Test Post 2
1.3.1 ARALL
All
four standard ARALL products employ a thermoset adhesive system impregnated
with unidirectional aramid fibers in a fiber to resin weight ratio of 50:50.
The fibers are oriented parallel to the aluminium sheet rolling direction.
ARALL laminates are produced under very rigid quality control standards. The
aluminium sheets are anodized and primed according to existing aerospace
industry specifications. Both chromic acid anodizing and phosphoric acid
anodizing treatments are used. The outer aluminium surfaces can be left bare
for later processing after an ARALL part has been formed. Table 6 summarizes
production parameters of ARALL laminates.
ARALL
panels are laid up and cured under the appropriate times and temperatures either
in mechanical press or an autoclave. After curing, the aluminium layers are in
a slight tensile residual stress state and the pregreg layers in a compensating
compressive residual stress state.
1.3.2 CARALL
CARALL
laminates has developed as an improvement of ARALL laminates. They contain
different amount of carbon/epoxy pregregs instead of aramid/ epoxy pregregs.
Compared with aramid/epoxy, carbon/epoxy composites possess higher specific
modulus, but relatively low values of specific strength, strain to failure and
impact resistance. In terms of fatigue, it was recognized that aramid fiber
composites have better low cycle fatigue performance but worse high cycle
fatigue performance than carbon fiber composites. Moreover, the high stiffness
of carbon fibers allows for extremely efficient crack bridging and therefore
very low crack growth rates. CARALL is produced similar to ARALL and GLARE
laminates. Before the curing process, aluminium surfaces are treated for an
optimal adhesion between aluminium alloy and epoxy resin. Then, they are cured
in hot press. The combination of high stiffness and strength with good impact
properties gives CARALL laminates a great advantage for space applications.
Other applications for this laminate are impact absorbers for helicopter struts
and aircraft seats.
1.3.3 GLARE
GLARE-
Glass Laminate Aluminium Reinforced Epoxy
Aircraft manufacturers worldwide are always
exploring and evaluating materials that can save weight. One of the most
exciting materials under evaluation for primary and secondary aircraft
components is GLARE laminate. GLARE is sandwich material composed of several very thin layers of metal interspersed with layers of glass-fiber "pre-preg", bonded
together with a matrix such as epoxy.It is constructed from alternating layers of
aluminum and Glass fiber with bond film.
Several types of glass reinforcements are suitable
for the manufacture of aircraft and helicopter composite components. E-glass
composites are used extensively in gliders and in non-structural components that
do not require high stiffness, such as radomes. S-glass composites have better
mechanical properties and therefore are used in more demanding applications. A
third type of reinforcement known as D-glass has good dielectric properties and
is occasionally used in aircraft to minimize the impact of lighting strikes. E-
and S-glass are used in the form of epoxy-based pre-preg or as fabrics
containing unidirectional, woven, or chopped strand filaments.
A major advantage of E-glass fibers over the other
types of fibers used in aircraft is their low cost. The costs are approximate
and do not include the expense of fabricating the composite into an aircraft
component, which is usually much higher than the raw material cost. E-glass
composites are by far the cheapest, particularly when chopped strand mat or
woven fabric is used. S-glass composites are much more expensive than the
E-glass composites and only marginally less expensive than carbon/epoxy. The
forms are chopped-stand mat, woven rovings, and unidirectional pre-preg
material.
The
biggest differentiation of GLARE compared to ARALL is that GLARE consists of
glass fibers instead of aramid fibers. This diversity gives superior properties
to GLARE laminates. The specific stiffness and strength in the fiber direction
of GLARE are enhanced over the high strength aluminium alloy used for the metal
layers, which significantly contributes to weight savings in the designs of
tension-dominated structural components.
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