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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.

Thesis (Ph. D.)--Lehigh University, 2001. / Includes vita. Includes bibliographical references (leaves 266-275).

Includes bibliographical references (p. 82-87).

Thesis (M.S.) in Mechanical Engineering--University of Maine, 2001. / Includes vita. Includes bibliographical references (leaf 141).

No description available.

No description available.

No description available.

Composite structures are subjected to multi-axial fatigue loads while in service. The objective of this thesis is to study the effect of load path (sequence of application of load) on biaxial fatigue behavior of multidirectional composites. The fatigue behavior of thin walled [±45]2s graphite fiber composite tube was experimentally studied under uniaxial and biaxial loading. The sequence of application of, and the phase difference between the tensile and torsional loads was varied. While an in-phase torque, superposed on to the tensile load, extended the fatigue life, an out-of-phase torque, superposed onto the tensile load, reduced the fatigue life, with respect to uniaxial fatigue life. An out-of-phase torque applied prior to the tensile load had the most impact on the fatigue life, when compared to the torque applied after the tensile load. These results establish the effect of load path on the fatigue life of composites under biaxial loading.

Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1990. / Vita. Abstract. Includes bibliographical references (leaves 64-74). Also available via the Internet.