Dislocation mechanics concepts

In the Sect. 8.2, one of the dislocation mechanisms was discussed-the pile-up concept-as the origin of brittle fracture due to the stress concentration occmring at the leading dislocation in the pile-up. However, in materials that is entirely brittle, as are most ceramics at RT and low temperatures, plastic deformation by dislocation motion does not occur or occurs to such a limited extent that cracks are sharp up to the atomic level. In order to understand the fracture behavior of ceramic materials, it is first necessary to understand the fracture mechanisms of materials that are entirely brittle. In such materials, the mechanism of fracture is associated with various flaws inherent or intentionally added to the ceramics. A fist of most of the flaws that may induce brittle failure by crack formation is given below, as indicated in Rg. 8.10 ... 

In crystalline polymer systems the tough response, besides cavitation and crazing, is crystallographic in natme. Crystallographic slips ai-e the main plastic deformation mechanisms that require generation and motion of crystallographic dislocations. The concepts of generation of monolithic and half-loop dislocations plausibly explain the observed yield stress dependences on crystal thickness, temperatm-e and strain rate. 

The essential difference between treatments of chemical processes in the solid state and those in the fluid state is (aside from periodicity and anisotropy) the influence of the unique mechanical properties of a solid (such as elasticity, plasticity, creep, and fracture) on the process kinetics. The key to the understanding of most of these properties is the concept of the dislocation which is defined and extensively discussed in Chapter 3. In addition, other important structural defects such as grain boundaries, which are of still higher dimension, exist and are unknown in the fluid state. 

The basic mechanisms by which various types of interfaces are able to move non-conservatively are now considered, followed by discussion of whether an interface that is moving nonconservatively is able to operate rapidly enough as a source to maintain all species essentially in local equilibrium at the interface. When local equilibrium is achieved, the kinetics of the interface motion is determined by the rate at which the atoms diffuse to or from the interface and not by the rate at which the flux is accommodated at the interface. The kinetics is then diffusion-limited. When the rate is limited by the rate of interface accommodation, it is source-limited. Note that the same concepts were applied in Section 11.4.1 to the ability of dislocations to act as sources during climb. 

The concept of a defect has undergone considerable evolution over the course of the last century. The simplest notion of a defect is a mistake at normal atom site in a solid. These stmcturally simple defects are called point defects. Not long after the recognition of point defects, the concept of linear defects, dislocations, was invoked to explain the mechanical properties of metals. In later years, it became apparent that planar defects, including surfaces, and volume defects such as rods, tubes, or precipitates, also have important roles to play in influencing the physical and chemical properties of the host matrix. More recently, it has become apparent that interactions between point defects are of considerable importance, and the simple model of isolated point defects is often inadequate with... 

Dislocation Based Fracture Mechanics by Johannes Weertman, World Scientific, Singapore Singapore, 1996. What I find especially engaging about this book is its idiosyncratic treatment of a number of problems. Weertman shows repeatedly how in adopting the notion of dislocation solutions as being fundamental, many interesting concepts may be explained concerning cracks and dislocations. 

Although the observed mechanical properties in FG ceramics have been related to the GBS mechanism, the actual deformation mechanism is stiU under debate and may be material-dependent. A concept of plastic deformation has been suggested as a mechanism involving the non-local, homogeneous nucleation of nanoscale loops of partial dislocations also unusual, nonlinear stress and grain-size dependence is assumed to facilitate nanocrystalline plasticity. However, the dominant mechanism is stiU GBS. The stress level required to nucleate a dislocation is much higher than usually encountered in experimental data. Therefore, dislocation gliding itself is not expected to contribute to total strain. 

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