MS Thesis Presentation by Amelia A. Case
Monday, November 15, 2004

(Dr. Steven Johnson, Chair)

"Permeability of Hybrid Composites Subjected to Extreme Thermal Cycling and Low-Velocity Impacts"

Abstract

The next generation of space launch vehicles would like to utilize composite materials for both fuel tanks and fuel feedlines in an effort to reduce the overall vehicle weight, which in turn, increases the weight of payload that can be sent into orbit. These polymer matrix composites are subjected to extreme thermal cycles and are vulnerable to impacts, both of which can cause the composite to leak thru micro-cracks in the matrix material, jeopardizing the performance and safety of the vehicle. A reusable launch vehicle’s composite fuel tanks are cycled from -253ºC (cryogenic fuel temperature) to 127ºC (reentry), which can cause matrix micro-cracking due to the thermal mismatch between the fibers and the matrix. These fuel tanks and feedlines are vulnerable to low velocity impacts, such as those due to dropped tools and inadvertent bumping during installation and maintenance, which can also cause matrix micro-cracking.

The main objective of this research is to develop hybrid polymer matrix composites that are able to withstand applications of extreme temperature fluctuations and moderate impacts while retaining structural integrity (i.e. low permeability and adequate load carrying capacity). Specifically, this research will determine if embedding a “barrier” layer within a graphite/epoxy composite during manufacture will:

1) allow a higher percent of composite structures to be leak-free after fabrication
2) allow a higher percent of composite structures to remain impermeable after extreme thermal cycling
3) allow the composite structures to withstand greater impacts while remaining impermeable (i.e. increase their critical impact energy)

“Barrier” layer candidates include aluminized Mylar, aluminum foil, and ß-Ti 15-3 film. The results of this research suggest that the addition of an embedded barrier layer can increase a graphite/epoxy composite’s resistance to thermal stresses and low-velocity impacts, thereby making the use of hybrid composites extremely promising for applications of extreme temperatures and stresses.