Structural health monitors, like strain gauges and temperature sensors, are built to withstand high temperatures, typically up to 750 C. As engineers look to place these devices in even hotter environments, the hunt is on for more durable options.
Flame on. Luna performed many tests to learn how fast its temperature sensors could be heated and still track the temperature. The sensors accurately tracked the temperatures of propane torches, wind tunnels, and machining laser beams.
Luna Innovations, Inc. (Blacksburg, VA), has developed fiber-optic-based strain and temperature sensors that significantly extend the temperature limits of structural health monitoring. The new sensors can operate between -269 C and 1400 C, a range that few, if any, sensors can match. Other important features include millisecond response time and compact size about the width of a human hair (about 160 microns). The sensors were developed with MDA SBIR funding to assess damage to stainless steel targets from high-energy laser weapons such as the Airborne Laser.
The first application of Luna's temperature sensors is in hypersonic aircraft research. The Air Force Arnold Engineering and Development Center is using the company's sensors to measure the very high rate of change in temperature on the surface of a hypersonic vehicle. The temperature of the air when it stops at the leading edge surface of hypersonic vehicles can range up to 3000 C. High-response-rate sensors are essential for making these measurements. Luna's temperature sensor has demonstrated the ability to track temprature changes at a rate of up to 2.4 million C per second.
Aircraft manufacturers that use carbon-carbon composite materials are using Luna's sensors to make strain measurements in environments that are more than 1100 C. An engine manufacturer may also use Luna's strain sensors to measure the deformation in and around gas turbine aircraft engines. Conventional strain sensors are based on piezo silicon carbide, which degrades at around 700 C to 750 C. A turbine engine structure approaches around 1000'C. Luna's strain sensors have a high-temperature measurement capability demonstrated at more than 1100 C.
The sensors and their associated cabling are fabricated using fiber optics. To measure both strain and temperature, a broadband light source is transmitted to the cleaved end of a single-mode fiber. To perform the strain measurements, upon reaching the end of the fiber, the light is partially reflected while the remaining light travels past the end of the fiber and is reflected off a secondary reflector. The reflectors (also fibers) are aligned with the main fiber in a capillary tube and attached to a substrate. The two reflected light signals interfere with each other forming a fringe pattern. As the substrate strains, the distance between the two fiber end-faces vary, causing the fringe pattern to change. Using a spectrometer, the changing gap is measured to obtain the strain. The sensor is less prone to failure because the fiber itself is not being strained by the substrate.
The temperature sensor has a small, single-crystal chip on the end of the fiber. The two faces of the chip are reflectors. Precise temperature can be obtained by measuring the temperature-dependent optical path length through the chip.
The company welcomes any inquiries regarding either sensor.
--T. Robinson