A process and a laboratory setup to implement the process (see figure) have been devised to enable the acquisition of time-resolved data on the thermal decomposition of a specimen of a solid material exposed to a heat flux comparable to the heat flux in a typical rocket engine. The process is called "RTR-TGA" because it includes a combination of real-time radiography (RTR) and thermogravimetric analysis (TGA). In the process, one specimen surface (e.g., representing a surface exposed to flames in a rocket engine) is heated by a continuous-wave CO2-laser beam while the interior temperature of the specimen is measured and the specimen is observed by an x-ray apparatus that produces video images that can be recorded. The major advantage of this process over older processes for observing thermal decomposition of material specimens is that the environment to which the specimen is exposed approximates more closely the heating environment in a full-scale rocket engine.
The specimen must be instrumented with a thermocouple for measurement of its interior temperature. Experiments at different heating rates can be performed by changing the power output of the laser and/or by changing the depth at which the temperature is recorded. The closer a thermocouple is to the irradiated surface of the specimen, the higher is the heating rate observed. Specimens tested to date have been made of a composite material with 90° ply angles and overall dimensions of 1.5 in. (≈38 mm) in height and width and 0.75 in. (≈19 mm) in thickness. Thermocouples have been installed in these specimens, oriented parallel to the irradiated surfaces in the cross-ply directions, at depths of 1/8 in. (≈3 mm), 1/4 in. (≈6 mm), and 3/8 in. (≈9.5 mm) from the irradiated surfaces.
The special fixture for holding the specimen is designed to exclude any extraneous material from the radiographic field of view of the specimen. The fixture is also designed to minimize any "funneling" of the photons and to restrain the specimen against any motion that might be induced by thermal expansion. The fixture is further designed to allow access for electrical connection to the thermocouple in the specimen.
In preparation for an experiment, a specimen containing a buried thermocouple is placed in the fixture. A C-shaped arm that is part of the radiographic apparatus is then positioned for scanning; guidance for positioning is obtained by turning on the radiographic apparatus and observing real-time x-ray images as the arm is maneuvered.
In the experiment, the specimen surface of interest is completely exposed to the laser beam. Exposures to date have been 20 seconds in duration with incident laser-beam power densities of 300 and 400 W/cm2 laser incidence. The radiographic and thermocouple data are recorded from about 5 seconds before to about 1 minute after turn-on of the laser beam.
The recorded radiographic images are digitized, then digitally processed to obtain a density profile of the specimen as a function of time. The density data at the depth of the thermocouples are then plotted against the temperatures measured by the thermocouples to obtain an industry-standard density-vs.-temperature plot.
This work was done by Tim Johnson of Thiokol Corp. for Marshall Space Flight Center.