| The requirements of engineering structures in terms of high performance,multifunctionality,multi-purpose,and landscaping have increased the uncertainty during construction and in-service,and the complex structural information and various shaped and narrow monitoring working surfaces have brought new challenges to structural health monitoring(SHM).A number of flexible strain sensors with excellent performances have been successfully developed,based on different sensing materials and sensing principles through rational structural design.However,piezoresistive sensors are still inadequate in responding to dynamic signals,and piezoelectric sensors also have shortcomings in sensing static loads and deformations.Therefore,attempts to combine piezoresistive and piezoelectric materials through rational material selection and structural design to exploit the complementary functions of both static and dynamic signal sensing,to prepare a sensor with high sensitivity,high flexibility,and dual-mode complex strain sensing capability.The main research elements are as follows:(1)The CNT dispersion slurry was applied to the cotton substrate by spraying.The result of four-probe test results showed that CNT adhered uniformly and the total resistance decreased with the increase of the number of spraying times,with the most uniform at 4 spraying times and the ratio of diagonal resistance RAB,CD/RBC,DA was closest to 1,at 1.047.The microwave ablation of CNT fiber cloth was then used to prepare a porous CNT ablative skeleton using the wave absorption properties of CNT and the pyrolysis properties of cotton fibers,and the G-peak of the ablated CNT was slightly red-shifted in the RM test but the intensity was basically unchanged,indicating that the CNT structure was not destroyed and the ablation reduced the defects instead,reducing ID/IG from 1.038 to 0.718.7cm × 7cm skeleton weighs only 0.0776g and SEM shows that the CNT skeleton has been given a unique laminar interweave structure through attachment and cotton fiber removal.(2)A soft PVDF film with a thickness of 0.18-0.2mm was obtained by the solution diffusion method.The polarization field intensity was set to 50MV/m,and the high piezoelectric film was obtained by controlling the polarization temperature.The dielectric constant,piezoelectric constant,FTIR,and XRD tests were carried out on the polarization results,and the results showed that:The polarization treatment significantly enhanced the piezoelectric properties of the film and the characteristic peak intensity of the β phase in the crystal phase.At 90℃,the relative dielectric constant,piezoelectric constant,and β phase content reached the maximum,respectively 14.28(low frequency),13.3pC/N,and 84.977%.(3)Dual-mode flexible sensors assembled from porous CNT ablative skeleton and PVDF film using PDMS elastomers can be bent to 180 ° and are approximately 2.2 mm thick.The special porous morphology and lamellar interlacing structure significantly improve the static piezoresistive performance of the piezoresistive layer of the dual-mode sensor.The maximum sensitivity GF coefficient of the ablative skeleton is 259.688,the maximum linearity is 0.9968,the maximum hysteresis is 6.669%,and the response time is 0.028s.The resistance change rate(△R/R)remained stable during 10,000 times loading and unloading and cyclic tests with different loading rates.The peak output voltage of PVDF film is correlated with the impulse,the minimum detection threshold is about 95kPa,the response time is 0.01s,and the PVDF film also shows different responses to different vibration speeds generated by knocking and impulse.Finger bend of 0-90° and strain of 0-1720 με loading process of complex strain test of dual-mode sensor showed good conformal ability,and in the monitoring strain pressure resistance and the piezoelectric signal can be synchronous output two signals at the same time,the pressure resistance signal △R/R reflect the specimen strain size,piezoelectric signal strain dynamic information such as the vibration amplitude of the specimen and the pulse height can be obtained.The deformation information of finger movement and different loading processes can be reflected by combining the two signals:When strain increases step by step from 0 to 1720με,△R/R increases from 0 to 13.28%,this process △R/R changes proportionally to the strain and the peak voltage signal is positive;When the strain decreases △R/R decreases,the peak voltage signal is negative.The sensor detects the free vibration of the specimen,at which time the voltage signal appears attenuation waveform.Dual-mode flexible sensors have a good application in the field of SHM due to their sensing performance,denaturability,and complementary signal acquisition functions,and are expected to play an important role in the monitoring of specialshaped structures and complex strains.The micro-structure microwave ablation forming process used in this paper is simple to use and the flexibility to change the size of the product by controlling the size of the cotton substrate helps to move the sensor from the laboratory to the factory. |
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