Rozprawy doktorskie na temat „Strain Sensing Application”

A fatigue test has been performed on 7075-T651 aluminum specimens which were bonded with polyimide coated optical fibers with discrete Bragg gratings. These fibers were bonded with AE-10 strain gage adhesive. The results indicate that lower strain amplitudes do not produce cause for concern, but that larger strain amplitudes (on the order of 3500 μ) may cause some sensors to become unreliable. The strain response of acrylate coated optical fiber strain sensors bonded to aluminum specimens with AE-10 and M-Bond 200 strain gage adhesives was investigated with both axial and cantilever beam tests. These results were compared to both the strain response of conventional strain gages and to model predictions. The results indicate that only about 82.6% of the strain in the specimen was transferred through the glue line and fiber coating into the fiber. Thus, multiplying by a strain transfer factor of approximately 1.21 was sufficient to correct the optical fiber strain output. This effect was found to be independent of the adhesive used and independent of the three-dimensional profile of the glue line used to attach the fiber. Finally, this effect did not depend on whether the fiber had a polyimide or an acrylate coating. Further investigation was conducted on the feasibility of using optical fiber strain sensors for monitoring subcritical damage (such as matrix cracks) in fiber reinforced composite materials. These results indicate that an array of optical fibers which monitor the strain profile on both sides of a composite panel may be sufficient for these purposes
Master of Science

Optic fiber sensors are employed in a variety of applications for the remote measurement of various parameters such as strain, pressure, or temperature. These sensors offer an array of benefits as well including light weight, compactness, and high resolution. In particular, Fabry-Perot interferometers (FPIs) maintain these benefits and can also be made to withstand extremely high temperatures. This advantage of the FPI allows it to be used in harsh environments where many other tools for parameter measurement could not survive. An FPI strain sensor is constructed and tested which has the capabilities to be used at high temperatures of over 1000°C for applications in gas turbine engine testing. This paper discusses the need for high temperature strain sensors in engine testing and this sensor’s capabilities in this application.

Gallium nitride (GaN) is a promising material for electronic sensing devices operating in harsh environments, thanks to its large energy band gap, superior mechanical properties and excellent chemical inertness. Among various wide energy band gap semiconductors such as 3C-SiC, 4H-SiC, 6H-SiC materials, GaN and its compounds are considered as the most suitable materials for Micro Electro-Mechanical Systems (MEMS) sensors for harsh environment applications, as it can be grown on both sapphire and Si substrates, which are compatible with conventional MEMS fabrication processes, while reducing the cost of GaN wafers. GaN-based electronic devices for high frequency and high power applications have been already commercially available. However, their application in sensing is still underdeveloped and under-commercialized. This research aims to experimentally investigate and theoretically analyze the physical sensing effects, such as piezotronic, Hall, pseudo-Hall, and phototronic effects on Al-GaN/GaN heterojunctions, and explores the potential of enhancing the sensitivity of AlGaN/GaN-based sensing devices through multi-physics coupling effect. The first purpose of this study is to examine the effect of external strain on the polarization and electronic properties of the p-GaN/AlGaN/GaN heterostructure (piezotronic effect) and evaluates the possibility to utilize the effect as a strain sensing mechanism. Theoretical analysis on the strain induced effect in the energy band structure is thoroughly conducted. p-GaN/AlGaN/GaN based sensing devices are fabricated and characterized, which exhibit high sensitivity, excellent linearity, and good repeatability, indicating the potential for pressure/strain sensing. In addition, the possibility of enhancing the sensitivity of an p-GaN/AlGaN/GaN heterostructure based piezotronic sensor by employing the photoexcitation-electronic coupling effect is also investigated. The research analyses the key parameters contributing to this tuneable giant piezotronic effect and figures out the physical mechanism leading to this phenomenon. The next goal is to characterize the performance of the AlGaN/GaN-based current sensor and AlGaN/GaN van der Pauw strain sensor utilizing Hall and pseudo-Hall effects, respectively. Both current sensor and van der Pauw sensor exhibit high sensitivity, excellent repeatability and linearity, while the current sensor operates at a temperature range from room to 200 degrees C with negligible changes in sensitivity. Combining these performances with the excellent mechanical strength, electrical conductivity, and chemical inertness of GaN, the proposed sensors are promising for strain and power monitoring in harsh environments. Furthermore, this research also intends to investigate the phototronic behaviors of the AlGaN/GaN heterojunction under UV illuminations (phototronic e ect). The characteristics of the heterojunction are also evaluated under broad spectral illuminations to prove its potential for high-performance visible-blind UV light detector. Moreover, the in-depth discussion about the carrier generation and transport mechanisms will provide vital information for the development of AlGaN/GaN optoelectronic sensing devices. This thesis is prepared in a \thesis by publications" format. The published and submitted journal articles are the main contents of chapters 3, 4, 5, and 6.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Eng & Built Env
Science, Environment, Engineering and Technology
Full Text

