Electromagnetic Stress Sensor for Bridge Cables and Prestressed Concrete Structures

This paper presents the design, construction, and test results of an electromagnetic stress sensor. The sensor uses the reverse magnetostrictive effect found in high elastic limit steels such as those used in cables and in prestressed concrete. This effect is characterized by the variation in the steel's magnetic permeability as a function of its internal stress. Consequently, the internal stresses in these high elastic limit steels can be found by measuring their permeability. The permeability can be measured indirectly by measuring the inductance of a coil placed around or near the cable. We designed a prototype of the sensor with a finite element program. We also used this program to optimize the sensing coil and the measurement frequency and to design the magnetic shielding around the sensor. We built and tested the prototype in our laboratory. We evaluated the precision, accuracy, linearity, and reliability of the sensor, and also the influence of external thermal and magnetic perturbations on the sensor measurements. This paper provides experimental data demonstrating that the prototype meets the desired performance criteria. The major advantages of this sensor are its robustness and its ability to operate continuously for several decades, even in hostile environments. Also, this sensor can be either embedded in or added to the structure (e.g., bridge cables and nuclear plant concrete foundations), which makes the task of monitoring stresses easier and less expensive.