TEST BANK FOR The Science & Engineering of Materials By Donald R. Askeland, Pradeep P. Fulay

1–4 Steel is often coated with a thin layer of zinc if it is to be used outside. What characteristics
do you think the zinc provides to this coated, or galvanized, steel? What
precautions should be considered in producing this product? How will the recyclability
of the product be affected?
Solution: The zinc provides corrosion resistance to the iron in two ways. If the
iron is completely coated with zinc, the zinc provides a barrier between
the iron and the surrounding environment, therefore protecting the
underlying iron. If the zinc coating is scratched to expose the iron, the
zinc continues to protect the iron because the zinc corrodes preferentially
to the iron (see Chapter 23). To be effective, the zinc should bond well to
the iron so that it does not permit reactions to occur at the interface with
the iron and so that the zinc remains intact during any forming of the
galvanized material. When the material is recycled, the zinc will be lost
by oxidation and vaporization, often producing a “zinc dust” that may
pose an environmental hazard. Special equipment may be required to
collect and either recycle or dispose of the zinc dust.
1–5 We would like to produce a transparent canopy for an aircraft. If we were to use a
ceramic (that is, traditional window glass) canopy, rocks or birds might cause it to
shatter. Design a material that would minimize damage or at least keep the canopy
from breaking into pieces.
Solution: We might sandwich a thin sheet of a transparent polymer between two
layers of the glass. This approach, used for windshields of automobiles,
will prevent the “safety” glass from completely disintegrating when it
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fails, with the polymer holding the broken pieces of glass together until
the canopy can be replaced.
Another approach might be to use a transparent, “glassy” polymer
material such as polycarbonate. Some polymers have reasonably good
impact properties and may resist failure. The polymers can also be
toughened to resist impact by introducing tiny globules of a rubber,
or elastomer, into the polymer; these globules improve the
energy-absorbing ability of the composite polymer, while being too
small to interfere with the optical properties of the material.
1–6 Coiled springs ought to be very strong and stiff. Si3N4 is a strong, stiff material.
Would you select this material for a spring? Explain.
Solution: Springs are intended to resist high elastic forces, where only the atomic
bonds are stretched when the force is applied. The silicon nitride would
satisfy this requirement. However, we would like to also have good
resistance to impact and at least some ductility (in case the spring is
overloaded) to assure that the spring will not fail catastrophically. We
also would like to be sure that all springs will perform satisfactorily.
Ceramic materials such as silicon nitride have virtually no ductility,
poor impact properties, and often are difficult to manufacture without
introducing at least some small flaws that cause to fail even for relatively
low forces. The silicon nitride is NOT recommended.
1–7 Temperature indicators are sometimes produced from a coiled metal strip that
uncoils a specific amount when the temperature increases. How does this work;
from what kind of material would the indicator be made; and what are the important
properties that the material in the indicator must possess?
Solution: Bimetallic materials are produced by bonding two materials having
different coefficients of thermal expansion to one another, forming a
laminar composite. When the temperature changes, one of the materials
will expand or contract more than the other material. This difference in
expansion or contraction causes the bimetallic material to change shape;
if the original shape is that of a coil, then the device will coil or uncoil,
depending on the direction of the temperature change. In order for the
material to perform well, the two materials must have very different
coefficients of thermal expansion and should have high enough modulus
of elasticity so that no permanent deformation of the material occurs.
1–8 You would like to design an aircraft that can be flown by human power nonstop for
a distance of 30 km. What types of material properties would you recommend?
What materials might be appropriate?
Solution: Such an aircraft must possess enough strength and stiffness to resist
its own weight, the weight of the human “power source”, and any
aerodynamic forces imposed on it. On the other hand, it must be as light
as possible to assure that the human can generate enough work to
operate the aircraft. Composite materials, particularly those based on a
polymer matrix, might comprise the bulk of the aircraft. The polymers
have a light weight (with densities of less than half that of aluminum)
and can be strengthened by introducing strong, stiff fibers made of glass,
carbon, or other polymers. Composites having the strength and stiffness
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of steel, but with only a fraction of the weight, can be produced in this
manner.
1–9 You would like to place a three-foot diameter microsatellite into orbit. The satellite
will contain delicate electronic equipment that will send and receive radio signals from
earth. Design the outer shell within which the electronic equipment is contained. What
properties will be required, and what kind of materials might be considered?
Solution: The shell of the microsatellite must satisfy several criteria. The material
should have a low density, minimizing the satellite weight so that it can
be lifted economically into its orbit; the material must be strong, hard,
and impact resistant in order to assure that any “space dust” that might
strike the satellite does not penetrate and damage the electronic
equipment; the material must be transparent to the radio signals that
provide communication between the satellite and earth; and the material
must provide some thermal insulation to assure that solar heating does
not damage the electronics.
One approach might be to use a composite shell of several materials.
The outside surface might be a very thin reflective metal coating that
would help reflect solar heat. The main body of the shell might be a light
weight fiber-reinforced composite that would provide impact resistance
(preventing penetration by dust particles) but would be transparent to
radio signals.
1–10 What properties should the head of a carpenter’s hammer possess? How would you
manufacture a hammer head?
Solution: The head for a carpenter’s hammer is produced by forging, a metalworking
process; a simple steel shape is heated and formed in several
steps while hot into the required shape. The head is then heat treated to
produce the required mechanical and physical properties.
