Electromagnetic stress sensor for bridge cables and prestressed concrete structures

This paper presents the design, construction, and test results of an electr omagnetic stress sensor. The sensor uses the reverse magnetostrictive effec t found in high elastic limit steels such as those used in cables and in pr estressed concrete. This effect is characterized by the variation in the st eel's magnetic permeability as a function of its internal stress. Consequen tly, the internal stresses in these high elastic limit steels can be found by measuring their permeability. The permeability can be measured indirectl y 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 als o used this program to optimize the sensing coil and the measurement freque ncy and to design the magnetic shielding around the sensor. We built and te sted the prototype in our laboratory. We evaluated the precision, accuracy, linearity, and reliability of the sensor, and also the influence of extern al thermal and magnetic perturbations on the sensor measurements. This pape r provides experimental data demonstrating that the prototype meets the des ired performance criteria. The major advantages of this sensor are its robustness and its ability to o perate continuously for several decades, even in hostile environments. Also , this sensor can be either embedded in or added to the structure (e.g., br idge cables and nuclear plant concrete foundations), which makes the task o f monitoring stresses easier and less expensive.