Solid metals fracture types

Creep and fracture in crystals are important mechanical processes which often determine the limits of materials application. Consequently, they have been widely studied and analyzed in physical metallurgy [J. Weertmann, J.R. Weertmann (1983) R.M. Thomson (1983)]. In solid state chemistry and outside the field of metallurgy, much less is known about these mechanical processes [F. Ernst (1995)]. This is true although the atomic mechanisms of creep and fracture are basically independent of the crystal type. Dislocation formation, annihilation, and motion play decisive roles in this context. We cannot give an exhaustive account of creep and fracture in this chapter. Rather, we intend to point out those aspects which strongly influence chemical reactivity and reaction kinetics. Illustrations are mainly from the field of metals and metal alloys. 

Solids that show considerable plastic deformation before fracture generally fail in a different way from brittle solids, discussed above. An important parameter that characterises this type of failure is the ductility of the material. Ductility is a measure of the degree of plastic deformation that can be sustained by a material at fracture. The ductility of metals can be estimated by measuring the percentage elongation of a sample after fracture, where ... 

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. 

These properties indicate that tire atoms are capable of slipping witii respect to one another. Ionic solids or crystals of most covalent compounds do not exhibit such behavior. These types of solids are typically brittle and fracture easily. Consider, for example, the difference between dropping an ice cube and a block of aluminum metal onto a concrete floor. 

The joint action of the thermal fluctuations and mechanical stresses results in a gradual decrease in the time required for the sample to fracture with an increase in stress. This was established by Zhuikov et al. for various types of solids [62], In the case of contact with the active medium, such as in the case of the contact of a metal with a melt, high stresses can cause a sharp decrease in durabUity, while low stresses will have practically no impact on it The durability curve reveals a kink related to the same mechanism as the brittle-to-plastic transition upon active elongation with a given deformation rate (Figure 7.26). 

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