Recession of FEP specimens from trays D11 and B7

We report work done at Boeing Defense and Space Group on analysis of silvered teflon specimens taken from selected locations of the Long Duration Exposure Facility under support from a contract provided by NASA LaRC. The samples discussed in this presentation were taken from the unexposed side of D11 and extended through the folded area of this blanket into the exposed area. Two similar areas were cut from blanket B7, one from the edge of the blanket near row six and one from the edge of the blanket near row eight and within a few centimeters of the copper grounding strap for B7. The specimens were each divided into three sections by cutting with a scapel. Two of the sections were mounted in a potting compound, which was cut and polished such that the cross-sectional thickness of each was exposed. One piece was mounted straight and the other was mounted in an attempt to configure the specimens such that it was bent with a radius of curvature similar to the on-orbit configuration. The third portion of each specimen was used for SEM images to help define the angle of exposure with respect to the ram at each location on the specimen. Photomicrographs were taken in cross section from the edge of the blanket through the curved transition region into the exposed area of the blanket. The thickness of the Fluorinated Ethylene Propylene (FEP) layer was determined at known distances from the edge of the blanket. SEM images were obtained at known distances to help define the angle with respect to ram and therefore establish the atomic oxygen fluence on each location and correlate this exposure with thickness. Thickness measurements made with a two to three centimeter distance minimized the uncertainty arising from variations in the as-manufactured thickness of each blanket. The nominal angle from ram of the exposed portion of each blanket, and the fact that the unexposed edge portions are approximately at right angles to the exposed portion were also used to help define the angles. Thickness measurements were taken at specified locations. An average thickness for the unexposed portion of the blanket was determined. Changes in thickness were then determined by difference.