Real-Life Communication
Metallurgical technicians need to be able to communicate their findings
in an easily understandable way. "If we get too technical, then nobody has
a clue what we found," says technician Linda Richards.
You're a metallurgical
technician in a development laboratory at a university known for its research
into steel making. Because of your expertise, you've been asked to give a
10-minute presentation to first-year engineering students about steel. One
of the key quests in your research is to try to find ways of making steel
stronger. But in order to do that, you need to know how steel breaks.
Here
are a couple definitions from the Metals Handbook:
Ductile
Fracture
Metals Handbook definition: "Fracture characterized by
tearing of metal accompanied by appreciable gross plastic deformation and
expenditure of considerable energy. Appreciable gross plastic deformation
is indicated by the cup and cone appearance."
Brittle Fracture
The
definition: "Separation of a solid accompanied by little or no macroscopic
plastic deformation. Typically, brittle fractures occur by rapid crack propagation
with less expenditure of energy than for ductile fracture. The surface of
a brittle fracture has characteristic cleave planes that are smooth."
Here's
some more information from the Manchester Materials Science Center:
Most
metals fail by ductile fracture, although brittle cleavage occurs in some
at low temperatures. Microvoids are nucleated around particles in the metal.
These grow and coalesce until failure occurs.
At low temperatures,
carbon steels suffer from a ductile to brittle transition in their fracture
behavior. Cracks, often initiated by metal fatigue, propagate rapidly and
catastrophically -- particularly in the wings of airplanes.
The opening
and closing of the crack leaves characteristic striations on the fracture
surface. If undetected, the crack may eventually cause catastrophic failure
by a brittle or ductile failure mechanism.
Low temperatures can severely
embrittle steels. The Liberty ships, produced in great numbers during the
Second World War, were the first all-welded ships. A significant number of
ships failed by catastrophic fracture. Fatigue cracks nucleated at the corners
of square hatches and propagated rapidly by brittle fracture. In earlier ships,
the riveted plates acted as natural crack arresters. These were absent in
the all-welded Liberty ships. The problem was solved by improvements in ship
design and steel quality.
At high temperatures, cracking may occur
as voids are nucleated at grain boundaries. This causes a characteristic brittle
intergranular (around the grains) crack path, in contrast to the transgranular
(through the grains) fracture that is associated with low temperature brittle
fracture.
Atomic force microscopy has been used to resolve fine features
associated with the slow crack growth mechanism. Atomic force microscopy is
a high-resolution technique for observing surface topography. Atomic resolution
can be achieved with some materials.
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With the help of these notes, definitions and pictures, prepare
your speech to give to the class. These pictures will be part of a slide presentation
to the class so study them carefully to make sure you've labelled them correctly.
Click on them to find out.