Organic polymeric materials 175 Polystyrene (PS) and polymethylmethacrylate (PMMA) are both glassy polymers that can readily form crazes, and both materials exhibit a well- defined 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 21/4Cr–
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.
Materials for engineering 176
Epoxy PMMA
PVC PC
PVDF
7075–T651 aluminum
21/4Cr–1 Mo steel
PA Nylon
66
0.2 1 10 20 40 60 80
∆K (MPa √m) (a)
da/dN (mm/cycle)
10–1
10–2
10–3
10–4
10–5
10–6
da/dN (mm/cycle)
10–1
10–2
10–3
10–4
10–5
10–6
21/4Cr–1 Mo
7075
Nylon 66
PA PMMA
PC PVDF PVC
Epoxy
10–4 2 4 6 8 10–3 2 4 6
∆K/E (√m) (b)
5.10 (a) FCG characteristics of some amorphous (dashed lines) and semicrystalline (solid lines) polymers. Comparative data for a steel and an aluminium alloy are shown. (b)FCG rates shown in Fig. 5.10a replotted as a function of ∆K/E.
Organic polymeric materials 177
Friction welding
Friction welding can be achieved by several techniques; the simplest is the spin welding of two thermoplastics at relative speeds of up to 20 m s–1 under pressures of between 80 and 150 kPa. Welds of high quality may be produced in a few seconds, although residual stresses may be generated. Tubes and hollow sections can be welded satisfactorily and, since the process can be carried out in liquids, it is also a useful method of encapsulation of liquids.
Relative movement of the components by vibration in linear oscillation may also be employed. This method of friction welding is widely used in the automotive manufacturing industry to produce large, complex joints. A development of this principle is ultrasonic welding, in which the parts to be joined are held together under pressure while mechanical vibrations perpendicular to the area of contact are applied by means of a piezo-electric transducer at frequencies in the range 20–40 kHz. As the energy output of these devices is limited, the size of possible weld is much smaller than that in normal vibration welding and tooling is expensive, but the method is well- suited to mass production and finds wide use in industry in the assembly of domestic products. No heat is required, and joint strengths approaching 100%
of that of the parent materials are readily achieved.
External heating methods
Hot tool welding employs an electrically heated flat plate which is sandwiched between the two pieces to be joined. When a temperature of 180–230°C (depending on the particular polymer) has been achieved, the plate is withdrawn and the surfaces are pressed together under a specified stress for sufficient time for a joint to be made. For items of large cross-section, such as large pipes, this time can be quite protracted (several tens of minutes is common), but very strong joints can be produced, with strengths at least 90% of the parent material. It is essential that the surfaces to be joined are clean, for a successful weld to be achieved.
A variant of this approach is the use of an ‘electrofusion connector’, for joining plastic piping. It consists of a coiled electric heating element embedded near the inside surface of a specially constructed joint made of the same plastic as the pipes to be joined. The joint is assembled, a current passed and the joint fuses with the pipe material. Rapid weld times can be achieved, although the joint strength may be impaired by the presence of the heating element.
Hot gas welding heats a filler rod and the edges of the workpiece to be welded by means of a stream of hot gas from a welding gun. Compressed air is normally employed, but if the polymer can undergo degradation by oxidation, then a nitrogen stream is used. Temperatures of 200–300°C are achieved and
Materials for engineering 178
the technique can be applied to a wide range of thermoplastics, if necessary in the form of large complex components. There is a danger of entrapped air pockets when this technique is used, with a resulting risk of fracture in service if the polymer is notch-sensitive.
5.5.2 Adhesive bonding
Adhesive bonding was discussed in Chapter 3 in the context of the joining of metals, but its use is also widely encountered in the joining of polymers.
Good wetting of the polymer by the adhesive is required and this will be achieved if there is a strong chemical bond formed between the adhesive and the adherands.
There are three important categories of adhesives which may be used for the joining of polymers:
1. Hot melt adhesives. These are thermoplastics such as PE or PET, which are melted and applied to the adherands, which are then squeezed together during the cooling cycle. Although good bond strengths may be achieved, the joint may creep in service if the temperature is not low.
2. Solvent-based adhesives. Amorphous plastics are the most likely to dissolve in appropriate solvents and these include materials such as PS, ABS (alkylbenzene sulphonate), PVC and PC. Solvent alone will form an adhesive joint, but polymer solutions have better gap-filling properties and are available in various viscosities depending on the application.
Many of these adhesives are based on rubber and are used as ‘contact’
adhesives to form tough joints of fairly low strength.
3. Reaction cured adhesives. Very high bond strengths are achievable with this class of adhesive: polymerization and cross-linking takes place after mixing low-viscosity precursors. Such adhesives are usually temperature and solvent resistant, the main types being:
(i) epoxies (ii) phenolics
(iii) cyanacrylates, widely used in the bonding of rubber, and
(iv) anaerobics, which cure when air is excluded (useful in thread- locking applications).
5.5.3 Mechanical fastening
Snap fitments
Designs of this nature are possible using semicrystalline thermoplastics (e.g. PP, PE and nylons) which exhibit resilience and high elastic strains.
Organic polymeric materials 179
Screw fitments
Self-tapping screws are the most commonly used form of mechanical fastener for polymers. Thermoplastics can employ thread-forming screws, where elastic relaxation processes ensure a tight fit. Thermosets are too brittle for this technique and they tend to crack in use, so recourse has to be made to thread- cutting screws for these materials.