mechanical Engineering
Pages: xxvii, 175 pages : color illustrations
Language: English
Abstract: Real-time, continuous condition monitoring of fibre-reinforced polymer (FRP) composites, from the onset of manufacturing, through service to the end of life, is of vital significance, to warrant the structural integrity, reliability and durability spanning the entire life cycle of composites. To accommodate such a need, prevailing integrity monitoring approaches use multiple types of sensors with different sensing philosophies, to monitor individual life cycle stage of composites. When conventional sensors, such as rigid lead zirconate titanate (PZT) wafers or brittle fibre Bragg grating (FBG) sensors, are embedded in composites, the sensors may, more or less, compromise the original structural integrity, regardless of the fact that the intended role of the sensors is to implement integrity monitoring of composites.
Envisaging such a deficiency and facilitated by the recent prosperity in nanotechnology, this PhD study explores and develops integratable nanocomposite piezoresistive sensors, which can precisely perceive strains induced by quasi-static loads, medium-frequency structural vibrations and high-frequency structure-guided ultrasonic waves (GUWs), to accommodate demanding monitoring requirements. To achieve such an objective, the present study embraces three key aspects, to hierarchically develop FRPs with integrated self-sensing and monitoring capabilities, namely:
1) the initial effort to graft glass fibres (GFs) with carbon nanotubes (CNTs) via chemical vapor deposition (CVD), endowing conventional glass fibre-reinforced polymer (GFRP) composites with the monitoring capability;
2) the further exploration to develop a new type of implantable nanocomposite piezoresistive sensor, with a morphologically optimized nanostructure, via spray coating - a cost-effective additive manufacturing approach;
3) the validation of self-continuous monitoring capability of FRPs with the above implantable nanocomposite sensors.
To address aspect 1) and enable conventional GFRPs to perform self-sensing and monitoring, sensing fabrics containing CNT-grafted GF sensors, referred as CNT-g-GF sensors in this study, are created via direct growing CNTs on GF fabrics through CVD. The high graphitization degree of grafted CNTs, synthesized in a low growth temperature at 500℃, is affirmed via scrutinizing their Raman spectra, with a D-band to G-band intensity ratio <0.89. The smooth synthesis of CNTs at such a low temperature is a joint result of selecting the cobalt (II) nitrate hexahydrate as the catalyst precursor, the ethanol as the hydrocarbon precursor and the 5 vol% H2/N2 gas mixture as the carrier and reducing gas. The loadings of grafted CNTs are regulated to distribute CNTs in a close proximity manner, upon which the quantum tunnelling effect can be triggered and promoted when GUWs traverse the CNT-g-GF sensors. Experimental results show the high accuracy, sensitivity and fast response of such-fabricated sensors to dynamic strains, with a frequency regime from 175 to 375 kHz. Single fibre tensile test (ASTM C1557) and fibre-reinforced polymer matrix composite tensile test (ASTM D3039) are launched to interrogate the possible influence of the CNT grafting process on tensile attributes, confirming no measurable variation in mechanical properties of hybrid composites due to the sensor integration. Thus-produced CNT-g-GF sensors can be used to calibrate the cure progress of epoxy, by measuring the dynamic variations in the electrical resistance (ER). The CNT-g-GF sensors also precisely sense in-service loads applied to the composites, with a gauge factor as high as 30.2. This type of sensor sheds light on the use of nanocomposite-driven fibre decoration towards the development of new functional composites.
To address aspect 2), a new sort of compatible nanocomposite piezoresistive sensors, which can be implanted into carbon fibre-reinforced polymer (CFRP) composites and networked for in situ acquisition of dynamic strains, is further explored. The nanocomposite ink, formulated with graphene nanoplatelets (GNPs) and polyvinylpyrrolidone (PVP), is tailored to acquire the percolation threshold of conductive nanofillers. The above ink is then deposited on partially precured (α = 0.4) B-stage epoxy films using spray coating. The sensors are electrified, circuited and networked using highly conductive CNT-film-made wires, to be implanted into CFRPs and form a sensor network. With a morphologically optimized nano-architecture in nanocomposites, the quantum tunnelling effect can be triggered in percolated networks, which enables the sensors to faithfully response to quasi-static loads (with a high gauge factor of 34.5), medium-frequency structural vibrations and high-frequency GUWs up to 450 kHz. Only ~45 µm in thickness (including wires), the implanted sensors exhibit high compatibility and nonintrusive attributes with host composite structures, as confirmed in tensile and bending tests.
To address aspect 3), such-fabricated implantable nanocomposite sensors are integrated into composites from the onset of manufacturing, to continuously monitor the cure progress and structural integrity of composites. In conjunction with differential scanning calorimetry (DSC) and a Sesták-Berggren autocatalytic kinetic model, cure behaviors of matrix are first comprehensively evaluated and understood. The matrix cure degree in manufacturing is correlated with subtle changes in propagation characteristics of GUWs perceived by the sensors. The implantable sensors successfully prove their capabilities of tracing the matrix cure progress and detecting the cure anomaly. Subsequent to cure monitoring, in-service integrity monitoring is continuously implemented by the same implanted sensors, to locate a transient impact.
Being sensitive, compatible and lightweight, both types of sensors developed in this study can be densely deployed in a composite structure - flat or non-flat, while not at the cost of mechanical attributes of the original FRPs. With the demonstrated sensing performance, these nanocomposite sensors usher in a concrete road to implement either ER-based or acousto-ultrasonics-based continuous monitoring of composites, from manufacturing through service, to the end of life.
Rights: All rights reserved
Access: open access

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