University Park, Pa.—The same material that sets off alarms in a department store when a salesperson forgets to remove the antitheft tag may make inexpensive temperature and stress sensors to ensure the safety of highways and concrete buildings, said Penn State researchers.

"These materials typically cost about $100 a mile, and each sensor is about an inch long," said Dr. Craig A. Grimes, associate professor of electrical engineering and member of Penn State's Materials Research Institute.

The material used in these sensors is an amorphous ribbon of alloy manufactured to be softly magnetic by quick cooling. In magnetically soft materials, the magnetic field switches back and forth, depending on the environment, and generates many higher-order harmonic frequencies.

"When an externally applied magnetic field reaches the sensors, they ring like a bell, emitting both magnetic flux and acoustic energy with a characteristic resonant frequency," said Grimes.

To use these strips as temperature and stress sensors, an activator must pass near the sensor strips. There are no wires or connectors, so the sensors are simply embedded.

A simple loop that generates a magnetic field activates the sensor from a distance. The materials in the road surface do not block this magnetic field, and iron, such as rebar in construction concrete, does not alter the results. A figure-eight loop senses the strips' response, reading the harmonics of the magnetic field.