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.
Crack orientation is a very important parameter for structural integrity analysis. Even though many sensors have been developed to detect crack presence and length, very few sensors can detect crack orientation. Recently, we have demonstrated a patch antenna sensor that can detect crack propagation with sub-millimeter resolution. In this paper, the capability of the antenna sensors for crack orientation detection is studied. An antenna sensor with a rectangular antenna patch radiates at two fundamental modes. The effect of a ground-plane crack on these two resonant frequencies depends on its orientation. Hence, the crack orientations can be monitored by analyzing the crack-induced shifts of both antenna resonant frequencies. The principle of operation will be discussed first, followed by detailed descriptions on the numerical simulation, sensor fabrication, experimental procedure, results and analysis. Simulation as well as experimental results confirmed that the antenna sensor is capable of providing quantified information about the crack orientation.
A major goal of structural health monitoring (SHM) is crack detection and monitoring. Because cracks are localized defects, quantifying the sizes and locations of the cracks would require placing many sensors over a large area. We present a crack sensor that is suitable for densely distributed sensor network because they can be remotely interrogated and do not need any wiring for data transmission or power supply. A rectangular patch antenna, consisting of a metallic patch on one side of a dielectric substrate and a ground plane on the other side of the substrate, is studied in this paper for crack detection and monitoring. The presence of a crack in the ground plane of the antenna changes its conductivity, which in turn reduces the resonant frequency of the antenna sensor. Experimental results demonstrated that the antenna sensor can monitor crack growth with sub-millimeter spatial resolution. Detailed experimental set-up and measurement results are presented.
A foot ulcer is the initiating factor in 85% of all diabetic amputations. Ulcer formation is believed to be contributed by both pressure and shear forces. There are commercially available instruments that can measure plantar pressure. However, instruments for plantar shear measurement are limited. In this paper, we investigate the application of antenna sensors for shear and pressure measurement. The principle of operation of both antenna sensors will be discussed first, followed by detailed descriptions on the antenna designs, sensor fabrication, experimental setup, procedure and results. Because the antenna sensors are small in size, can be wirelessly interrogated, and are frequency multiplexable, we plan to embed them in shoes for simultaneous mapping of plantar shear and pressure distributions in the future.
A microstrip patch antenna sensor was studied for shear sensing with a targeted application of measuring plantar shear distribution on a diabetic foot. The antenna shear sensor consists of three components, namely an antenna patch, a soft foam substrate and a slotted ground plane. The resonant frequency of the antenna sensor is sensitive to the overlapping length between the slot in the ground plane and the antenna patch. A shear force applied along the direction of the slot deforms the foam substrate and causes a change in the overlapping length, which can be detected from the antenna frequency shift. The antenna shear sensor was designed based on simulated antenna frequency response and validated by experiments. Experimental results indicated that the antenna sensor exhibits high sensitivity to shear deformation and responds to the applied shear loads with excellent linearity and repeatability.
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