Real-time hoop strain monitoring is known as an important parameter for the evaluation of pipeline safety and integrity. Internal corrosion and consequently variation of wall thickness directly reflects in hoop strain variation. In addition, leakage causes a pressure drop and strain reduction due to negative pressure wave. Due to their promising features such as extremely low cost, relatively high sensitivity, compatibility with harsh environmental conditions, distant and non-contact sensing with negligible power consumption, microwave resonator-based sensors achieved great deals of interest during the last decade. In this work, a chipless flexible microwave sensor for pipeline hoop strain real-time monitoring is presented. The sensor structure comprises a flexible chipless split ring microwave tag resonator attached to the pipeline and electromagnetically coupled to a pair of gap coupled transmission lines form the reader located at a certain distance from the tag strain sensor. Strain variations as the results of the mentioned pipeline defects change the overall length of the attached tag sensor which consequently causes a shift in its resonance frequency. For assuring the tag sensor to mechanically follow the strain variation of the pipeline, the Young modulus of its structural material should be much lower than that of the pipeline. This condition also important for the integrity of the sensor-pipe system because their connection will be accomplished by an adhesive. Since copper as the standard microwave conductive material is relatively highly stiff, it is not an appropriate candidate for such an important application. For addressing this issue, the chipless tag structure is fabricated by a conductive rubber layer in this work with extremely low Young modulus guaranteeing the length of the tag strain sensor to exactly follow the strain variation of the pipeline and forms a reliable and precise pipeline strain sensor. The spectrum of the tag sensor is reflected on the reader structure spectrum which could be measured to monitor the resonance frequency shift of the tag resulted from length variation of the tag sensor directly related to the pipeline strain fluctuation.

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