Brittle materials fracture

Veldkamp, J.D.B., Hattu, N., and Snijders, V.A.C., Crack formation during scratching of brittle materials. Fracture Mechanics of Ceramics, Vol. 3 Flaws and Testing, R.C. Bradt et al.. Plenum Press, 1978. 

In general, it is found that the tensile strengths of materials are very much lower than E/10 because brittle materials fracture prematurely due to the presence of flaws (Section 5.6.2) and ductile materials undergo plastic deformation through the motion of dislocations. However, fine whiskers of glass, silica and certain polymer crystals which do not contain any flaws and are not capable of plastic deformation have values of fracture strength which are close to the theoretically predicted ones. 

Ceramics and refractories are inherently brittle materials. The reason for this behavior is that the bonding in them is predominantly ionic or predominantly covalent. For plastic deformation, which is required for ductile fracture, there should be dislocation movement. In ionic compounds, formation of dislocation itself is difficult, because, for neutrality of the material, a pair of dislocations should simultaneously form. One should carry negative charge, and the other, positive. This is a difficult thing. If at all a dislocation forms, it requires simultaneous movement of the oppositely charged dislocations. This is still more difficult. In the case of covalent bonds, they are directional and strong. There is no question of any line defect, such as a dislocation, forming. Therefore, any question of dislocation movement does not arise. Ceramic and refractory materials fail by the sudden fracture of their atomic or ionic bonds. Hence, the failiue of ceramic and refractory materials can be discussed in terms of the failure of brittle materials. In other words, the theory of brittle materials fracture can be applied to ceramics and refractories. 

It is very important, from one hand, to accept a hypothesis about the material fracture properties before physical model building because general view of TF is going to change depending on mechanical model (brittle, elasto-plastic, visco-elasto-plastic, ete.) of the material. From the other hand, it is necessary to keep in mind that the material response to loads or actions is different depending on the accepted mechanical model because rheological properties of the material determine type of response in time. The most remarkable difference can be observed between brittle materials and materials with explicit plastic properties. 

Hydraulic piston-type compactors for collection vehicles, on-site compactors, and transfer-station compactors roll crushers used to fracture brittle materials and to crush tin and aluminum cans and other ductile materials... 

The importance of inherent flaws as sites of weakness for the nucleation of internal fracture seems almost intuitive. There is no need to dwell on theories of the strength of solids to recognize that material tensile strengths are orders of magnitude below theoretical limits. The Griffith theory of fracture in brittle material (Griflfith, 1920) is now a well-accepted part of linear-elastic fracture mechanics, and these concepts are readily extended to other material response laws. 

The parameters for the model were originally evaluated for oil shale, a material for which substantial fracture stress and fragment size data depending on strain rate were available (see Fig. 8.11). In the case of a less well-characterized brittle material, the parameters may be inferred from the shear-wave velocity and a dynamic fracture or spall stress at a known strain rate. In particular, is approximately one-third the shear-wave velocity, m has been shown to be about 6 for various brittle materials (Grady and Lipkin, 1980), and k can then be determined from a known dynamic fracture stress using an analytic solution of (8.65), (8.66) and (8.68) in one dimension for constant strain rate. 

This diagram also helps to illustrate why the inherent fracture toughness of a material is not the whole story in relation to brittle fracture. For example. Table 2.2 shows that polystyrene, which is known to be a brittle material, has a K value of about 1 MN However, LDPE which has a very high... 

Jayatilaka, Ayal de S., Fracture of Engineering Brittle Materials. Applied Science Publishers... 

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