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 -253℃(cryogenic fuel temperature) to 127℃(reentry), which can cause matrix micro-cracking due to the thermal mismatch between the fibers and the matrix. These fuel tanks and feedlines are also 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 was 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 investigated the effects of the addition of a 'barrieer'layer, embedded during manufacture, on a PMCs permeability after thermal cycling and low velocity impact events. The baseline composite material was an eight ply IM7/977-2 epoxy system. Barrier layer candidates include a nano-clay reinforced epoxy, aluminized Mylar, aluminum foil, and two т-Ti 15-3 films. Thermal cycling was performed on the hybrid composites to simulate the extreme temperatures experienced by the cryotanks and to determine if the interleaved composites exhibited reduced permeability after thermal cycling. Drop-weight impact tests were performed to determine the effect of interleaving on the critical impact energy of the graphite/epoxy composites. 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. This research indicates that hybrid composites are extremely promising materials for applications of extreme temperatures and stresses.