Study Of Sensing Behavior And Its Mechanisms Of Conductive Fibers/Fabric

As a typical kind of smart material, intelligent textiles have been widely used inmany fields such as aeronautics, medical health care, sports, entertainment anddetection and sensing area. The functions of intelligent textiles have developed fromtransmitting the electrical signal to detect and sense the force, strain or parameters ofgas. In recent years, the detection and sensing functions of flexible sensors have gotmuch more attention and one of them basing on the smart textile are flexible, folding,wearable and washable. Comparing with the traditional sensors, the flexible sensorshave a wider work range for its better elasticity. In despite of many researchers havefocused on the flexible sensor, most studies concentrate on the sample preparation andexperimental tests, while the sensing mechanisms are limited referred to. It is importantfor design and development of the sensor to understand the sensing mechanism, so inthis paper, the typical coated conductively electrical fiber and fabric are taken asexamples to study the mechanism of flexible sensor. The discussion of sensingbehavior consists of experimental and theoretical studies. No matter the fiber sensor orfabric sensor is included of two parts, the substrate (yarns or fabric) and the coatingmaterial. The substrate is made of the common fibers. The fabric is knitted by Nylonand LycraTMyarns. The conductive polymer Polypyrrole (PPy) and the conductivecomposite carbon-black filled silicon rubber are selected as the coating materialsrespectively.In the first part, the object of the study is the LycraTMfiber coated with PPy. Macroand micro experimental researches are carried on the sample respectively. Theresistance-strain curve obtained from the macro tensile test presents the sensingbehavior of samples and a sequence of Scanning Electron Microscope (SEM) imagesfrom micro test presents the surface morphology variation with tension. With the helpof image process in Matlab software, the parameters of microcracks are extracted fromthe SEM images. Basing on the results the function between the relative resistancevariation and strain of the sensor is established and it agrees well with the experimental.Subsequently, the parameters sensitivity of microcracks is analyzed and it proves thatthe width and length are the key factors influencing the resistance variation. In addition,the sensing behavior of PPy-LycraTMfiber is compared with PPy-PA6and PPy-XLATM fiber. All the three conductive fibers can be used as strain sensors, but the sensitivityand work range are different for the different mechanics behaviors of the substrates.Moreover, the shifts of resistance are also different under cyclic loading.Basing on the analytical results of PPy coated fiber, the sensing behavior ofPPy-coated plain weft knitted fabric is discussed later. The resistance variation withstrain can be analogized to unidirectional tensile process. It includes four deformationtypes, slipping, contact, bending and tension of the yarns. If the couplings of the fourprocesses are ignored, both the deformation and resistance variation can be divided intofour steps. During the tension of the yarn, the microcracks caused on the surface haveobvious effects on the resistance variation, so that the tension process is considered asthe most important step. Because of the lesser variation in former three steps, therelative resistance variation is seemed as linear functions of the strain. According to theresults of fiber sensor, a segment function can be established to describe the resistancevariation with strain. All the parameters undetermined in this function are related withthe knitted fabric structure. Furthermore the prestretching and coating on the arcsegment also have effect on the sensing process. Since the anisotropy of the knittedfabric, the sensing behaviors are different in the wale and course directions.With the development of conductive composite, it has been applied in the flexiblesensor. The latter part of the thesis is focused on the sensing behavior of fabric strainsensor with carbon-black filled silicon rubber. Referring to the references, theresistance variation of carbon-back filled silicon rubber is decided by percolationtheory and conductive path theory together. The I-V curves indicate that the sensorpresents ohmic type conductive behavior during the work range, so the effect offrequency can be ignored. The relative resistance-strain of fabric obtained by thesimple tensile test indicates an almost linear correlation during60%strain level. Sincethe fabric strain sensor has good linearity, sensitivity and wide woke range, it can beapplied to measure large stress/strain under high speed impact, such as the ballisticimpact or car crash etc.The work of the last part is to build a theoretical model reflecting the sensingmechanism of the fabric strain sensor. Analogizing to the mechanics constitutiveequation of viscoelastic materials, a strain rate-dependent sensing behavior model isestablished. The model consists of hyperelastic and viscoelastic response, which theparameters can be determined by fitting the tensile test results in low strain rate andhigh strain rate respectively. In the fitting process the relaxation time is proved not a constant, and it can be regarded as a power function of the strain rate. Finally thetheoretical model is verified reliable by comparing the prediction and experimentalresults.