Dynamic stress monitoring for RC beam using PMN-PT-based three-direction normal stress sensor

Reinforced concrete structures are widely used in bridge engineering and various infrastructures. However, material aging, poor load, and a complex service environment will lead to internal damage and resistance decay of the structure. Therefore, real-time monitoring of internal stresses is critical for evaluating structural health, providing timely warnings of potential failures, and ensuring reliable operations. This paper presents the design and fabrication of an embedded three-direction normal stress sensor based on PMN-PT piezoelectric single crystals, capable of simultaneously monitoring normal stresses in three orthogonal directions within a structure. The sensor comprises a high-strength concrete cube base, cover plate, copper sheet, PMN-PT patch, epoxy resin package layer, and shielding wire. To test the sensor's sensing performance, sinusoidal reciprocating, square wave, random, and sweep loads were applied, and the sensor output response and frequency response characteristics tests were carried out. Moreover, to assess the feasibility and accuracy of the sensor in monitoring internal three-direction normal stresses in actual structures, three sensors were embedded in a reinforced concrete beam, and dynamic stress monitoring experiments were conducted. The process was also simulated using COMSOL software. The results demonstrated that the sensor has a regular shape, is simple to fabricate, has a uniform sensitivity of 1.53 V/Mpa, and provides stable output signals with superior frequency response characteristics. The experimental and simulation results agree well, with the embedded sensors sensitively detecting internal stress variations and the voltage and stress outputs exhibiting a maximum error of less than 4.5 %. The research results can serve as an essential reference and guide for monitoring internal stresses in large-scale structures.