Deformation homogeneous plastic

PBA shells are cavitated and fibrillated the PMMA cores (dark) are unchanged in the stress concentration fields the matrix strands are plastically deformed (homogeneously or with crazes) ... 

The plastic deformation patterns can be revealed by etch-pit and/or X-ray scattering studies of indentations in crystals. These show that the deformation around indentations (in crystals) consists of heterogeneous rosettes which are qualitatively different from the homogeneous deformation fields expected from the deformation of a continuum (Chaudhri, 2004). This is, of course, because plastic deformation itself is (a) an atomically heterogeneous process mediated by the motion of dislocations and (b) mesoscopically heterogeneous because dislocation motion occurs in bands of plastic shear (Figure 2.2). In other words, plastic deformation is discontinuous at not one, but two, levels of the states of aggregation in solids. It is by no means continuous. And, it is by no means time independent it is a flow process. 

This chapter is concerned with the influence of mechanical stress upon the chemical processes in solids. The most important properties to consider are elasticity and plasticity. We wish, for example, to understand how reaction kinetics and transport in crystalline systems respond to homogeneous or inhomogeneous elastic and plastic deformations [A.P. Chupakhin, et al. (1987)]. An example of such a process influenced by stress is the photoisomerization of a [Co(NH3)5N02]C12 crystal set under a (uniaxial) chemical load [E.V. Boldyreva, A. A. Sidelnikov (1987)]. The kinetics of the isomerization of the N02 group is noticeably different when the crystal is not stressed. An example of the influence of an inhomogeneous stress field on transport is the redistribution of solute atoms or point defects around dislocations created by plastic deformation. 

Comprehensive investigations into brittleness of some crystals determined with a Vickers pyramid led Ikornikova and Khrenova (1951) to establish that crystals of mosaic structure with traces of plastic deformation are more brittle than similar crystals of homogeneous structure. Moreover it has been found (Glazov and Vigdorovich, 1969) that as the mean square displacement of the lattice structural components diminishes, in other words, as the mobility of these components diminishes with propagation of elastic waves, the ultimate effect is increased material brittleness. 

Here, we shall consider several macroscopic features of the plastic deformation of glassy epoxy-aromatic amine networks. Mostly, the tensile or compression deformation has an inhomogeneous character. Usually, diffuse shear zones (or coarse shear bands) are clearly seen at room temperature deformation. Shear zones start from the defects on the sample boundaries or voids (dust) in the bulk. At higher temperatures, the samples are homogeneously deformed with neck formation (DGER-DADPhS, P = 1) 34>. 

The strength properties of solids are most simply illustrated by the stress-strain diagram, which describes the behaviour of homogeneous brittle and ductile specimens of uniform cross section subjected to uniaxial tension (see Fig. 13.60). Within the linear region the strain is proportional to the stress and the deformation is reversible. If the material fails and ruptures at a certain tension and a certain small elongation it is called brittle. If permanent or plastic deformation sets in after elastic deformation at some critical stress, the material is called ductile. 

Another important fact revealed by electroetching is that plastic deformation of a metal (polycrystalline or even single crystal) is extremely heterogeneous. This heterogeneity depends upon the type of deformation cold-rolling for example produces some slips which are not homogeneous in the bulk of each grain, but rather localized. 

Failure can develop under alternating loading, in both homogeneous and rubber modified glassy polymers, at stress values well below those required for fracture in simple tension. Also, in failure under cycUc stress, plastic deformation is limited and highly localized. 

There have been many efforts for combining the atomistic and continuum levels, as mentioned in Sect. 1. Recently, Santos et al. [11] proposed an atomistic-continuum model. In this model, the three-dimensional system is composed of a matrix, described as a continuum and an inclusion, embedded in the continuum, where the inclusion is described by an atomistic model. The model is validated for homogeneous materials (an fee argon crystal and an amorphous polymer). Yang et al. [96] have applied the atomistic-continuum model to the plastic deformation of Bisphenol-A polycarbonate where an inclusion deforms plastically in an elastic medium under uniaxial extension and pure shear. Here the atomistic-continuum model is validated for a heterogeneous material and elastic constant of semi crystalline poly( trimethylene terephthalate) (PTT) is predicted. 

In polyclusters, at a sufficiently high temperature, when the atomic diffusion at the boundaries becomes appreciable, there occurs a diffusion-viscous flow, similar to that of polycrystals. In this case, in a macroscopically homogeneous solid, the rate of the plastic deformation, eik, and the tensor aik = aik — Sp[Pg.239]

The diffusion-viscous flow changes to an inhomogeneous slip at a rather high strain rate or with temperature decrease. The boundary between homogeneous and inhomogeneous plastic deformations of a polycluster on the (a, T) plane is given by [6.29]... 

Fig. 6.17. Map of polycluster mechanical states. Region I elastic and anelastic (shaded area) deformations Region II inhomogeneous plastic deformation Region III homogeneous diffusional-viscous flow. Curves 1-3 show the temperature dependence of the stress at different constant strain rates...

---