The Design And Fabrication Of Isotropic Flexible Strain Sensors For Skinned Structures

With the development of materials science and the pressing requirements for multifunctional types of equipment,flexible deformable structures instead of rigid structures have taken the essential position in the fields of unmanned equipment,aerospace,and biomedicine,due to their excellent adaptability.Since the flexible skin structure(FSS)is one of the most commonly used flexible structures,the research on FSS sensing has become one of the hot spots in this field.The FSS suffers large deformations with uncertain directions and strong environmental coupling,which brings a great challenge to the accurate monitoring of structural deformation.Therefore,flexible strain sensing technology with high sensitivity,a large working range,multi-directional monitoring,and multidimensional measurement is highly essential.This thesis mainly focuses on the design and fabrication technology of flexible strain sensing from sensing films,sensing units,and their expanded structures.Firstly,sandwiched film structures based on graphene and silver nanowires are designed,followed by the quantitative modeling of the sensing film.As a result,an isotropic flexible strain sensor and its expanded structures are optimized and fabricated based on the sandwiched sensing film,which settles the deformation and health monitoring problem for FSS in different application scenes.1.Design and Test for the Sandwiched Sensing FilmsThe large deformation of FSS was hardly accommodated by the traditional flexible strain sensors with limited working range and sensitivity.In this thesis,a sandwiched sensing film based on silver nanowires and graphene is proposed to overcome the mutually restricted relationship of the working range and sensitivity.The sensing mechanism of the sandwiched film to achieve the large working range and high sensitivity in the meantime is analyzed.To fabricate the sensing film,vacuum filtration and film transfer procedures are introduced and followed by optimizing the fabrication parameters.Then the structures of the produced films are guaranteed by the micrographic evaluation.And their sensing properties are measured by a customized test platform.Sensing films with distinct thickness,width,and structures are produced and tested for comparison purposes.Results show that the sandwiched sensing film meets the monitoring requirements for the FSS,surpassing the sensing film with other types of structure.2.Modeling of the Sensing Films on the Strain-resistance PropertiesIt was a massive challenge for the sandwiched film to obtain the quantitative relationship between the strain-resistance(S-R)property and the sensing mechanism.In this thesis,a characterization method is proposed based on the Fourie transformation and power spectrum of the dynamic crack images under different strains.The fractal and the percolation phenomenon are demonstrated from the characterization method,respectively.From the fractal property,the equivalent circuit models of the cross-section and the longitudinal section are constructed,achieving the quantitative characterization of the sensing mechanism of the sandwiched film.From the percolation phenomenon,the resistance networks models are introduced to calculate the S-R property of the sandwich sensing film.Results show that the above models are accurate and effective for characterizing the S-R property of the sandwiched sensing film.3.Design and Optimization of the Isotropic Strain Sensor Unit Based on the S-R ModelThe multidirectional sensing properties of the traditional strain sensor were inconsistent and accompanied by calibration difficulty.In this thesis,the design and optimization method is proposed to obtain an isotropic strain sensor based on the S-R model,as the feasibility of the isotropic sensing property derived by the shape design is also analyzed.In the method,the shape control and its deformation calculation are employed by introducing isogeometric analysis(IGA),which significantly decreases the design parameters with accurately controlling the designed shapes.Then an algorithm based on design parameters and the least square method is proposed to obtain isotropic property in 0°to 30°.Simulated results show that the sensor unit with the optimized shape endows isotropic properties in 0°to 30°,which gives evidence to the effectivity of the proposed optimization method.4.Expanded Arrays of the Isotropic Strain Sensor Unit and Their Sensing PropertiesThe isotropic and multifunctional requirements for sensor units varied with different applications and were influenced by the coupling phenomenon.In this thesis,the expanded method for the isotropic sensor unit is proposed by introducing symmetrical and latticed structures,which expands the range of isotropic property to 360°.Then pressure/tactile sensing array is integrated by designing the non-uniform stiffness substrate with cavities,which raises the types of measured parameters and obtains parameters isolation.The sensing and coupling properties of the isotropic sensor unit and its expanded structures are calculated based on the Kirchhoff equations.And the decoupling method for the expanded sensing array is proposed on the basis of the electrical impedance tomography(EIT)technology,achieving the absence of data coupling.Moreover,the fabrication method based on the flash stamp machine and the film transfer is designed to obtain the isotropic sensor unit and its expanded structures,followed by the measurement and evaluation of sensing properties.Experimental results show that the designed sensor unit endows isotropic sensing property in 30°and can be expanded to 360°as well as the accompanying multifunction.5.Applications for the Istropic Strain Sensor Unit and Its Expanded ArraysFor the practical application of the isotropic sensor unit and its expanded structures,the proper types of sensor structures are chosen according to the sensing requirements of wearable devices,flexible mechanic structures,and integrated sensing systems.The high sensitivity,wide working range,and isotropic property are demonstrated in the above applications,verifying the applicability and superiority of the isotropic sensor unit and its expanded structures.In conclusion,this thesis aims at the requirements for strain sensing and health monitoring of FSS in diverse applications.A sandwiched sensing film that endows the high sensitivity and a large working range is newly designed,followed by the quantitive modeling of its sensing properties based on crack images and resistance networks.Then,an isotropic flexible strain sensor unit derived from the above sensing film is designed and optimized on the basis of isogeometric analysis.Moreover,the isotropic sensor unit is expanded by introducing the structures with symmetry,lattice,and cavity,which naturally avoids the coupling stemmed from multidirectional monitoring and realizes the data decoupling based on the EIT.As a result,the isotropic sensor and its expanded structures demonstrate the isotropic sensing properties in 360 and the multifunctional sensing properties.The theoretical method and research findings of this thesis provide great potentials for the practical applications of flexible strain sensors.