Crack sensor based on patch antenna fed by capacitive microstrip lines

When sensors based on a single patch antenna are used to monitor crack width, the issues of incomplete strain transfer ratio, insufficient bonding strength, and randomness of crack propagation will inevitably compromise sensor sensitivity and make the calibration uncontrollable. To circumvent these difficulties, this study presents a novel crack-measuring sensor based on a rectangular patch antenna fed by a pair of microstrip lines, which form a parallel plate capacitor as a crack-sensing unit. The sensing mechanism was analyzed using the transmission-line model. Then two crack sensor prototypes were designed and the dimensions were optimized. A crack opening simulator was customized to test the sensors by using different width resolutions. The experiment results validated that the resonant frequency shifts are linearly proportional to the applied crack width; moreover, the crack sensor proved capable of detecting crack widths as small as one-hundredth of a millimeter in an object. (paper)