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Stealth Furnace - High Temperature TSA Furnace
Stress Photonics has recently released a furnace designed to address the specific problems of elevated-temperature TSA. Under Air Force funding, the "Stealth Furnace" was used in combination with the DeltaTherm stress measurement system to monitor crack growth at elevated temperatures. Although previous work performed by Enke and Lesniak demonstrated the ability of TSA to work at elevated temperatures as high as 1100°C (2000°F), there was not a complete understanding of thermal radiation or the problems that impede high-temperature testing. Applying TSA to extreme environments was still very much a black art that would often fail to yield results. To improve on this, a deeper understanding of the relationship between thermographic methods and the test environment was necessary.
As seen in the Thermoelastic Stress Analysis fundamentals section, the thermoelastic equation describes that as the specimen temperature rises so does the differential temperature induced via the thermoelastic effect. Because of this, the signal-to-noise ratio (S/N) can actually improve as the temperature increases. The material properties in this equation should be evaluated at the operating temperature.
Although the thermoelastic effect increases at elevated temperatures, there are practical considerations that must be addressed. At high temperatures, pseudo-signals have a greater effect on data, and need to be minimized. Pseudo-signals are temperature changes that occur in sync with the loading, and therefore are detected by the DeltaTherm system as thermoelastic signal. The main causes of pseudo-signals are:
Stealth Furnace Control Electronics (Close up front view)
Stealth Furnace Control Electronics (Close up back view)
The Stealth Furnace is designed considering all of the potential pseudo-signals . The furnace is laid out in eight zones six of which control wall temperatures and two of which control the temperature of the specimen, compensating for the conduction of heat through the specimen and into the grips. Temperatures can be controlled to within ±2 C, which is adequate for minimizing the pseudo-signals described above. Keeping the zonal temperatures similar solves the problems of thermal gradients, angular motion, and edge effects. Wall reflections are minimized by the construction and shape of the furnace, hence the name "Stealth Furnace."
(Contact Stress Photonics for detailed references).
The above document is an excerpt from "An Elevated-Temperature TSA Furnace Design" by Jon Lesniak and Brian Bartel. The paper originally appeared in the March/April '96 issue (Vol.20 #2) of "Experimental Techniques." ET is a publication of the Society for Experimental Mechanics (SEM).
Click here to view the PDF of "An Elevated-Temperature TSA Furnace Design".
Unfamiliar with PDFs? Click here for simple directions.
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