The measurement of structural properties of materials at cryogenic temperatures is a daunting task. The measurement of thin films at low temperatures is even harder. Current practices of immersing the material in a cryogenic fluid and then removing and testing it does not work due to the rapid warming of the low-mass thin film. Similarly, exposing it to a cold vapor environment, unless well controlled procedurally, does not yield data at a known temperature (the temperature of the material is unknown). Thus, gathering basic material properties of thin film materials at low temperature requires a new approach.
One approach used at room temperature to measure thin film mechanical properties is the bulge test, where the thin sheet is secured tightly over an opening, and its deflection is measured as pressure is applied to one side of the film. This type of measurement puts the film into the same or similar configuration as NASA would use it when designing for cryogenic fluid containment. In order to measure the mechanical properties at cryogenic temperatures, a new apparatus was designed and fabricated (based on the room temperature approach) that utilizes pressurized liquid nitrogen to deform a polymer film disk, and a laser displacement sensor to measure the resulting deformation. In this method, the liquid nitrogen pressure is slowly increased to provide incremental loads. As the film is directly exposed to the liquid, it is maintained at a known temperature for the entire experiment. The differential displacement with differential load provides several interesting properties that can be evaluated. Some of these mechanical properties include the biaxial modulus, the onset of plasticity, and the ultimate strength of the film.
A number of different materials were tested, ranging from novel composite laminates to those with known bulk properties at cryogenic temperatures. The results for the materials with known properties compared well with the known data.
This work was done by Wesley Johnson, Robert Youngquist, Martha Williams, and Katherine Holland of Kennedy Space Center; Tracy Gibson of QinetiQ North America Inc.; and Scott Jolley of Stinger Ghaffarian Technologies. KSC-13792