Everything about Brittleness totally explained
A
material is
brittle if it's liable to
fracture when subjected to
stress. That is, it has little tendency to deform (or
strain) before fracture. This fracture absorbs relatively little
energy, even in materials of high
strength, and usually makes a snapping sound.
When used in
materials science, it's generally applied to materials that fail in
tension rather than
shear, or when there's little or no evidence of
plastic deformation before failure.
When a material has reached the limit of its strength, it usually has the option of either deformation or fracture. A naturally
malleable metal can be made stronger by impeding the mechanisms of plastic deformation (reducing
grain size,
dispersion strengthening,
work hardening, etc.), but if this is taken to an extreme, fracture becomes the more likely outcome, and the material can become brittle. Improving material
toughness is therefore a balancing act.
Toughening
This principle generalizes to other classes of material. Naturally brittle materials, such as
glass, are not difficult to toughen effectively. Most such techniques involve one of two mechanisms: to deflect or absorb the tip of a propagating crack, or to create carefully controlled
residual stresses so that cracks from certain predictable sources will be forced closed. The first principle is used in
laminated glass where two sheets of glass are separated by an interlayer of
polyvinyl butyral, which as a
viscoelastic polymer absorbs the growing crack. The second method is used in
toughened glass and
pre-stressed concrete. The least-brittle structural ceramics are
silicon carbide (mainly by virtue of its high strength) and transformation-toughened
zirconia.
Effect of pressure
Generally, the
brittle strength of a material can be increased by
pressure. This happens as an example in the
brittle-ductile transition zone at an approximate depth of 10 km in the
Earth's crust, at which rock becomes less likely to fracture, and more likely to deform
ductilely.
Crack growth
Supersonic fracture is crack motion faster than the speed of sound in a brittle material. This phenomenon was first discovered by scientists from the
Max Planck Institute for Metals Research in
Stuttgart (
Markus J. Buehler and
Huajian Gao) and
IBM Almaden Research Center in
San Jose, California (
Farid F. Abraham).
Further Information
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