Carbon nanotubes (CNTs) have excellent electrical, mechanical and electromechanical properties. When CNTs are incorporated into polymers, electrically conductive composites with high electrical conductivity at very low CNT content (often below 1% wt CNT) result. Due to the change in electrical properties under mechanical load, carbon nanotube/polymer composites have attracted significant research interest especially due to their potential for application in in-situ monitoring of stress distribution and active control of strain sensing in composite structures or as strain sensors. To sucessfully develop novel devices for such applications, some of the major challenges that need to be overcome include; in-depth understanding of structure-electrical conductivity relationships, response of the composites under changing environmental conditions and piezoresistivity of different types of carbon nanotube/polymer sensing devices. In this thesis, direct current (DC) and alternating current (AC) conductivity of CNT-epoxy composites was investigated. Details of microstructure obtained by scanning electron microscopy were used to link observed electrical properties with structure using equivalent circuit modeling. The role of polymer coatings on macro and micro level electrical conductivity was investigated using atomic force microscopy. Thermal analysis and Raman spectroscopy were used to evaluate the heat flow and deformation of carbon nanotubes embedded in the epoxy, respectively, and related to temperature induced resistivity changes. A comparative assessment of piezoresistivity was conducted using randomly mixed carbon nanotube/epoxy composites, and new concept epoxy- and polyurethane-coated carbon nanotube films. The results indicate that equivalent circuit modelling is a reliable technique for estimating values of the resistance and capacitive components in linear, low aspect ratio-epoxy composites. Using this approach, the dominant role of tunneling resistance in determining the electrical conductivity was confirmed, a result further verified using conductive-atomic force microscopy analysis. Randomly mixed CNT-epoxy composites were found to be highly sensitive to mechanical strain and temperature variation compared to polymer-coated CNT films. In the vicinity of the glass transition temperature, the CNT-epoxy composites exhibited pronounced resistivity peaks. Thermal and Raman spectroscopy analyses indicated that this phenomenon can be attributed to physical aging of the epoxy matrix phase and structural rearrangement of the conductive network induced by matrix expansion. The resistivity of polymercoated CNT composites was mainly dominated by the intrinsic resistivity of CNTs and the CNT junctions, and their linear, weakly temperature sensitive response can be described by a modified Luttinger liquid model. Piezoresistivity of the polymer coated sensors was dominated by break up of the conducting carbon nanotube network and the consequent degradation of nanotube-nanotube contacts while that of the randomly mixed CNT-epoxy composites was determined by tunnelling resistance between neighbouring CNTs. This thesis has demonstrated that it is possible to use microstructure information to develop equivalent circuit models that are capable of representing the electrical conductivity of CNT/epoxy composites accurately. New designs of carbon nanotube based sensing devices, utilising carbon nanotube films as the key functional element, can be used to overcome the high temperature sensitivity of randomly mixed CNT/polymer composites without compromising on desired high strain sensitivity. This concept can be extended to develop large area intelligent CNT based coatings and targeted weak-point specific strain sensors for use in structural health monitoring.

