To solve the defect where a normally working traditional crack sensor requires a power supply,a passive crack sensor design based on an unstressed combined patch antenna is proposed in this paper,which utilizes a U-shaped re-sonator to realize crack sensing and further change the resonant frequency of the resonant unit inside the sensor.The res-onant frequency of the sensor can be extracted by observing the amplitude and phase of the echo signals to realize moni-toring of cracks.Based on the theory of antenna,an equivalent circuit model of the sensor was established and its reso-nant frequency was derived for the study of the effects of the crack width on the resonant frequency of the sensor.Fur-thermore,numerical simulations were conducted for sensors in the three-dimensional electromagnetic simulation software CST,and the influence of crack expansion on the resonant frequency was studied through experiments.Both simulation and experimental results show that the resonant frequency of the sensor increases with the crack expansion and that there is a good linear relationship between the crack width and the resonant frequency.
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