Homogeneous deformation bands

Fibrillated crazes are usually thinner than the homogeneous deformation bands. Therefore, the whole content of pol5uneric material, which is plastically deformed, is smaller in the crazes and larger in the shear bands or deformation zones. The result is shown macroscopically in the unusual effect of a decrease of toughness and an embrittlement with increasing temperature. 

AT= Tg- T larger than 80 K) is connected with homogeneous deformation bands with higher deformation temperatures below Tg an effect of a thermally Induced disentanglement occurs, which creates fibrillated crazes [3, 12, 13] ... 

A very broad and several narrow homogeneous deformation bands (homogeneous crazes) in front of a crack tip in SAN copolymer ... 

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. 

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>. 

Fig. 29a and b. Structure of the shear zone at various temperatures in PP a T = —196 °C, two sets of discrete shear band A and B, b T = —40 °C, diffuse shear zone with quasi-homogeneous deformation of spherulites... 

Finally, when the testing temperature is near the glass transition temperature, a broad shear zone instead of discrete coarse shear bands develops at an angle of 45° to the direction of the external stress (Fig. 29 b). Inside this zone the spherulites are homogeneously deformed in direction to the main shear stress. It is expected that... 

Jenkins (1990) and Poliakov, et al. (1993) of the formation of shear bands with the findings of Mulhaus and Vardoulakis (1987). A shear band is a narrow zone of intense shear of a thickness which is a small multiple of the mean grain size of the rock or soil in which it was formed. It is understood to be the product of spontaneous localisation of a previously homogeneous deformation. 

How the homogeneous deformation (Fig. Ic) develops between shear bands, in response to the stresses acting on it from the just formed shear band and from the tool, determines segmentation shape — such dimensions as hj, i2, L, and 0. Softening in the shear band thus influences segmentation shape as well as shear-band thickness. 

In Fig. 4 we have seen typical deformation bands in PET. The observed extinction directions in the matrix (IDD) and in the bands (EDB) are marked showing the angle oc between them. From Fig. 9 it can be seen that a homogeneous shear could not explain the observed reorientation if all the chains were aligned parallel to the IDD, as the direction of maximum refractive index would then rotate in the opposite sense to that observed. 

The stress-strain curves simulate a homogeneous deformation process of the polymer. However, on the microscale above the linear part of the stress-strain curve (see Fig. 1.15, curves (b), (c), (d)), localized heterogeneous deformation mechanisms occur. Depending on the polymer chemical structure and entanglement molecular weight Mg and on the deformation conditions (temperature and strain rate), several types of heterogeneous deformation are observed micro plastic zones, crazes, deformation zones, and shear bands. Their main features are sketched in Fig. 1.18. 

Rgure 9.7 Deformed semithin section of an eight-layered PET/PC (50/50) composite (four layers were removed during specimen preparation) showing shear bands in the PET layers that initiate homogeneous deformation zones in adjacent PC layers [two-component behavior) the area indicated by a rectangle in (a) is magnified in (b) the arrows indicate the deformation direction TEM [15]... 

This macroscopically detected loss of toughness is explained by the reduction of all deformation zones in the total volume. It is caused by the change that occurs in polycarbonate subsequent to annealing below its glass transition temperature ductile deformation zones (shear bands, homogeneous deformation zones) are replaced by strongly localized deformation zones and fibrillated crazes [768]. 

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