With the development of modern industrial engineering technology, increasing demands of multifunctional materials drive the exploration of new applications of electrical conductive polymer nanocomposites (CPNCs). Toward applications of smart materials, sensing performance of CPNCs has gained immense attention in the last decade. Among them, strain sensors, based on piezoresistive behavior of CPNCs, are of high potential to carry out structural health monitoring (SHM) tasks. Poly(vinylidene fluoride) (PVDF) is highly thought to be potential for SHM applications in civil infrastructures like bridges and railway systems, mechanical systems, automobiles, windgenetors and airplanes, etc. because of its combination of flexibility, low weight, low thermal conductivity, high chemical corrosion resistance, and heat resistance, etc. This work aimed to achieve high piezoresistive sensitivity and wide measurable strain ranges in carbon nanotube based poly(vinylidene fluoride) (PVDF) nanocomposites. Four strategies were introduced to tune the sensitivity of the relative electrical resistance change (ΔR/R0) versus the applied tensile strain for such nanocomposites. Issues like the influence of dispersion of multi-walled carbon nanotubes (MWCNTs) on initial resistivity of PVDF nanocomposites and conductive network structure of MWCNTs, as well as piezoresistive properties of the nanocomposites, were addressed when using differently functionalized MWCNTs (strategy 1). In addition, the effects of crystalline phases of PVDF, mechanical ductility of its nanocomposites and interfacial interactions between PVDF and fillers on piezoresistive properties of PVDF nanocomposites were studied. Using hybrid fillers, to combine MWCNTs with conductive carbon black (strategy 2) or isolating organoclay (strategy 3), piezoresistive sensitivity and sensing strain ranges of PVDF nanocomposites could be tuned. Besides, both higher sensitivity and larger measurable strain ranges are achieved simultaneously in PVDF/MWCNT nanocomposites when using the ionic liquid (IL) BMIM+PF6- as interface linker/modifier (strategy 4). The detailed results and highlights are summarized as following: 1. The surface functionalization of MWCNTs influences their dispersion in the PVDF matrix, the PVDF-nanotube interactions and crystalline phases of PVDF, which finally results in different ΔR/R0 and the strain at the yield point (possibly the upper limit of sensing strain ranges). As a whole, regarding to the fabrication of strain sensors based on PVDF/MWCNT nanocomposite, in contrast to pristine CNTs, CNTs-COOH and CNTs-OH, CNT-NH2 filled PVDF nanocomposites possess not only high piezoresistive sensitivity but also wide measurable strain ranges. Gauge factor, i.e. GF, is ca.14 at 10% strain (strain at the yield point) for the nanocomposites containing 0.75% CNTs-NH2. 2. Using hybrid fillers of CNTs and CB to construct strain-susceptible network structure (conductive pathway consisting of string-like array of CNTs and CB particles) enhances the piezoresistive sensitivity of PVDF nanocomposites, which is tightly associated with the CNT content in hybrid fillers and mCNTs/mCB. The best piezoresistive effect is achieved in PVDF nanocomposites with fixed CNT content lower than the ΦC (0.53 wt. %) of PVDF/CNT nanocomposites. 3. ΔR/R0 and possible sensing strain ranges of PVDF nanocomposites were tailored by changing crystalline phases of PVDF and PVDF-MWCNT interactions. Besides, the increase of the strain at yield point in PVDF nanocomposites filled by CNTs-OH is more obvious than that in the nanocomposites containing the same amount of clay and CNTs. The nanocomposite consisting of 0.25% clay and 0.75% CNTs-OH have ca. 70% increase of the strain at the yield point (17%) and the GF at this strain is ca. 14, while GF for the nanocomposite filled by only 0.75% CNTs-OH is ca. 5 at 10% strain. 4. IL BMIM+PF6- served as interface linker for PVDF and MWCNTs, which significantly increased the values of ΔR/R0 and strain at the yield point of PVDF nanocomposites simultaneously. Besides, this increases with increasing IL content. With the aid of IL, the dispersion of nanotube and toughness of the nanocomposites are greatly improved, but the electrical conductivity of the nanocomposites is decreased with the incorporation of IL, which is related to the IL modified PVDF-MWCNT interface connection or bonding. GF reaches ca. 60 at 21% strain (the strain at the yield point) for PVDF nanocomposites filled by 10% IL premixed 2%CNTs-COOH.

