A “Toolbox” for Forensic Engineers

40 Forensic Materials Engineering: Case Studies
product has been subjected to gross overstress by the milky white translucent
area that develops around the point where the damaging force was applied
due to the scattering of light by the myriad interfaces.
In amorphous (noncrystalline) elastomeric materials the long molecular
chains are not cross linked by strong chemical cross-link bridges but are
simply tangled together, held by very weak bonds. Such materials include
natural rubber and the large range of synthetic elastomers such as NBR (used
in fuel pipes) and fluoroelastomers such as Viton (which became more widely
known after the Challenger space disaster). Their inherent flexibility and
reversible elasticity make them key engineering materials because they can
absorb vibration, and so reduce the debilitating effects of fatigue, for example.
The same property makes them ideal seals against fluid movement in engines
and motors.
Thermoplastic materials such as polyethylene or polypropylene are usually
partly crystalline, which makes them much more rigid and thus suitable
for use in engineered products. However, they are still much less stiff than the
same geometry in a typical metal, so their design to withstand imposed loads
must be changed quite drastically. They can also be transformed into very
strong flexible products by straightening out the molecular chains. This shows
up well in their tensile force-extension curves, as in Figure 2.5C.
When the force is first applied there is only slight resistance so the
extension increases rapidly while the applied force is doing nothing more
than straightening out the chain. Extensions of several hundred percent may
occur without any sign of fracture, though the cross-sectional area of course
becomes correspondingly smaller as the strain increases. Eventually, all the
chains become straightened out so the tensile force is now acting along the
line of the molecular chains. Because these are strong chemical bonds the
force rises steeply as very little strain is possible. Eventually the test piece
breaks where the chains have the least overlap. Narrow strips may be split
away easily to give very strong fibers (e.g., polypropylene), which can subsequently
be woven into products like string, binding tape and rope, which
have high tensile strength and are flexible. Even stronger materials such as
carbon fiber can be made by simultaneously heating and stretching, while
other high performance polymers can be made by control of chain orientation
during manufacture (e.g., UHMPE fibers such as Spectra and Dyneema)
or synthesis (e.g., aramid fibers like Nomex, Twaron and Kevlar).
One disadvantage of some common thermoplastic materials for loadbearing
applications is that they soften and deform slowly under quite modest
loads at high temperatures. There are two thermal measures that are important:
T g (the glass transition temperature) and T m (the melting temperature)
if the polymer is partly crystalline. T m is always above T g and many engineering
thermoplastics have a T m above 200∞C. The ultimate use temperature lies