Science and Technology
Science and Technology,
the study of materials, nonmetallic as well as metallic, and how they can
be adapted and fabricated to meet the needs of modern technology. Using
the laboratory techniques and research tools of physics, chemistry, and
metallurgy, scientists are finding new ways of using plastics, ceramics,
and other nonmetals in applications formerly reserved for metals.
The rapid development of semiconductors
for the electronics industry, beginning in the early 1960s, gave materials
science its first major impetus. Having discovered that nonmetallic materials
such as silicon could be made to conduct electricity in ways that metals
could not, scientists and engineers devised ways of fashioning thousands
of tiny integrated circuits (see Integrated Circuit) on a small chip of
silicon. This then made it possible to miniaturize the components of electronic
devices such as computers.
In the late 1980s, materials science research
was given renewed emphasis with the discovery of ceramics that display
superconductivity at higher temperatures than metals do. If the temperature
at which these new materials become superconductive can be raised high
enough, new applications, including levitating trains and superfast computers,
Although the latest developments in materials
science have tended to focus on electrical properties, mechanical properties
are also of major, continuing importance. For the aircraft industry, for
instance, scientists have been developing, and engineers testing, nonmetallic
composite materials that are lighter, stronger, and easier to fabricate
than the aluminum and other metals currently used to form the outer skin
Mechanical Properties of Materials
Engineers must know how solid materials
respond to external forces, such as tension, compression, torsion, bending,
and shear. Solid materials respond to these forces by elastic deformation
(that is, the material returns to its original size and form when the external
force is lifted), permanent deformation, or fracture. Time-dependent effects
of external forces are creep and fatigue, which are defined below.
is a pulling force that acts in one direction; an example is the force
in a cable holding a weight. Under tension, a material usually stretches,
returning to its original length if the force does not exceed the material's
elastic limit . Under larger tensions,
the material does not return completely to its original condition, and
under even greater forces the material ruptures.
is the decrease in volume that results from the application of pressure.
When a material is subjected to a bending, shearing, or torsional
(twisting) force, both tensile and compressive forces are simultaneously
at work. When a rod is bent, for example, one side of it is stretched and
subjected to a tensional force, and the other side is compressed.
is a slowly progressing, permanent deformation that results from a steady
force acting on a material. Materials subjected to high temperatures are
especially susceptible to this deformation. The gradual loosening of bolts,
the sagging of long-span cables, and the deformation of components of machines
and engines are all noticeable examples of creep. In many cases the slow
deformation stops because the force causing the creep is eliminated by
the deformation itself. Creep extended over a long time eventually leads
to the rupture of the material.
can be defined as progressive fracture. It occurs when a mechanical part
is subjected to a repeated or cyclic stress, such as vibration. Even when
the maximum stress never exceeds the elastic limit, failure of the material
can occur even after a short time. With some metals, such as titanium alloys,
fatigue can be avoided by keeping the cyclic force below a certain level.
No deformation is apparent during fatigue, but small localized cracks develop
and propagate through the material until the remaining cross-sectional
area cannot support the maximum stress of the cyclic force. Knowledge of
tensile stress, elastic limits, and the resistance of materials to creep
and fatigue are of basic importance in engineering.
"Materials Science and Technology," Microsoft®
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