James H. Williams

Pressure Effects on Oxygen Concentration Flammability Thresholds of Polymeric Materials for Aerospace Applications

Spacecraft materials selection is based on an upward flammability test conducted in a quiescent environment at the highest expected oxygen concentration. However, NASA’s advanced space exploration program is anticipating using various habitable environments. Because limited data are available to support current program requirements, a different test logic is suggested to address the expanded atmospheric environments through the determination of materials self-extinguishment limits. This paper provides additional pressure effects data on oxygen concentration and partial pressure self-extinguishment limits under quiescent conditions. For the range of total pressures tested, the oxygen concentration and oxygen partial pressure flammability thresholds show a near linear dependence on total pressure, and appear to increase with increasing oxygen concentration (and oxygen partial pressure) thresholds. For the Constellation Program, the flammability threshold information will allow NASA to identify materials with increased flammability risk from oxygen concentration and total pressure changes, minimize potential impacts, and allow for development of sound requirements for new spacecraft and extraterrestrial landers and habitats.

Pressure Effects on the Self-Extinguishment Limits of Aerospace Materials

The Orion Crew Exploration Vehicle Module (CM) is being designed to operate in an atmosphere of up to 30% oxygen at a pressure of 10.2 psia for lunar missions. Spacecraft materials selection is based on an upward flammability test conducted in a closed chamber under the worst expected conditions of pressure and oxygen concentration. Material flammability depends on both oxygen concentration and pressure but, since oxygen concentration is the primary driver, all materials are certified in the 30% oxygen, 10.2 psia environment. Extensive data exist from the Shuttle Program at this condition which used relatively the same test methodology as currently used in the Constellation Program. When the CM returns to Earth, a snorkel device will be activated after splashdown to provide outside air to the crew; however, for operational reasons, it is desirable to maximize the time the crew is able to breathe cabin air before the snorkel device is activated. To maximize this time, it has been proposed to raise the partial pressure of oxygen in the CM immediately before reentry while maintaining the total cabin pressure at 14.7 psia. In addition, it has been proposed to leak-test the Orion CM with ambient air at a maximum pressure of 17.3 psia. No data exist to assess how high the cabin oxygen concentration can be at 14.7 psia or 17.3 psia. One is to re-test a large number of materials at these pressures at a significant cost. However, since the maximum oxygen concentration (MOC) at which a material will self-extinguish has been determined for a variety of spacecraft materials as a function of pressure, a second alternative is to use existing data to estimate the MOC at 14.7 psia and 17.3 psia. This data will be examined in this paper and an analysis presented to determine the oxygen concentrations at the increased pressures that will result in self-extinguishment of a material. This analysis showed that the oxygen concentration for the Orion CM at 14.7 psia cannot be set higher than 25.6% without potentially invalidating the materials flammability certification in 30% oxygen at 10.2 psia for some materials. Materials certified under these conditions would still be self-extinguishing in ambient air at 17.3 psia. alternative

Acoustic emission during unloading of elastically stressed magnesium alloy

Acoustic emission (AE) has been recorded for a magnesium alloy that has been quasi-statically cycled elastically between zero load and tension. Delayed AE was observed during both loading and unloading, and the stress delay during unloading is further studied. An analytical expression is written for the AE unloading stress delay which is an elastic constitutive parameter. The potential use of these results for the acoustic emission monitoring of elastic stress states is discussed.

Dynamic Failure and Arrest in Large Space Structures

Abstract Some concepts associated with fracture mechanics and some aspects of wave propagation theory are combined to produce an analysis of dynamic failure in a simple lattice structure. For the given model, the conditions for failure propagation and failure arrest in the lattice are derived and the location of failure arrest (if any) is computed. It is shown that failure arrest may be achieved by the introduction of a wave deflector that has material properties different from those of the remainder of the structure. With the proper choice of the material properties of the deflector, the failure process is arrested after it has passed into the deflector system. This paper is an example of one of the applications of the study of wave propagation in lattice structures and may serve as a basis for more complicated models of dynamic failure in such structures