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NEWS RELEASE

October 3, 2002

Media Contact:
   Troy Anderson, (858) 822-3075 or tdanderson@ucsd.edu

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UCSD DEVELOPS SENSORS FOR SAFER BRIDGES

UCSD engineers have developed magnetostrictive sensors to improve bridge safety and performance.

The University of California, San Diego (UCSD) has received a grant from the National Science Foundation (NSF) to develop a system of low-cost, nondestructive sensors to detect damage and ensure the safety of cable-stayed bridges. There are currently no well-established ways to inspect the steel tendons and cables that are critical to the performance of these structures. In cable-stayed bridges, the cables are attached to the towers, which alone bear the load.

Principle investigator Francesco Lanza di Scalea, a structural engineer at the UCSD Jacobs School of Engineering, along with co-PI and interim dean Frieder Seible and graduate student Piervincenzo Rizzo, are using the three-year $180,000 NSF grant to develop and test magnetostrictive sensors which will probe the strands and provide important health data.

A pair of in-situ sensors (one transmitter and one receiver), comprised primarily of simple electrical coils, are fastened around the cables. The sensors use a magnetic field and rather simple physics principles to gain the information needed to determine performance. Essentially, one sensor sends an ultrasonic signal and another sensor receives it. Signal features including velocity and amplitude provide the sensors with data that can reveal various defects and load levels. UCSD’s sensor development effort is being conducted in collaboration with the Federal Highway Administration (FHWA) NDE Validation Center in McLean, VA.

“When activated by an alternating voltage signal, the magnetostrictive transmitter modulates a static magnetic field that induces an alternate deformation in the steel,” explains Lanza di Scalea. In other words, the steel actually expands and contracts. “This results in the launch of an ultrasonic or stress wave that travels along the strand. In receiving the wave, the inverse mechanism is used - the incoming wave modulates the magnetic field and this produces a voltage in the receiving coil which can be read, as with any signal, using a device like an oscilloscope or computer acquisition card.” Monitoring can be done as frequently as every three seconds for real-time information, or as sporadically as desired.

While the use of magnetostrictive sensors in ultrasonic health monitoring techniques is well established, the application to steel tendons presents unique challenges that have not been fully solved to date.

The magnetostrictive sensors serve two purposes in this application. First, they monitor loads, such as from heavy traffic, in which case the speed of the ultrasonic wave is analyzed. Because loads are nominally the same throughout the cable, only one pair of sensors is required and can be placed anywhere along the strand. “Monitoring loads allows us to check alignment of components during construction and distribution of forces throughout the structure’s operating lifetime,” explains Lanza di Scalea. “If we detect a load that is substantially different from its design value, that will indicate some serious anomalies in the overall behavior of the structure (for example failure of some other cables that are not being monitored) that could lead to collapse.”

The sensors can also be used to find defects. In this instance, two pairs of sensors can be used to monitor the critical areas near both ends of a cable, where it is anchored and wedged— the location where most defects occur. Wave amplitude or strength reveals problems such as corrosion or cracks in the cables when compared to baseline data for the material. Lanza di Scalea notes, “If a cable breaks, the structure will often fail. This project is part of the broader topic of structural health monitoring that has become a major focus of the School.”

Lanza di Scalea, Rizzo and Seible have been invited to submit a paper to the Journal of Materials in Civil Engineering of the American Society of Civil Engineers (ASCE). The paper, titled “Stress measurement and defect detection in steel strands by guided stress waves” will appear in the February 2003 Special Issue on non-destructive evaluation and structural health monitoring. In addition, Rizzo recently received a $15,000 American Society for Nondestructive Testing (ASNT) Research Fellowship Award to further study and develop the new sensors.

A portable, self-contained, dedicated unit based on National Instruments PXI and LabVIEW technology is being developed for the sensor excitation, sensor probing and data analysis. The system will be able to interface to the Web for broad access to the data. Undergraduate student researcher Michael Palmer has been responsible for assembling the hardware and writing the dedicated software.

Large-scale testing in UCSD’s Charles Lee Powell Structural Research Laboratories should begin by the end of the year. The first bridge to use the sensors will be the much anticipated cable-stayed I-5 Gilman Bridge in San Diego, CA.

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