| In the fields of large equipment and key engineering, it is necessary to detect the contact stress in the connection status and the use procedure of shell components, piping components and so on by real-time, of which the important information of active condition and inventory performance for the key materials and its parts can be grasped accurately. So the contact stress sensitivity measurement for the components with curved surface structure is becoming an important topic in the field of force sensor.A series of novel flexible stress sensitive composite film of FeCuNbSiB soft magnetic powder/silicone rubber ("FeCuNbSiB/SiR composite film") were successfully fabricated by mechanical blending and precise cast pressure hot forming method, using FeCuNbSiB soft magnetic powder as functional filler particles and ternary block silicone rubber as matrix. The preparation technics, microstructure, physical and chemical properties of FeCuNbSiB/SiR composite film were studied systematically in the paper. Then, the measuring technique and characterization methods for the stress sensitive characteristics of the composite film were designed and developed step by step. On this basis, the stress sensitive characteristics of the composite film under different loading mode was studied systematically and the composite films with excellent stress sensitive property were optimized through a large number of experiments. The stress sensitive mechanism of the FeCuNbSiB/SiR composite film was preliminary discussed based on the peculiar stress sensitive characteristics. Finally, the flexible contact stress sensor prototype system was trial-produced according to the optimized stress sensitive film. The major results obtained in the study are as follows.(1) On the respect of composite film preparation. FeCuNbSiB/SiR stress sensitive composite film was researched and developed based on FeCuNbSiB soft magnetic powder as functional filler particles with stress sensitivity and ternary block silicone rubber as matrix, of which the length, width and thickness of the composite film can be controlled in the range of less than200mm, less than100mm and 50-2000μm, respectively. The effects of filler particle types, phase structure, particle size and surface modification on the physical and chemical properties of FeCuNbSiB/SiR composite film were studied in detail and the rubber compounding was also optimized through vulcanization technology. Thus, such problems as uniform dispersion of soft magnetic powder into the rubber matrix, the interface control between rubber matrix and soft magnetic powder and the thickness control of the composite film, were solved successfully. It shows that the physical and mechanical properties of FeCuNbSiB/SiR composite film can meet the requirements of flexible stress sensitive sensor with4.73MPa tensile strength,168%elongation at break,3.28%compression permanent deformation,51.8%rebound resilience and75A shore hardness.(2) On the respect of the measuring technique and characterization methods for the stress sensitive characteristics of composite film. For the stress-impedance measurement of the composite film under axial forces, the stress-impedance measurement system was designed and developed from static loading, dynamic loading to the static and dynamic loading system, which can eliminate the instability of testing system, and contact error effectively."LCR testing puls inductive reactance compensation analysis" method was adopted to measure the stress sensitive characteristics of the composite film, Z-σ standard equivalent curve to represent the stress sensitive characteristics, standard deviation of Z-σ standard equivalent curve to evaluate the stability of stress sensitive characteristics and the stress impedance effect SI%to represent the sensitivity of the composite film. The optimized system for stress sensitive property measurement is composed by TH2816LCR digital electric bridge, LYYL-500N high-range computer control compression-testing machine, inductance compensation circuit and so on, and the composite film system was connected by capacitance mode. The measurement conditions are set as20mm×20mm for size,1kHz for frequency, less than0.5mm/min for the pressuredynamic loading/unloading speed and120s for the compressive load time.(3) On the respect of the stress sensitive characteristics’research of the composite film. The results show that the stress sensitive characteristics of the composite film are excellent when the test frequency is less than100kHz. The stress sensitivity of the composite film decreases with the increase of the test frequency, and the impedance Z and stress sensitivity of the composite film also decrease with the pressureincreasing. When the pressureis in the range of0.1MPa～0.54MPa, the stress sensitivity presents linear increase and the amplification reaches to maximum value. When the pressureis in the range of0.54MPa～1.5MPa, the stress sensitivity increases slowly and presents nonlinear variation. The thickness of the composite film, the soft magnetic powder’s size, content and its dispersion uniformity are the major factors that can affect the stress sensitivity of the composite film. It is also found that the factors above have strong relationship between each other, so the optimization should be overall considered by the complexity, operability, quality stability of the preparation technics for soft magnetic powder and composite film, especially the limiting conditions for the thickness of the composite film in practical application. Through the optimization of internal and external influencing factors for stress sensitive characteristics of the composite film, the optimum conditions are as follows: the test frequency should be less than100kHz, the pressureless than0.5MPa, the particle size of FeCuNbSiB soft magneitic power in the range of (10～30) μm, the heat treatment condition480℃xlh, the content of FeCuNbSiB in the range of (22.5～35)vol%, the thickness of the composite film less than300μm and the environment temperature13℃～50℃. The optimized composite film with27.5vol%powder content and200±2μm thickness possesses excellent stress sensitivity, stability at the conditions of0.5mm/min continuous load/unload speed or pressure maintaining1kHz frequency and0.2MPa～1.0MPa pressurerange. The value of SI%is6%～23%, the standard deviation is0.02～0.05and the stress sensitivity is more excellent when the pressureis less than0.3MPa.(4) On the respect of stress sensitivity mechanism research of the composite film. It was found that the composite film showed stress-capacitive and stress-resistance characteristic during alternating test circuit by the analysis of spectral characteristics for impedance Z, inductance L, capacitance C, resistance R and phase angle θ. The equivalent circuit of the whole test system is in the structure of total capacitance C and total resistance R’s parallel connection, and series connection with inductance L of the circuit. The key factors to affect the stress-capacitive and stress-resistance characteristic of the composite film are the modulus and powder content. It is special to see that the flexible stress sensitive composites present remarkable double percolation characteristics at the frequency of50kHz.(5) On the respect of the flexible contact stress sensor prototype system. The optimized FeCuNbSiB/SiR composite film was packaged by the flexible circuit printing and hot-press packaging technique. It was found that the stress sensitivity of the composite film after packaging was not been affected obviously. Thus, the flexible contact stress sensor prototype system is provided with good stress sensitivity and impedance creep property produced by the flexible circuit printing and hot-press packaging technique above.FeCuNbSiB/SiR composite film is in accord with the essential requirements of flexible contact stress sensitive sensor, and possessing the prominent dominant of environmental friendly, insulation, thickness control and so on, which is expected’to become a new potential generation of sensitive material used in contact stress sensor field with excellent application prospect. |
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