Mechanical properties - Orthopaedic Biomaterials in Research and Practice - page 60

such as window glass, which can be bent significantly but fails at
very low external load if scratched, as with a glass cutter. Similar
effects exist in ductile materials, but the ability to dissipate energy
by local plastic deformation at the tip of the defect tends to pro-
duce much less degradation in strength than in brittle materials.
Thus, defects significantly reduce the strength of metals, polymers,
and ceramics, but the inherent plasticity of polymers tends to protect
them. Composites are, by design, highly defective, owing to the large
area of interface between the components involved. Accurate strength
predictions are nearly impossible since the nature and strength of bond-
ing across these interfaces are unknown except in a few special cases.
In any case, fracture of a material is caused by the formation and
propagation of a crack until material integrity is lost. Any material fea-
ture that either produces cracks or reduces the energy needed to produce
or propagate them will weaken the material, whereas, conversely, any
feature that reduces the number of cracks or increases the energy needed
to form or propagate them will strengthen the material.
One may further characterize materials by comparing the ease of
defect formation to defect (crack) propagation. If it is difficult to pro-
duce a defect but easy to propagate it in a material, then that material is
said to be “notch (or crack) sensitive.” There is no comparable term for
the opposite situation (easy production, difficult propagation), but simple
reflection suggests that such a material would be easy to scratch or mar
but would be relatively tough. Glasses and brittle materials tend to be
notch sensitive, whereas polymers are softer (easier to scratch) and rela-
tively tough (in terms of retention of predicted ultimate strength).
Strengthening mechanisms
The “art” of strengthening materials by changing composition, struc-
ture, or processing is suficiently complex to merit a topic in its own
right. All such variations involve increasing the difficulty of either crack
initiation or crack propagation. Some simple examples are as follows:
Surface hardening. Increasing the hardness of the surface in an oth-
erwise relatively defect-free but notch-sensitive material, such as
a glass, can greatly strengthen it. Such surface hardening may be
brought about by alloying or, in the case of metals, by cold work-
ing. In some cases, heat treatments can produce phase transfor-
mations selectively at the material's surface to produce the same
effect.
Increased cross-linking. In polymers, strength may be remarkably
increased by increasing the volume density of cross-linking (cova-
lent) bonds. This may be restricted to the surface, if it is desired to
retain bulk plasticity, or produced throughout to increase overall
toughness. Cross-linking may be produced by a variety of means,
including heat, ultraviolet illumination, or ionizing radiation.
In some cases, gases of unsaturated organic molecules, such as