Thesis (M. S.)--Electrical and Computer Engineering, Georgia Institute of Technology, 2009.
Committee Chair: Brand, Oliver; Committee Member: Adibi, Ali; Committee Member: Allen, Mark G.; Committee Member: Bottomley, Lawrence A.; Committee Member: Degertekin, F. Levent.

La polyvalence de la technique de track-etching a permis d’étudier plus avant l’effet piezoélectrique direct et indirect d’un film polarisé en poly(fluorure de vinylidène) PVDF en créant des membranes nanostructurées hybrides de nanofils de nickel (Ni NWs)/PVDF. Les propriétés magnétiques du nanofil de nickel, telle que la magnétorésistance anisotrope (AMR), ont été exploitées afin d’étudier la réponse de l’aimantation à la déformation mécanique de la matrice PVDF. En particulier, les déformations ont été induites soit par contrainte thermo-mécanique, soit par contrainte électromécanique (effet piezoélectrique indirect). La sensibilité d’un nanofil unique a permis de déterminer l’amplitude et la direction de la contrainte mécanique exercée à l’échelle nanométrique par la matrice PVDF. La résistance exceptionnelle de la réponse piezoélectrique directe du film PVDF polarisé à l’irradiation, telle que l’irradiation aux ions-lourds accélérés et aux électrons (domaine de doses < 100kGy) a été observée. Mis à part la conservation de la réponse piezoélectrique, les défauts engendrés par l’irradiation dans ce domaine de dose (scissions de chaines, augmentation de phase crystalline, réticulations) ont eu un impact significatif sur la structure du matériau polymère. L’ensemble de ces défauts, les uns prépondérants en-dessous de la dose-gel ( 10kGy), les autres au-dessus, forme une compensation d’effets antagonistes qui mènent à une réponse piezoélectrique globalement inchangée. Stimulé par la grande résistance du PVDF à l’irradiation en termes de réponse piezoélectrique, l’idée a été d’exploiter, en vue d’une application dans la récupération d’énergie, le réseau de nanofils de nickel inclus dans la membrane en PVDF polarisé pour étudier l’influence des nanofils de nickel sur la l’efficacité piezoélectrique. La présence du réseau de nanofils de nickel mène à un accroissement non négligeable de l’efficacité piezoélectrique. Reliée à la présence des nanofils, une augmentation de la permittivité diélectrique dans le PVDF nanostructuré a également été enregistrée. Une polarisation interfaciale entre les nanofils de nickel et la matrice PVDF pourrait expliquer cette valeur accrue par rapport au PVDF nanoporeux sans nanofils
The versatility of the track-etching technique has allowed to investigate deeper the direct and inverse piezoelectric effect of a polarized Poly(vinylidene fluoride) (PVDF) film in building nanostructured hybrid Nickel nanowires (Ni NWs)/PVDF membrane. The magnetic properties of the Ni NW, such as anisotropic magneto resistance (AMR), are exploited to investigate the response of the magnetization to a mechanical deformation of the PVDF matrix. In particular, the deformations were induced either by thermo-mechanical or an electro-mechanical (inverse piezoelectric effect) stress. The sensitivity of the single NW has allowed to determine the amplitude and direction of a mechanical stress exerted at the nano-scale by the PVDF matrix. The outstanding resistance of the direct piezoelectric response of polarized PVDF film to radiation, such as SHI and e-beam, (doses range < 100kGy) was reported. Beyond the conservation of the piezoelectric response, in this dose range, irradiation defects (chain scissions, increase of the crystalline -phase, crosslinking) had a significative impact on the polymer material. All these defects, ones predominant above the gel dose (herein 10 kGy), and the other ones below, compensate their antagonistic effects towards the globally unchanged piezoelectric responses. Motivated by the high radiation resistance of the PVDF in terms of piezoelectric response, the idea was to exploit Ni NWs array embedded in the polarized PVDF membrane to study the influence of the Ni NWs on the piezoelectric response in view of harvesting energy application. The presence of the Ni NWs array leads a non-negligible increase of the piezoelectric efficiency. Related to the presence of the NWs, an increase of the dielectric permittivity in the nanostructured PVDF was also reported. An interfacial polarization between the Ni NWs and the PVDF matrix could explain the higher efficiency value respect to nanoporous PVDF, without NWs

