Martha K. Williams

Characterization of polyimide foams after exposure to extreme weathering conditions

The weathering degradation of three closely related polyimide foams was studied by X-ray Photoelectron Spectroscopy (XPS), Fourier Transform Infrared (FT-IR) spectroscopy, Raman spectroscopy, Thermogravimetric Analysis (TGA) and Thermomechanical Analysis (TMA) after exposure at the NASA Kennedy Space Centers (KSC) Beach Corrosion Site. These foams were developed by NASA Langley Research Center for applications such as cryogenic insulation, flame retardant panels and structural subcomponents. The degradative environmental conditions at the KSC corrosion site include exposure to sunlight, exposure to changes in temperature and humidity, mechanical erosion by wind and rain, and high sodium chloride content due to the close proximity of the ocean. Other possible atmospheric contaminants include hydrogen sulfide and hydrogen chloride (the latter originating with exhausts from the launching of space vehicles). The foams were studied for a total of 17 months exposure, with samples taken at 3, 9 and 17 months. Data analyses of the weathered foams showed that chemical structure and density effects were the key variables in weathering performance. The carbonyl linkage in the dianhydride of the TEEK-L series polyimide foams is the most important factor in degradation. TEEK-H series foams, which contain an ether linkage in the dianhydride, showed much less degradation or more resistance to weathering in comparison to the TEEK-L series. In the same chemical series, the lower density foams were more degraded in comparison to higher density foams.

Apparatus and method for low-temperature training of shape memory alloys

An apparatus and method for the low-temperature thermo-mechanical training of shape memory alloys (SMA) has been developed. The experimental SMA materials are being evaluated as prototypes for applicability in novel thermal management systems for future cryogenic applications. Alloys providing two-way actuation at cryogenic temperatures are the chief target. The mechanical training regimen was focused on the controlled movement of rectangular strips, with S-bend configurations, at temperatures as low as 30 K. The custom holding fixture included temperature sensors and a low heat-leak linear actuator with a magnetic coupling. The fixture was mounted to a Gifford-McMahon cryocooler providing up to 25 W of cooling power at 20 K and housed within a custom vacuum chamber. Operations included both training cycles and verification of shape memory movement. The system design and operation are discussed. Results of the training for select prototype alloys are presented.