Brittle and Tough Fracture

Probably the most basic information about a material that we need to know is its strength. But what precisely do we mean by the term strength In practice, the answer that engineers give to this question is to define the strength of a material as the force experienced at the point where it fractures. This topic is one that is complex from a theoretical point of view, largely because fracture is a point of discontinuity, and hence cannot readily be interpreted in terms of events leading up to it. 

Whilst these extremes of behaviour can be readily distinguished, there are transitional types of fracture behaviour that lie between them, and in these cases, judgement about fracture mode can be difficult. 

Strictly the terms brittle and tough fracture can only be applied to failure under carefiilly specified test conditions. That is to say that the statement that a glassy polymer, such as poly(methyl methacrylate), undergoes brittle fracture at ambient temperatures needs qualifying test conditions must be stated. These are usually that the material has been formed into a dumbbell shaped specimen. 

The large region of yield in materials that fail by tough fracture arises as the molecules of the polymer rearrange themselves in response to the applied stress. This is different from the mechanism of yield in metals, where planes of metal atoms slide over one another. In polymers, the molecular movement 

The fracture strength and mode of fracture of a material have been found to be related to a number of characteristics of the polymer molecules of which it is made up. These include, among others, constitution, molar mass, polydispersity, crystallinity, and degree of crosslinking. Other factors which also affect fracture strength and mode of fracture are temperature, strain rate, and geometry of the specimen, all of which are decided upon prior to testing the material. 

The large region of yield in materials that fail by tough fracture arises as the molecules of the polymer rearrange themselves in 

Both of these individual strengths increase with increase in the rate at which strain is applied. Yield strength has heen found to be more sensitive to change in strain rate than brittle strength, and thus increases more quickly. For a material showing tough fracture at a low strain rate, this means that there comes a point at which Oy and are equal, and this corresponds to the tough-brittle transition. Further increase in the strain rate causes a, to 

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The phenomena of brittle and tough fracture give rise to fairly characteristic stress-strain curves. Brittle fracture in materials leads to the kind of behaviour illustrated in Figure 7.1 fairly uniform extension is observed with increasing stress, there is minimal yield, and then fracture occurs close to the maximum on this graph. 

Figure 1 Effect of thickness on fracture toughness (the fracture surface sketches arc more typical of metals, adopted here to indicate brittle and ductile fractures).

Adhesive manufacturers have made significant efforts over the last several years to develop toughened systems that can pass these cold impact tests. The first step toward meeting this requirement is fine-tuning the curing process of the adhesive to avoid brittleness and improve fracture toughness. The use of specially formulated accelerators can help adjust the cure rate and improve toughness. 

If the load is increased further, the failure behaviour depends mainly on the fracture toughness of the matrix and the properties of the interface. If the matrix is brittle and the fracture toughness of the interface is large, the stress concentration in front of the crack tip is transferred to the fibre, causing it to break. In this case, the crack propagates on load increase, starting from the site of first fibre fracture. If, on the other hand, the stress concentration in front of the crack tip is not sufficient to cause fibre fracture, another weak fibre somewhere else in the material will fail first, at a position that is completely independent of the first failure position. Thus, fibres will fracture at arbitrary positions in the material, and the load on the material will increase... 

The ultimate (fracture) properties of a wholly amorphous polymer are strongly dependent on temperature. At low temperatures, in the elastic region, the fracture is predominantly brittle and the fracture toughness is low. A considerable increase in the fracture toughness accompanies the onset of the subglass process when approaching the anelastic region. 

The glass-fibre nylons have a resistance to creep at least three times as great as unfilled polymers. In the case of impact strength the situation is complex since unfilled nylons tend to break showing tough fracture whereas the filled polymers break with a brittle fracture. On the other hand the glass-filled polymers are less notch sensitive and in some tests and service conditions the glass-filled nylons may prove the more satisfactory. 

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