Composite Materials Research Progress

Major Trends in Polymeric Composites Technology 117
in various directions to accommodate the loading and because of this characteristic, it is truly
a composite laminate with tailorable in-plane properties, and with the capability to add the
aluminum plies in various locations and arrangements through the stack up, but with
processing properties similar to bulk aluminum sheet metal. Its major advantages over
conventional aluminum are lower density, better fatigue resistance (cracks are inhibited in
growth due to the restraining effects of the adjacent composite plies) and better resistance to
impact. An important consideration is the matching of the cure temperature of the composite
with the thermal effects on the metal. For example, if the composite requires a 350°F degree
cure, the aluminum alloy must be able to sustain its performace after exposure to that
temperature because it is the total laminate that must go through the autoclave process. The
original GLARE® used 250°F curing fiberglass composite, with an appropriate aluminum
alloy, but in situations where a higher Tg is required for design thermal exposure, a 350F
version was created and has been dubbed “New GLARE”.
Another interesting version of FML is titanium-graphite (TiGr) laminates which consist
of layers of titanium interleafed through the thickness of a Carbon Fiber Reinforced Plastic
(CRFP) laminate. TiGr offers advantages over metallic structures in terms of weight, fatigue
characteristics, damage tolerance, and design flexibility, and also advantages over traditional
composite materials through higher bearing capabilities, greater toughness, and an expanded
design space. There has been extensive testing in the industry to support development of
mechanical properties for TiGr with various epoxy prepreg systems and success in
optimization of surface preparation has resulted in extremely robust and environmentally
durable TiGr. Production-related issues such as scale-up, compound contours, drilling and
trimming, NDI, repair methods and even automation have been addressed for feasibility and
optimization.
These extreme hybrid composites have great potential for use in many applications
including aircrafts. However, due to the large difference in thermal expansion coefficient of
both the fiber and metal, and the anisotropy of the composites layers combined with isotropic
metals, large residual stresses can be built up during the curing cycle which could cause an
unsuitable residual stress system that may seriously hinder its outstanding performance. To
regulate the residual stresses with controlled layups is necessary. In addition, there are many
factors which influence the performance of these FMLs due to three kinds of material
constituents involved, and the two interfaces: fiber/resin and metal/resin. To control the
quality of the FML materials is critically important for the applications in practice.
Other types of hybrids include co-mingling of different fibers for multifunctionality of
the composite ply. Co-mingling of carbon fibers and glass fibers can add a softness to a
laminate in places that need dimensional flexibility. Co-mingling of carbon and
thermoplastic fibers can add toughness to strength and stiffness in a part. In some
commercial products, such as the Cytec Priform technology, the thermoplastic actually melts
during the cure and disperses in a controlled way throughout the matrix to form a minor phase
of toughening material.
A new trend is the coating of polymeric composites with metals for multiple purposes
ranging from electrical conductivity to thermal management to surface hardness. MesoScribe
Technologies has a Direct Write Thermal Spray in which fine powders are injected into a
small thermal plasma and accelerated through an aperture to make patterns on composites
without a cure cycle or masking, and is adaptable to large and highly contoured surfaces.