Several genera of bacteria residing in the sea surface microlayer and in the near-surface layer of the ocean have been found to be involved in the production and decay of surfactants. Under low wind speed conditions, surfactants can suppress short gravity capillary waves at the sea surface and form natural sea slicks. These features can be observed with both airborne and satellite-based synthetic aperture radar (SAR). Using a new microlayer sampling method, a series of experiments have been conducted in the Straits of Florida and the Gulf of Mexico in 2013 to establish a connection between the presence of surfactant-associated bacteria in the upper layer of the ocean and sea slicks. In a number of cases, sampling coincided with TerraSAR-X and RADARSAT-2 satellite overpasses to obtain SAR images of each study site. Samples collected from slick and non slick conditions have been analyzed using real time PCR techniques to determine Bacillus relative abundance in each area sampled. Previous work has shown that the sea surface microlayer plays a role in air-sea gas exchange, sea surface temperature, climate-active aerosol production, biochemical cycling, as well as the dampening of ocean capillary waves. Determining the effect of surfactant-associated bacteria on the state of the sea surface may help provide a more complete global picture of biophysical processes at the air-sea interface and uptake of greenhouse gases by the ocean.

博士
國立高雄應用科技大學
機械與精密工程研究所
104
In this paper, we utilize lithography technique along with etched optical fibers and SU-8 3050 photoresist to product long-period fiber grating (LPFG). The periodic comb-type structure of photoresist is patterned onto the LPFG. Then, we adopt the PDMS, which is a kind of polymeric organosilicon compound, to coat the grating structure of LPFG, and the packaged comb-type LPFG (PCLPFG) is fabricated. Subsequently, we use the electroforming process to deposit the nickel within the patterned periodic structure and we will obtain the metallic comb-type LPFG (MCLPFG). First, we use the PCLPFG sensors to detect the variations of the longitudinal and transverse loading. According to the results of the experiments, we can find that when the longitudinal and transverse loading are strengthen, the phenomenon of the transmission loss agrees with the coupled-mode theory and the dips of transmission loss perform as a quadratic function of cosine. We also execute the experiment of the axial loading test for three times to ensure the feasibility and reproducibility of the proposed optical fiber grating sensors. The better sensitivities of sensors are -53.892 dB/N and 0.00946dB/με, and the corresponding period of the fiber gratings are 600 and 620 μm. During the process of the magnetic field sensing experiment, we use the MCLPFG sensors to detect the strength of external applied magnetic field. In order to avoid thermal effect, we employ the NdFeB magnet to generate the magnetic field. The strength of magnetic flux density is determined by adjusting the vertical distance between the NdFeB magnet and metallic grating structure of MCLPFG. We also use the Gaussmeter to execute the real-time probing of the magnetic flux density. According to the results of the experiments, we can find that when the magnetic flux density is strengthen, the magnetic force would attract the metallic grating structure and induce the variation of strain induced refractive index. The phenomenon of the transmission loss agrees with the coupled-mode theory and the dips of transmission loss would descend or ascend. The best sensitivity is 0.04528 dB/Gauss at the grating period of 630 μm.

