Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
Toughening of polymers
The moderate ductility apparent in curve 3 in Fig. 5.4 is more difficult to
achieve. One approach is to prepare a two-phase material by a technique
known as copolymerization. An example of this is acrylonitrile–butadiene
styrene which consists of spheroidal inclusions of the soft elastomeric
polybutadiene in a matrix of the relatively rigid styrene acrylonitrile (SAN)
copolymer. When the material is strained, stress concentrations form in the
matrix around the inclusions. Defects then grow from the inclusions, absorbing
extra energy as they do so, the process being known as elastomer toughening,
although the overall tensile modulus of the material is reduced by the change
in microstructure.
Another approach has been to synthesize new polymers based on different
kinds of repeat units, for example the use of amorphous polycarbonate. This
contains rigid aromatic groups in the molecular backbone and exhibits
mechanical toughness well below its glass-transition temperature. This may
again arise by the formation of energy-absorbing defects as happens in
‘elastomer toughening’ described above.
5.4.4 Material data for polymers
In contrast to the properties of metals and alloys, the properties of a given
polymer made by different manufacturers may well differ significantly. This
arises for several reasons: firstly, all polymers contain a range of molecular
lengths (see Fig. 1.22) and slight changes in processing will change this
spectrum and also the degree of crystallinity in the product. The properties
will also be changed by mechanical processing. Again, as already discussed,
the mechanical properties of polymers are time- and temperature-dependent,
so that the data obtained will depend on the testing conditions of temperature
and strain-rate. The following compilation of data (where they are available)
must therefore be regarded as approximate and, in designing a product,
reference should always be made to the data provided by the individual
manufacturers. We have included the representative polymers listed at the
start of this chapter, Table 5.1
5.4.5 Fracture
Crazing
If a brittle transparent polymer such as polystyrene (widely used for the
manufacture of rulers and the bodies of ballpoint pens) is loaded in tension
at room temperature, plastic deformation leads to the appearance of small,
white, crack-shaped features called crazes, typically some 5 µm in thickness.
Crazes usually develop at orientations which are at right-angles to the principal
stress axis, but they are not in fact cracks rather a precursor to fracture.