The Effects of Ionized Gas Exposure on the Toughness and Fatigue Properties of Aluminum Alloys and Composites.

The effects of an oxygen ionized gas from simulated space exposure on the toughness and fatigue properties of several aerospace aluminum alloys and aluminum composites have been analyzed. The test matrix consisted of four aluminum systems: the 6000 and 1100 series and two 8090 aluminum-lithium alloys. The test specimens were prepared as Charpy V-notched impact and disk-shaped compact fracture toughness specimens. A small specimen size is used for the compact tension specimens to facilitate exposure in a Radio Frequency (RF) Plasma Prep II unit. Radio frequency plasma, sometimes referred to as the "electrodeless plasma", is used in disassociative ionization of molecular oxygen to simulate high fluence, relatively low energy, low earth orbit (LEO) space atomic oxygen. The plasma reacts with the aluminum alloy systems and forms a thin scale of alumina a few microns thick. Monolayer sensitive Variable Angle Spectroscopic Ellipsometry (VASE) is used to determine the growth characteristics and effect on optical constants, the reflective index, n and absorption coefficient, k. Atomic Force Microscopy (AFM) was also employed to determine the effect of short exposure to the oxygen plasma environment. AFM indicates that sharp spikes of oxygen rich material are produced above the aluminum specimen surface resulting in stress concentrations with gradual roughing of the surface. After 10 years of simulated exposure, the increased roughness results in a 34% increase in the fatigue crack growth, da/dN, for the 6000 series; the crack growth rate of the aluminum-lithium material was increased about 28%. The results of instrumented Charpy V-notched impact specimens indicated that exposure reduced the energy for fracture initiation by 29%. Plane strain fracture toughness effects could not be established because the influence of oxygen was concentrated at the surface of the specimens.