Aero engines are one of the most complex machines on this planet and have propelled the necessity of advanced material technologies. Health monitoring of the engine is performed by a variety of sensors and amongst these strain sensor is very important as it aids in evaluating the stresses experienced by the body. Unlike conventional foil gauge which tends to debond under hostile environments in the engine like high rpm of blades, temperature, mass flow, etc, thin film based strain gauges are likely to exhibit better adhesion on the substrates. The usage of Ni-Cr thin films in strain gauge sensor has been proven for static application, wherein the substrate does not experience the fluctuating loads. Material and mechanical aspects of thin films for design and development of thin film based strain gauge sensor for aero engine application was taken up as a research work. One of the objectives of the work was to characterize the Ni-Cr thin films with varying composition deposited by sputter deposition process and characterize the films for its microstructural features and mechanical properties. The correlation of these properties is performed and amongst the film compositions investigated the film with alloy composition of Ni-Cr:80-20 at% exhibits the most distinct columnar structure, highest electrical resistivity (2.037 μΩm), hardness (5.8 GPa) and the modulus (180 GPa). This Ni-Cr: 80-20 at% film exhibits no surface cracks when loaded in the elastic region of the titanium alloy GTM-Ti-64. Resistance to deformation under the action of externally applied load on a body results in stress within the body. In single or multilayer film stacking the stress experienced in the film by virtue of substrate deformation needs to be investigated quantitatively. The substrate stresses are transferred to the films by shear stresses at the interface. In order to measure the surface strain by change in the electrical resistance of the gage it is important to quantitatively evaluate the stresses in the films. Are these stresses very high to cause delamination and film cracking or are these stresses too less to be measured. In order to understand the stress evolution and transfer mechanism, an analytical approach, numerical simulation and experimental validation were performed. Thin film strain gauge device architecture has been engineered such that an insulating layer of alumina is deposited on substrate and a sensing layer is deposited on the insulating layer to avoid thermal mismatch and maintain the strain compatibility. A alumina of 45 micron thick alumina layer was successfully deposited on Titanium alloy (GTM-Ti-64) by sputter deposition without any edge delamination and microcracks. Finite Element Analysis (FEA) results showed that the axial and shear stress profiles at the Ti alloy-alumina interfaces for both single and multilayer architecture are similar and higher when compared with the stresses in alumina-NiCr. The shear stress profile for single layer and multilayer architecture follows the modified shear lag model with peak shear stresses at the extremes and peak axial stress at the centre of the film. The axial stresses in the alumina film is found to be significant in both FEA and validated by experimental findings with film fracture strength of 814 MPa. Similarly, the shear stresses were found to be minimal by FEA studies and the experimental finding suggests the film fracture under tensile mode. Complete strain transfer was observed from substrate to these thin films under both tensile and vibratory fatigue, suggesting proper adhesion of the alumina film on the Ti alloy substrate. The maximum strain compatibility of thin film alumina on Ti alloy substrate was found to be 0.22 %. A Goodman correction for the fatigue data under axial mode was performed and on combining the entire fatigue data for R = -1 linear fit was observed across all the data points wherein the Basquin equation was considered for data analysis and the fatigue strength coefficient and exponent are found to be 872.56 and -0.054 for alumina thin film on Ti alloy substrate. Thin film strain gauges (TFSG) with these characteristics were deposited on the compressor rotor blade of one of the typical aero engines. Thick contact pads and a new bonding technique are used for taking the lead wires. The entire multilayer structure with wire bonding was tested under static and dynamic (vibratory fatigue) conditions and TFSG exhibited a reproducible strain when compared against foil based strain gauge under both tension and compression. TFSG device was tested for a duration of 2200 seconds with a blade vibration frequency of 406 Hz i.e. 8.9x105 cycles. During the entire test duration, TFSG successfully measured strains from the aero engine blade.

碩士
國立聯合大學
光電工程學系碩士班
102
In this study, we made a stretched abrupt-tapered micro fiber interferometers by two-step processing tapered fiber, in order to sense strain and measure temperature characteristics. Firstly, we fabricated an abrupt-tapered fiber by discharging electrode to excite higher order cladding mode and then used hydrogen flame to make abrupt fiber stretched, enabling the fiber core and high order cladding mode to recombine and generate interference. In this experiment, by using Corning standard single-mode fiber (single mode fiber 28, SMF-28) and Erbium and Ytterbium co-doped fiber (Er/Yb codoped fiber, EYDF), we made sensors to compare results in different diameters. In case of using optic strain sensors made by EYDF fiber, with diameter of 2.8 μm, strain sensitivity reached up to 7.71 nm/mɛ, while using sensors made by SMF-28, with diameter of 4.2 μm, strain sensitivity reached 3.96 nm/mɛ and became temperature-insensitive from 30 to 70 degree Celsius. In this study, we provide a sensor with compact size, easy fabrication and cost efficiency. Hope in the future it will be widely applied in housing construction, machinery and equipment, environmental conservation, earthquake prediction and biomedical engineering.

