Materials for engineering, 3rd Edition - (Malestrom)

Organic polymeric materials 175
Polystyrene (PS) and polymethylmethacrylate (PMMA) are both glassy
polymers that can readily form crazes, and both materials exhibit a welldefined
region I. At the higher ∆σ end of region II, slow growth of crazes and
their transformation into cracks are dominant mechanisms of failure. In
contrast, the fatigue life in region III is controlled by the incubation time for
the nucleation of microscopic flaws.
Fatigue crack growth
The fatigue crack growth (FCG) behaviour of a wide variety of amorphous
and semicrystalline polymers can be characterized in terms of the stress
intensity factor, ∆K, Fig. 5.10(a) for PMMA, epoxy, polycarbonate (PC),
Nylon 66, polyacetal (PA), and poly(vinylidene fluoride) PVDF (after
Hertzberg, Nordberg and Manson, J. Mater. Sci., 1970, 5, 521–526, and
Hertzberg, Skibo and Manson, Spec. Tech. Publ., 1979, 675, 471–500).
Since these materials have very different elastic moduli, for comparison
purposes it is instructive to plot da/dN against ∆K/E, where E is the relevant
Young’s modulus. An example is shown in Fig. 5.10(b), (after Hertzberg,
Skibo and Manson, 1979, and Manson, Hertzberg and Bretz, 1981, in: Advances
in Fracture Research (ed. D. François), 1, 443–448). In both diagrams, data
for two engineering alloys are included for comparison, namely a 2 1 / 4 Cr–
1Mo steel and a 7075 aluminium alloy, and it is evident that plastics will
exhibit superior or inferior FCG resistance compared with metals depending
on whether cycling is conducted under strain-control or stress-control,
respectively.
Among the polymers, it appears that in general semicrystalline polymers
exhibit a superior FCG resistance to amorphous ones.
5.5 Joining of polymers
The question of joining similar or dissimilar polymers often arises, as it is
frequently necessary to assemble two sub-components when a product is
manufactured. Again, items are commonly fabricated from sheet or semifinished
sections, which requires the employment of fastening or welding techniques.
We will consider three modes of joining polymers: welding, adhesives and
mechanical fasteners.
5.5.1 Welding of polymers
Since welding involves melting and flow of the surfaces to be joined, the
technique will only be applicable to thermoplastic polymers. The workpiece
may have its temperature raised either by performing local mechanical work
of friction or by the application of thermal energy.