Literature Review - Finite Element Analysis for Satellite Structures - page 5

ratio is comparable to steel; however, the strength to weight ratio is typically
higher. The disadvantages are low hardness, low strength to volume ratio, and a
high coefficient of thermal expansion (CTE). Aluminum alloys are typically
tempered to increase the material strength. Two typical alloys used in manufac-
turing are 6061-T6 and 7075-T7. Aluminum 6061-T6 contains silicon and mag-
nesium which strengthens the alloy during tempering. This alloy has good
machinability and corrosion resistance. Aluminum 7075-T7 contains zinc and
trace amounts of magnesium. The alloy exhibits higher strength than 6061-T6, but
is more difficult to machine.
Titanium and titanium alloys are used for applications requiring very high
strength materials or high strength at high temperatures. The materials have high
strength to weight ratios, low coefficients of thermal expansion, and excellent
corrosion resistance. However, they are hard to machine, and some alloys have
poor fracture toughness. Ti-6Al-4V, which contains 6 % aluminum and 4 %
vanadium, is the most popular titanium alloy used in aerospace applications. The
alloy has heritage in wings and missile bodies. Perhaps its most famous applica-
tions are the castings used to connect the external fuel tank to the space shuttle and
its boosters.
Beryllium is used for very high-stiffness aerospace applications. Its specific
stiffness, which is the ratio of Young's modulus to density, is about six times that
of most metals. The material is nonisotropic due to its grain alignment, and
therefore has low ductility and fracture toughness in its short-grain direction. It is
commonly used in lightweight optics and mirrors because it performs well at
cryogenic temperatures (i.e., low coefficient of thermal expansion and high ther-
mal conductivity). However, beryllium is expensive, difficult to machine, toxic,
and requires special machining equipment. Beryllium parts are machined in a
controlled environment because its powder is a known carcinogen when inhaled.
The parts may be safely handled once machined.
Steel is mainly used in aerospace applications where low-volume strength and
stiffness are important. It has a broad range of strength, hardness, and ductility.
Steel provides high wear resistance; is generally easy to machine, and wieldable.
However it is not efficient for structural stability because it provides low buckling
strength vs. weight. Steels are combined with many trace elements to address a
wide range of needs. Austenitic stainless steel is by far the most abundant steel
alloy used in spacecraft. It contains 12 % chromium which results in a tough
chromium-oxide coating that protects parts from corrosion. Stainless steels are
generally used for fasteners and mechanisms whereas many heat-resistant alloys
are used for heat shields, rocket nozzles, and other high-temperature applications.
1.3.2.2 Advanced Composite Materials
Composite materials consist of a matrix and reinforcement. The matrix (metal,
epoxy) binds the reinforcing fibers (carbon, graphite) together into a continuous
system. The efficiency of composite structures is due their high specific modulus