碩士
國立臺灣大學
工程科學及海洋工程學研究所
100
This paper presents a pulsed-biasing strain gauge measurement system for wireless sensing applications. The use of pulsed-biasing on the strain gauge can reduce the power consumption of the sensor, which is crucial for low-power wireless sensors. Moreover, without using a constant bias current, power dissipated on the resistive bridge that includes the strain gauge can be reduced. Such reduction can also reduce measurement errors from an increased strain gauge temperature. A strain gauge wireless sensor was constructed using commercially available components. Measurement results show that the pulsed-biasing technique can be implemented with a duty cycle down to around 40% with negligible measurement error at a data rate of 32 Hz. The power saved is equivalent to an increased operation time of 50%.

Of late, the demand of wearable sensors has exponentially risen. For example, strain sensors that can be worn and are skin mountable play a significant role in the areas of human motion detection, healthcare, soft robotics, electronic skin, and sport as well as fitness tracking. It is known that traditional strain sensing devices made of semiconductors and metals exhibit low sensitivities (GF< 2), low strain sensing range (< 5%) and exhibit rigidity which are undesirable characteristics for wearable sensing applications. In order to address these issues, metal nanomaterials such as AgNWs-AgNPs nanohybrids are now employed as active and functional sensing devices for the fabrication of wearable strain sensors. Further, inkjet printing process has been utilized for fabricating wearable devices (together with advantages of wide sensing range and high sensitivity) due to its cost effectiveness and capability of large scale production. The stretchable, wearable, and skin-mountable strain sensors have garnered significant research attention in consumer and medical products which may be attributable to various aspects like cost-effectiveness and ergonomics fuelled by the development in miniaturized electronics, growing consumer awareness for health related issues, and constant need for medical practitioners to obtain quality medical data from patients. Recently, the fabrication of strain sensors with high sensitivity and high stretchability, which can precisely monitor subtle strains and large mechanical deformations exhibited by the human bodily motions, is critical for healthcare, human-machine interfaces, and biomedical electronics. However, a significant challenge still exists i.e. achieving strain sensors with high sensitivity, high linearity, and high stretchability by a facile, low-cost and scalable fabrication technique. Herein, in this research, we achieve AgNWs-AgNPs/Ecoflex based composite strain sensors via inkjet printing technique which precisely deposits functional materials in a rapid, non-contact and maskless approach allowing high volume production. Noteworthily, the fabricated strain sensor display many fascinating features, including high sensitivity (a gauge factor of 96.26), high linearity, a broad strain sensing range greater than ~ 55%, excellent stability and reliability (>1000 cycles), and low monitoring limit (<1% strain). These remarkable features allow the strain sensor to effectively monitor various human motions. This work opens up a new path for fabricating elastomer based strain sensors for wearable electronics. Next, a simple, eco-friendly and scalable fabrication process is presented for manufacturing flexible epidermis-like sandwich structured (i.e.AgNWs-AgNPs conductive network embedded between two slabs of Dragon skin) strain sensors based on AgNWs-AgNPs conductive network incorporated in a Dragon skin elastomeric polymer substrate. The AgNWs-AgNPs conductive network-Dragon skin nanocomposite based strain sensors demonstrate superior sensitivity with a gauge factor of 5.6 at an applied strain range of 40%-60% and a broad sensing range of up to 80%, while exhibiting superior performance and durability for more than 500 stretch-release cycles. The applicability of our high performance strain sensors for the detection of face expressions and large-strain joint motions is demonstrated in this work. Next, we fabricate electronic device comprising of highly conductive AgNWs-AgNPs network embedded onto highly stretchable natural rubber supporting material. When mechanical strains are applied, the disconnection of adjacent AgNWs-AgNPs along with opening-closing of microcracks in a reversible manner results in the variation of electrical resistance of the sensor exhibiting high sensitivity with discernible gauge factors. The inkjet printed strain sensor exhibit ultrasensitivity with a gauge factor of 170.8 coupled with a wide and linear sensing range of over 120%. Moreover, the sensor exhibit fast responsiveness to applied strains, low hysteresis, and remarkable cyclability. We demonstrate that the skin-mounted strain sensor can be used for multiscale sensing to monitor electrical resistance signals ranging from small-scale human face expressions to large-strain human joint motions. Furthermore, functional resistive-type strain sensor based on inkjet printedAgNWs-AgNPs conductive network on stretchable nitrile elastomer supporting material has been fabricated. The novel strain sensor employed disconnection of AgNWs-AgNPs and expansion of porous structure to accommodate the applied mechanical strains. The as-fabricated strain sensor exhibited low contact resistance, high sensitivity (GF~751.07), broad strain sensing range (>60%), fast responsiveness and remarkable long-term durability. The brittle nature of nanowires endows the wearable strain sensor with remarkable sensitivity and the elastomeric supporting material enhances the deformation ability of the strain sensor. Moreover, various demonstrations have been carried out for the detection of small-scale mechanical strains such as expressions of the face and monitoring of large-scale deformations like human joint motions. This study presents a unique sensor which can be useful for multi-scale sensing. The key novelty and contribution of this work is the chemistry for silver nanomaterial deposition with inkjet printing and the systematic testing of dynamic sensing performance of strain sensors under different conditions has been conducted. For example, the critical sensing properties of the wearable strain sensors have been evaluated such as stretchability or strain sensing range, sensitivity, linearity, hysteresis performance, and reproducibility.

