180°F) of the Army’s Future Combat System ground and tactical vehicles as well as Navy ship systems. Vacuum-assisted resin transfer molding (VARTM) has been identified as an affordable process and is used to fabricate structural armor for ground vehicle hull structures containing integral ceramic composites, as well as large-scale topside ship and hull structures. Desirable resin attributes for these applications include relatively low viscosity at room temperature to enable room-temperature infusion as well as the lowest cure temperature possible to meet hot-wet glass transition temperature requirements. Resins meeting these needs enable low-temperature tooling materials to be used, providing significant cost savings. However, the performance demands on these composites remain high. For example, structural armor requires resins that have high elongation to failure to survive ballistic impact, but current VARTM resins fall short of the service temperature requirement. Higher-cure-temperature resins meet service temperature requirements, but ballistic performance is degraded. Additional research in resins is needed to balance processing ease and performance for these important Department of Defense (DoD) applications.
Higher-temperature performance can be achieved (250 to 400°F) with epoxies, bismaleimides, and polyimide resins using traditional prepreg or towpreg2 and autoclave, filament winding, and fiber placement process technologies. Formulations of these materials have been developed to enable the use of resin transfer molding (RTM) processes for smaller-scale components.
The need for more damage-tolerant aerospace structures has led to the development of toughened thermosets and thermoplastic matrices that are resistant to impact damage and delamination growth. High matrix toughness has also been proven to be a key property in ballistic performance of tank munitions such as the M829A2 (toughened thermoset) and the M829A3 (polyetherimide thermoplastic) carbon fiber sabots (see Figure 2.1). The capability of electron-beam (e-beam) processing for non-autoclave cure of large-scale structures such as rocket motors and fuel tanks has been demonstrated. Improvements in resin toughness and interface optimization for e-beam resins are needed to improve properties and resistance to microcracking.
The role of the matrix in the long-term durability of a composite is a critical issue. Durability is affected by the state of the resin, which may undergo physical aging or environmental degradation, as well as changes in the interaction with the fiber at the interface. In addition, the stress state within the matrix due to processing, thermal and fatigue cycling, and other mechanical loads is critical to the long-term performance. Microcracking is one of the first damage modes observed in the matrix phase. Microcracking can initiate fiber fracture, interface debonding, and delamination that can limit the lifetime of the component. An even more severe case occurs when microcracks provide pathways for accelerated
Prepregs, or preimpregnated fiber assembies, are commonly used in many applications and are fabricated by spreading an array of fiber tows and impregnating the tows with a thermoset or thermoplastic resin to produce a thin sheet of material. In this material form, the fibers are continuous and aligned, providing high stiffness and strength in the fiber direction and low matrix-dominated properties in the transverse direction. For structures subjected to multiaxial loadings, the prepreg is laminated to tailor the properties.