Stress-strain behaviour glassy polymers

L. Nicolais, M. Narkis (1971) Stress-strain behaviour of styrene-acrylonitrile/glass bead composites in the glassy region, Polym. Eng. Sci. 11, 194. 

Ideal yielding behaviour is approached by many glassy polymers well below their glass-transition temperatures, but even for these polymers the stress-strain curve is not completely linear even below the yield stress and the compliance is relatively high, so that the deformation before yielding is not negligible. Further departures from ideality involve a strain-rate and temperature dependence of the yield stress. These two features of behaviour are, of course, characteristic of viscoelastic behaviour. 

The stress-strain curve in Fig. 7.24b first of all exhibits elastic and preplastic behaviour. It then reaches a maximum whose sharpness depends on the polymer and also the deformation rate. Beyond this point, the stress remains almost constant over a certain region, before suddenly increasing to fracture. This is brittle fracture, perpendicular to the load. Many semi-crystalline polymers, such as polyethylene, polypropylene, polyamide 6 and polyamide 6,6 exhibit this type of behaviour at ambient temperature. However, among amorphous polymers in the glassy state, polycarbonate is one of the rare examples to behave in this way. 

So far we have considered only deformation which takes place at constant rate and temperature, but plastic deformation, like other aspects of the mechanical behaviour of polymers, has a strong dependence upon the testing rate and temperature. Typical behaviour is illustrated in Fig. 5.29 for a glassy thermoplastic deformed in tension. At a given strain-rate the yield stress drops as the temperature is increased and a, falls approximately linearly to zero at the glass-transition temperature when the polymer glass becomes a rubber. If the strain-rate is increased and the temperature held constant the yield stress increases (cf. time-temperature superposition (Section 5.2.7)). 

Tg especially wl en deformed under the influence of an overall hydrostatic compressive stress. This behaviour is illustrated in Fig. 5.37 where true stress-strain curves are given for an epoxy resin tested in uniaxial tension and compression at room temperature. The Tg of the resin is 100°C and such cross-linked polymers are found to be brittle when tested in tension at room temperature. In contrast they can show considerable ductility in compression and undergo shear yielding. Another important aspect of the deformation is that glassy polymers tend to show strain softening . The true stress drops after yield, not because of necking which cannot occur in compression, but because there is an inherent softening of the material. 

Structural adhesives are used at temperatures below or at worst near to their glass transition temperatures. The stress properties of interest are therefore those of the material in its glassy or crystalline condition. The characteristic of long chain amorphous polymers above their glass transition, namely their S-shaped stress-strain relation and very high extensibility, sometimes more than 100%, will not be discussed. Nevertheless, in terms of metal behaviour, the extension from which... 

Dimensional stability is one of the most important properties of solid materials, but few materials are perfect in this respect. Creep is the time-dependent relative deformation under a constant force (tension, shear or compression). Hence, creep is a function of time and stress. For small stresses the strain is linear, which means that the strain increases linearly with the applied stress. For higher stresses creep becomes non-linear. In Fig. 13.44 typical creep behaviour of a glassy amorphous polymer is shown for low stresses creep seems to be linear. As long as creep is linear, time-dependence and stress-dependence are separable this is not possible at higher stresses. The two possibilities are expressed as (Haward, 1973)... 

FIG. 13.44 Typical tensile creep behaviour of a glassy amorphous polymer (a modified PMMA at 20 °C), where strain is plotted vs. log time, for various values of tensile stress. From bottom to top 10, 20, 30, 40, 50 and 60 MPa. From Haward (1973). Courtesy Chapmann Hall. 

To describe the creep behaviour of glassy or tough polymers where the creep strains involved are small ( 5% say), as distinct from elastomers where the deformations are large ( 100% say), separable stress and time functions have been proposed. 

---