Fabric strain sensor integrated with CNPECs for repeated large deformation

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Fabric strain sensor integrated with CNPECs for repeated large deformation

 

Author: Yi, Weijing
Title: Fabric strain sensor integrated with CNPECs for repeated large deformation
Degree: Ph.D.
Year: 2012
Subject: Detectors -- Materials.
Hong Kong Polytechnic University -- Dissertations
Department: Institute of Textiles and Clothing
Pages: xx, 158 leaves : ill. (some col.) ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2615867
URI: http://theses.lib.polyu.edu.hk/handle/200/6969
Abstract: Wearable strain sensors have wide applications in rehabilitation, detection of human posture and gesture, and personal protection in sports and workplaces. They should be flexible, soft and able to measure large strain with good fatigue resistance. Conventional strain gauges like metallic foil are accurate, but they are too rigid to be wearable and the working range is limited to only several percent, which cannot satisfy the requirement of repeated large strain measurement in smart textiles. Carbon nanoparticle (CNP) filled elastomer composites (CNPECs) are of great interest due to their flexibility and sensing behavior at large deformation. On the other hand, knitted fabrics made from elastic fibers have good repeatability under cyclic extension without fracture at large deformation. Hence, the objective of this thesis is to investigate the fabric strain sensors integrated with CNPECs for repeated large deformation. Conductive CNPECs were fabricated and studied. High porous but low structured CNPs and room temperature vulcanized (RTV) silicone elastomer (SE) as well as silicone oil (SO) were selected as materials for the conductive composites based on a literature review. The mechanical and electrical properties of the composites were studied. The results show that the introduction of SO decreases modulus of the composites to less than 1 MPa without affecting their deformability. After volatilization of the excessive SO with a heat treatment, the composites showed good stability, which was confirmed by thermal gravimetric analysis. The electrical resistivity of the composites was dependent on CNP concentration. With the increase of CNP concentration, percolation appeared. The ranges of insulating, percolation and post-percolation were divided at the CNP concentration of 0.5 wt% and 2.5 wt%, respectively. The conductivity of the composites was explained by an equivalent circuit consisting of resistors and capacitors. I-V curves and impedance spectra confirmed the domination of electron hopping conductive mechanism of the composites with CNP loading in the percolation range, where the strain sensitivity of the composite was high, but the workable strain range is narrow and electromechanical behavior is obvious nonlinear with a large capacitive effect. They limited strain sensing applications in smart textiles where a large strain range is essential. In the post-percolation range, the sensitivity of the composites decreases. However, the workable strain sensing range increased and linearity improved simultaneously without increasing the modulus of the composite. The composite with 9 wt% CNPs could be extended up to 100% strain with good repeatability in strain measurement up to 50%. The composite also showed marginal strain rate dependence from 0.0017-0.17 /s and small humidity effect of 0.63%. The temperature had a significant effect on the strain sensitivity of the CNPECs. In addition, the fatigue resistance of the CNPECs was not sufficient for repeated applications in smart textiles.
Electrically conductive fabrics were fabricated by coating CNPECs on knitted fabrics via screen printing process. The influences of processing parameters, such as printing times, CNP concentrations, on the variation of electrical resistance were systematically studied. The electromechanical behavior of the conductive fabrics was studied for both DC and AC conditions. When subject to direct current, the conductive fabrics possessed linear I-V curves when the applied voltage is in the range of -1 V/mm and 1 V/mm, the slope varied with the strain applied from 0 to 60%. The impedance spectra of the CNPEC coated fabrics demonstrated that the CNPEC fabric with higher CNP concentration was less dependent on frequency. The capacitive behavior could be neglected up to 10³ Hz even with strain of 60% for the sample of 9.0CNP. Compared to the CNPECs, the strain sensitivity of the conductive fabric increased by about 100%. A mechanical pretreatment has been examined, and the results showed that the pretreatment increased significantly the strain sensing stability of the conductive fabric. The resistance change of the conductive fabric was found to be dependent on strain rate in the range of 0.02-4 /s. The effects of temperature and humidity were also studied experimentally at constant temperatures and relative humidity. The electrical resistance had a variation of 5% when temperature changed from 0 to 60 °C. The resistance varied only approximate 2% when the relative humidity changed from 20 to 90%. The strain distribution of knitted fabric under uniaxial tensile strain was analyzed using digital image correlation analysis (DIC) method, which demonstrated gradient distribution with the highest strain between two neighboring stitches and the lowest at the center of yarn. After coated on the knitted fabrics, the CNPECs also show gradient distribution under extension due to the good adhesion between composites and fabrics. The combination of nonlinear resistance-strain behavior of the CNPECs and non-even distribution of strain of the coated fabrics was attributed to the higher strain sensitivity of the coated fabrics compared to that of CNPECs. Extended on the studies of CNPECs and the coated conductive fabrics, a fabric strain sensor was designed and fabricated. The performance of the fabric strain sensor was evaluated experimentally. The Young's modulus of the packaged fabric strain sensor was less than 1 MPa; the strain gauge factor was 4.76 within the strain range of 0-40% and the hysteresis was 5.5%; the resistance relaxation was 5.56% with a constant strain of 40%; the fatigue life of the sensor was more than 100,000 cycles.

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