The striking face and claws of the hammer should be hard—the metal
should not dent or deform when driving or removing nails. Yet these
portions must also possess some impact resistance, particularly so that
chips do not flake off the striking face and cause injuries.
1–11 The hull of the space shuttle consists of ceramic tiles bonded to an aluminum skin.
Discuss the design requirements of the shuttle hull that led to the use of this combination
of materials. What problems in producing the hull might the designers and
manufacturers have faced?
Solution: The space shuttle experiences extreme temperatures during re-entry into
earth’s atmosphere; consequently a thermal protection system must be
used to prevent damage to the structure of the shuttle (not to mention its
contents!). The skin must therefore be composed of a material that has
an exceptionally low thermal conductivity. The material must be capable
of being firmly attached to the skin of the shuttle and to be easily
repaired when damage occurs.
The tiles used on the space shuttle are composed of silica fibers bonded
together to produce a very low density ceramic. The thermal
conductivity is so low that a person can hold on to one side of the tile
while the opposite surface is red hot. The tiles are attached to the shuttle
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skin using a rubbery polymer that helps assure that the forces do not
break the tile loose, which would then expose the underlying skin to
high temperatures.
1–12 You would like to select a material for the electrical contacts in an electrical switching
device which opens and closes frequently and forcefully. What properties should
the contact material possess? What type of material might you recommend? Would
Al2O3 be a good choice? Explain.
Solution: The material must have a high electrical conductivity to assure that no
electrical heating or arcing occurs when the switch is closed. High purity
(and therefore very soft) metals such as copper, aluminum, silver or gold
provide the high conductivity. However, the device must also have good
wear resistance, requiring that the material be hard. Most hard, wear
resistant materials have poor electrical conductivity.
One solution to this problem is to produce a particulate composite
material composed of hard ceramic particles embedded in a continuous
matrix of the electrical conductor. For example, silicon carbide particles
could be introduced into pure aluminum; the silicon carbide particles
provide wear resistance while aluminum provides conductivity. Other
examples of these materials are described in Chapter 17.
Al2O3 by itself would not be a good choice—alumina is a ceramic
material and is an electrical insulator. However, alumina particles
dispersed into a copper matrix might provide wear resistance to the
composite.
1–13 Aluminum has a density of 2.7 g/cm3. Suppose you would like to produce a composite
material based on aluminum having a density of 1.5 g/cm3. Design a material
that would have this density. Would introducing beads of polyethylene, with a
density of 0.95 g/cm3, into the aluminum be a likely possibility? Explain.
Solution: In order to produce an aluminum-matrix composite material with a
density of 1.5 g/cm3, we would need to select a material having a
density considerably less than 1.5 g/cm3. While polyethylene’s density
would make it a possibility, the polyethylene has a very low melting
point compared to aluminum; this would make it very difficult to
introduce the polyethylene into a solid aluminum matrix—processes
such as casting or powder metallurgy would destroy the polyethylene.
Therefore polyethylene would NOT be a likely possibility.
One approach, however, might be to introduce hollow glass beads.
Although ceramic glasses have densities comparable to that of
aluminum, a hollow bead will have a very low density. The glass also
has a high melting temperature and could be introduced into liquid
aluminum for processing as a casting.
1–14 You would like to be able to identify different materials without resorting to chemical
analysis or lengthy testing procedures. Describe some possible testing and sorting
techniques you might be able to use based on the physical properties of materials.
Solution: Some typical methods might include: measuring the density of the
material (may help in separating metal groups such as aluminum,
copper, steel, magnesium, etc.), determining the electrical conductivity
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of the material (may help in separating ceramics and polymers from
metallic alloys), measuring the hardness of the material (perhaps even
just using a file), and determining whether the material is magnetic or
nonmagnetic (may help separate iron from other metallic alloys).
1–15 You would like to be able to physically separate different materials in a scrap recycling
plant. Describe some possible methods that might be used to separate materials
such as polymers, aluminum alloys, and steels from one another.
Solution: Steels can be magnetically separated from the other materials; steel (or
carbon-containing iron alloys) are ferromagnetic and will be attracted by
magnets. Density differences could be used—polymers have a density
near that of water; the specific gravity of aluminum alloys is around 2.7;
that of steels is between 7.5 and 8. Electrical conductivity measurements
could be used—polymers are insulators, aluminum has a particularly
high electrical conductivity.
1–16 Some pistons for automobile engines might be produced from a composite material
containing small, hard silicon carbide particles in an aluminum alloy matrix. Explain
what benefits each material in the composite may provide to the overall part. What
problems might the different properties of the two materials cause in producing
the part?
Solution: Aluminum provides good heat transfer due to its high thermal
conductivity. It has good ductility and toughness, reasonably good
strength, and is easy to cast and process. The silicon carbide, a ceramic,
is hard and strong, providing good wear resistance, and also has a high
melting temperature. It provides good strength to the aluminum, even at
elevated temperatures. However there may be problems producing the
material—for example, the silicon carbide may not be uniformly
distributed in the aluminum matrix if the pistons are produced by
casting. We need to assure good bonding between the particles and the
aluminum—the surface chemistry must therefore be understood.
Differences in expansion and contraction with temperature changes may
cause debonding and even cracking in the composite.
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