July 23, 2002
Bernie Graf asked me to summarize the discussions and outcome of a telecon we had with Brad Parker of the GSFC Materials Engineering Branch last week. The telecon dealt with Action Item ART-8: Bernie Graf to investigate material failure analysis capability at GSFC.
Bernie and I discussed with Brad the construction of the lower tray and our understanding of the reasons for using carbon-carbon (C-C) sidewalls. We also discussed the composition, structure, and properties of the C-C. In particular, I presented a list of material properties reported in an SBIR for the GLAST upper trays, which I assume is the same C-C being used in the lower tray. I have attached these properties to this e-mail for your information. The SBIR C-C, which is used for aircraft brake pads, has properties that suggest its fibers are aligned in preferential directions (radial and hoop). The fibers in this material are discontinuous and are formed into a mat or felt. Typically, it has pseudo reinforcement in the thickness direction, which results from the mat fibers being intentionally deformed and "locked" together out of plane. My major concerns are the interlaminar (through-the-thickness) tensile strength of the material and the possibility of it being weak in an off-fiber direction, which may potentially rule out use of the material for load bearing. To answer these concerns, we discussed doing some simple but meaningful tests on the material. Since Hytec has a very limited amount of material in stock, we decided on obtaining a small sample (6" x 6" x largest thickness) of the lower tray C-C. With this sample, Brad and his lab would be able to section and photograph the material to determine its microstructure and presence of any preferential fiber directions. In addition, the sample would provide test samples for interlaminar strength (flatwise tension) and beam-flexure strength. Both of these tests are simple to perform. Furthermore, Brad's lab has the capability to measure modulus (in plane and in bending) with a resonance method that uses the beam flexure specimens.
In addition, we discussed inspection techniques for the C-C with Brad, who is the GSFC expert on non-destructive testing. His opinion is that the material would attenuate sound too much to be inspected by ultrasonic methods and that x-ray inspection would be very difficult to assess, with only extremely gross flaws readily discernable. Brad was against doing black-light dye-penetrant inspection for surface flaws because of difficultly in removing the penetrant solution from the pores of the C-C (contamination concern). We all believed that the less machining done to the C-C the better, since the chances for damaging the material are lessened. I am relieved that Hytec has proposed to eliminate light-weighting the walls of the C-C in the lower tray.
Feel free to contact me if you have any questions about this e-mail.