This thesis reports the research and industry-related works carried out from the development of a fibre-optic strain sensor system for Civil Engineering applications. A sensor system consists of a number of core components, including the sensing element, interrogation/demodulation, multiplexing, signal processing and hardware equipment. In the process of development, a number of issues have been identified and investigated, which resulted in the improvement of the system performance, as well as the proposal of new techniques for the sensor system. First, an improved demodulation technique for a type of sensor, namely the fibre Fizeau interferometer (FFI), is presented. The technique is based on the improvement of the Fourier transform peak detection method, which suffers severely from the poor resolution and accuracy of finding the sensor cavity length. The improvement over the original method has been compared and verified through simulations and experiments. Second, a simultaneous demodulation technique for multiplexed FFI and fibre Bragg grating (FBG) sensors using the discrete wavelet transform (DWT) is proposed. Third, a multiplexing technique using amplitude-modulated chirped FBGs and the DWT is proposed. These two proposed techniques have been demonstrated experimentally through strain measurements. The strain resolution, crosstalk and limitations are investigated. In addition, simultaneous quasi-static strain and temperature sensing of different metal plates are performed. Fibre-optic sensors have found numerous applications in different areas. In this thesis, the use of FBG sensors in Civil Engineering applications is demonstrated in four experimental studies, including: (i) long-term measurement of drying shrinkage and creep of structural grade concrete; (ii) simultaneous measurement of shrinkage and temperature of reactive powder concrete (RPC) at early-age; (iii) measurement of coefficients of thermal expansion of cement mortar and RPC; and (iv) field-trial on the strain monitoring of the world?s first RPC road bridge. In addition, the experimental and practical issues of using FBG sensors are considered.

The inherent nature of distributed acoustic sensing technology is a direct result of two key components: optical fiber and the speed of light. Because the speed of light is constant and optical fiber is an isolated medium, combining the two creates a mechanism insulated from environmental interference that effectively “moves” at the speed of light. This process is most visible in the telecommunications industry where the technology transports large amounts of data over significant distances at very high speeds. The same factors that make optical fiber excellent for transporting data (high speed and low environmental interference) also make the technology very applicable for precise measuring applications. Because optical fiber is insulated, a change to the fiber will have a pronounced (measurable) effect. These measurable effects manifest themselves as changes in the amount of light that is reflected within the optical fiber. This change in reflected light can be measured and quantified to indicate both the specific location along the fiber where the change in reflection occurred and the magnitude of the change in reflection. Knowing both the location of the affected area and the extent to which the reflection changed allows for precise measuring and subsequently, educated inferences about what caused the changes initially. The ability of optical fiber to detect changes at myriad intervals over long distances has particular appeal for functions involving remote and hard to get to environments. Both of these conditions are inherent to the petroleum industry and provide substantial incentive for investigating DAS for oilfield applications.