Fatigue cracking is one of the most common failure modes of various load-bearing structures. Even though sensors of many different types have been developed for crack detection, very few can monitor crack growth with a high sensitivity. This paper presents an antenna sensor that is capable of monitoring the growth of fatigue cracks with a sub-millimeter resolution. According to microstrip patch antenna theory, the resonant frequencies of a dual-frequency patch antenna are inversely proportional to the electrical lengths of the corresponding antenna radiation modes. The presence of a crack in the ground plane or the elongation of the antenna patch due to crack opening increases the electric length, thereby causing a shift in its corresponding resonant frequency. As a result, crack propagation and opening can be monitored from the resonant frequency shifts of the patch antenna. The patch antenna's capability of monitoring crack growth was validated using fatigue testing of a compact tension specimen. The specimen preparation, sensor fabrication, and experimental procedure are presented. The experimental results demonstrated that the corresponding resonant frequency of the antenna sensor shifted linearly with crack growth. On average, 1 mm crack growth caused the antenna frequency to shift by 22.1 MHz. The orientation of the crack and the effect of crack closure on the resonant frequencies of the antenna sensor are also discussed.
This paper presents a novel pressure sensor consisting of a low-cost microstrip patch antenna placed a distance from a metal reflection plate. The pressure applied on the plate changes the distance between the metal plate and the patch antenna, which shifts the resonant frequency of the antenna sensor. The operation principle of the pressure sensor is firstly presented. Subsequently, the design and fabrication of the antenna sensor as well as the electromagnetic (EM) simulations of its response to the applied pressure are described. Finally, static experiments are performed to validate the performance of the pressure sensor and the results are discussed.
This paper presents a microwave antenna sensor that has the ability to monitor fatigue cracks between lap joints where compressive force is present or in any other areas where a crack is hidden between two parts of a structure. The sensor is designed to be low profile, light weight, and conformal, making it ideal for aerospace applications where lap joints are a common design feature, and thus minimizing manual inspection times. The design of the fatigue sample for simulating a lap joint will be presented. The experiment results validated the microwave antenna sensor’s capability to effectively monitor fatigue cracks under lap joints.
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