Brittle polymers fracture behaviour

Depending on the chemical structure of the polymer and on the experimental conditions (T and e), polymer solids can present a brittle behaviour, a ductile behaviour, or an intermediate fracture behaviour. 

The effect of crystallinity on the PP fracture behaviour was observed from tests on the neat polymer, by using different crystallisation temperatures and annealing treatment spherulite sizes range from 20 pm to 80 pm, and crystallinity X. from 64% to 75% [20 -21]. As the crystallinity is increased, the elastic modulus is enhanced and the toughness (both critical energy Jq 2 and propagation energy) is considerably reduced a ductile to brittle transition is observed at Xg > 70% This is coherent with results from Ouedemi [22]. 

There is, however, no generally accepted theory for predicting the brittle-ductile transition or relating it to other properties of the polymer, although for some polymers it is closely related to the glass transition. The type of failure is also affected by geometrical factors and the precise nature of the stresses applied. Plane-strain conditions, under which one of the principal strains is zero, which are often found with thick samples, favour brittle fracture. Plane-stress conditions, xmder which one of the principal stresses is zero, which are often found with thin samples, favour ductile fracture. The type of starting crack or notch often deliberately introduced when fracture behaviour is examined can also have an important effect ... 

Hardness values, indentation moduli, strain hardening exponents and viscoelastic properties can be measured with the instrumented indentation test, also the fracture toughness of very brittle polymers as well as the influence of residual stresses. If needed and a suitable device provided measurements can be done with high spatial resolution and with very small indentation depths. A special application of the testing devices is the characterization of the elastic behaviour of miniaturized components or the realization of micro compression tests, i.e. using the machines like a small universal testing machine. 

Figure 5.17 provides the key to understanding the effects of test conditions and materials structure upon the fracture behaviour of polymers. All materials contain small defects of some kind, but whether they are large enough to cause brittle fracture depends upon the relative positions of the yield and fracture lines on the diagram, and therefore upon rr and Kiq. 

The slow growth of cracks in poly(methyl methacrylate) is an ideal application of linear elastic fracture mechanics to the failure of brittle polymers. Cracks grow in a very well-controlled manner when stable test pieces such as the double-torsion specimen are used. In this case the crack will grow steadily at a constant speed if the ends of the specimen are displaced at a constant rate. The values of Kc or % at which a crack propagates depends upon both the crack velocity and the temperature of testing, another result of the rate- and temperature-dependence of the mechanical properties of polymers. This behaviour is demonstrated clearly... 

The incorporation of rubber particles into a brittle polymer has a profound effect upon the mechanical properties as shown from the stress-strain curves in Fig. 5.66. This can be seen in Fig. 5.66(a) for high-impact polystyrene (HIPS) which is a blend of polystyrene and polybutadiene. The stress-strain curve for polystyrene shows brittle behaviour, whereas the inclusion of the rubbery phase causes the material to undergo yield and the sample to deform plastically to about 40% strain before eventually fracturing. The plastic deformation is accompanied by stress-whitening whereby the necked region becomes white in appearance during deformation. As will be explained later, this is due to the formation of a large number of crazes around the rubber particles in the material. 

The widespread use of Izod and Charpy impact tests to evaluate plastics is, to an unprejudiced eye, rather difficult to justify. Many structural polymers us in load-bearing applications do show a range of fracture behaviour from ductile to brittle . Most thermoplastics can show either kind of behaviour, and may suffer an abrupt tough-to-brittle transition with any of a number of parameters — one of which is the rate of loading at a notch. In order to select a polymer for a specific application it may be important to know its sensitivity to this kind of impact embrittlement. However, it is difficult to see how one might learn this fiem conventional impact strength data. 

The most frequently quoted example to illustrate this behaviour is the children s toy Silly Putty , which is a poly(dimethyl siloxane) polymer. Pulled rapidly it shows brittle fracture like any solid but if pulled slowly it flows as a liquid. The relaxation time for this material is 1 s. After t = 5t the stress will have fallen to 0.7% of its initial value so the material will have effectively forgotten its original shape. That is, one could describe it as having a memory of around 5 s (about that of a mackerel ). Many other materials in common use have relaxation times within an order of magnitude or so of 1 s. Examples are thickened detergents, personal care products and latex paints. This is of course no coincidence, and this timescale is frequently deliberately chosen by formulation adjustments. The reason is that it is in the middle of our,... 

The ultimate stress is very much time-dependent, as may be understood from the viscoelastic behaviour of polymers. At very high velocities there is, even in ductile materials, a change from ductile to brittle fracture. 

Fig. 6.2 Possible forms of the load-extension curve for a polymer (a) low extensibility followed by brittle fraction (b) localised yielding followed by fracture, (c) necking and cold drawing, (d) homogeneous deformation with indistinct yield and (e) rubber-like behaviour.

As discussed in section 6.2.2, the values of Young s modulus for isotropic glassy and semicrystalline polymers are typically two orders of magnitude lower than those of metals. These materials can be either brittle, leading to fracture at strains of a few per cent, or ductile, leading to large but non-recoverable deformation (see chapter 8). In contrast, for rubbers. Young s moduli are typically of order 1 MPa for small strains (fig. 6.6 shows that the load-extension curve is non-linear) and elastic, i.e. recoverable, extensions up to about 1000% are often possible. This shows that the fundamental mechanism for the elastic behaviour of rubbers must be quite different from that for metals and other types of solids. 

---