Problem 1
Write a brief essay highlighting key characteristics of the mechanical behavior of metals,
ceramics, polymer, and reinforced composite materials. Discuss similarities and
differences.
The underlying structure of metals is the crystalline structure. Atoms form rigid ionic
bonds between each other to form a tightly knit crystal. These crystals can be arranged several
different ways, such as face-centered cubic, body-centered cubic, and hexagonal close packed.
Each gives the metal structure different properties and density. Metals tend to have the most
desirable mechanical properties when it comes to applications dealing with high strength and
durability. The reason behind this is in its crystalline structure, this gives metals their tensile and
yield strengths, along with their hardness and toughness. Alloying a metal, or hardening it such
as by cold working, will cause it to have different mechanical properties. For example, bringing
a metal into the plastic deformation region of a stress-strain curve will cause it to actually have a
higher tensile and yielding strength due to its enhanced ability to resist the motion of
dislocations. On the downside, the metal becomes more brittle – consideration of such
consequences must be in the forefront of the plan. In addition to being strong and durable,
metals are also ductile and excellent electricity and heat conductors.
Ceramics are similar to metals in regards to containing similar elements. Ionic and
covalent bonding can be present in a ceramic material. The differing feature that sets ceramics
apart from metals is the fact that they can be either crystalline (like metals) or amorphous.
Ceramics are typically chemically inert and are poor conductors of heat and electricity; however,
similar to metals, they can be strong and stiff, to a degree. If applying a strain hardening method
to a ceramic containing a crystalline structure similar to that of a metal, one would find it would
fracture before ever reaching the plastic deformation region. Since ceramics tend to be porous,
this leads to what are known as stress concentrators, or “stress razors.” These areas have greatly
reduced toughness and tensile strengths, which could lead to spontaneous and catastrophic
failure.
Polymers consist of a backbone, a long chain of usually a single atom, such as carbon.
Branching off of the carbon atoms are either regular hydrogen atoms (such as the case for
hydrocarbons) or different atoms or molecules, known as repeating units. Polymers tend to be
bendable and pliable, such as rubber bands or silicone molds. The mechanical properties of
polymers can be very fascinating. When investigating the ability of a material such as rubber to
withstand an impact against a flat surface, an immense force must be applied for it to even show
signs of fracture. The deformation of a polymer can cause it double or triple its length and still
be able to fully recover. That is not to say they do not deform. In fact, the mechanical properties
of polymers are similar to that of metals; they have a modulus of elasticity along with tensile and
yielding strengths. It depends on the polymer’s composition where along the stress-strain curve
they will fracture, and whether it is brittle, plastic, or an elastomer.
Reinforced composite materials join two or more materials into one. Unlike in metals,
where metallic bonding creates an alloy with different mechanical and chemical properties from
the original constituents, the reinforced composite material can retain the individual properties of
the original materials, effectively creating a material with desirable attributes of all original
materials. Reinforced composite materials can also consist of a matrix and a reinforcement
constituent; the matrix constituent provides the support for the reinforcement constituent. A
popular example is fiberglass, where a plastic matrix provides support for glass fibers, creating a
material stronger than its individual constituent properties; fiberglass is stronger than plastic and
more robust than glass. Metals (tungsten carbide drill bits), ceramics (heat shield tiles on the
space shuttle), and polymers (carbon fiber bicycle frames) can all be used to create various
reinforced composite materials.