Shear bands morphology

Visual observations of the tensile-tested samples show a square pattern, corresponding to shear banding of the PVA. Such a shear-band pattern obtained from the 0.4 vol% SiC NW reinforced composite is shown in figure 9. The shear-band morphology is found generally to diminish with increasing V), In the case of 0,8 vol% SiC and 0.4 vol% A1203 NW composites, with near-zero ductility, the shear-band pattern is absent,... 

Figure 10.8 Shear bands morphology. Reproduced with permission from Ref. [62] 1993, John Wiley and sons.

Figure J.4 (a) Heterogeneous flow in indented metallic glass. (Reprinted from Ramamurty et al., 2005, with permission from Elsevier.) (b) The shear band morphology...

These theoretical calculations predict that the generation of a porous morphology leads to a decrease in modulus and yield strength, and are in good agreement with experimental data. Furthermore, the calculation of stress distribution, which takes into account the interaction of randomly dispersed voids, predicts the buildup of local stress concentrations which in turn can initiate shear banding. 

Strain rate, test temperature and the thermal history of the specimen all affect the appearance of shear bands in a particular glassy polymer [119]. The differences in morphology of shear bands was proposed to be due to different rates of strain softening and the rate sensitivity of the yield stress. Microshear bands tend to develop in polymers with a small deformation rate sensitivity of Oy and when relatively large inhomogeneities exist in the specimen before loading. This is sometimes characterized by a factor e j, introduced by Bowden in the form [119] ... 

At 25 °C, a diffuse shear morphology is observed, without any craze. In aged samples (30 h at 130 °C), fine bands (ca. 100 A thick) that grow in both the maximum shear directions have a tendency to collect and localise the shear deformation into ca. 3000 A-wide diffuse shear bands, as indicated by the arrow D in Fig. 76a. In the case of un-aged sample, the fine bands are less distinct and more delocalised, as shown in Fig. 76b. 

Polymer Morphology and Failure Mechanisms. A failed tensile bar of unmodified piperidine-cured epoxy resin shows shear deformation before tensile failure when strained slowly (0.127 cm/sec). We could not produce stable crazes in specimens of unmodified epoxy resins. At all stress levels, temperatures, and conditions of annealing only fracture occurred after shear band formation. The failure to observe crazes in unmodified epoxy resins may be explained by a fast equilibrium condition which exists between crazing on loading and recovery on unloading. 

The formation of shear bands under compression is found in crystalline polymers when loaded at temperatures lower than 0.75 T. Under such a condition the shear bands interact with certain morphological features such as spherulite boundaries or lamellar arrangements inside the spherulites. The band initiation stress, ct, increases and the strain at break, Cp, decreases with decreasing temperature and increasing stiffness of the tested polymer, i.e. increasing degree of crystallinity. 

A final comparison of low temperature crazes with shear bands reveals that both deformation phenomena are related. The surface morphologies are quite similar because both modes of plastic deformation depend upon the relative displacement of domains of a size of 10 to 100 nm. However, crazing is controlled by a tensile stress and the fibrous matter contains voids. Shear banding, on the other hand, is controlled by a shear stress which encourages lateral movements without voiding. The final breakdown process may then be initiated in both cases by a random rupture at the upper or lower edge of the deformation zone (Fig. 39 a, b). 

Zhao, C.T. Zhang, G.L. Cai, B.L. Xu, M. Solvent composition dependence of band morphology in sheared lyotropic ethyl cellulose liquid crystals. Macromol. Chem. Phys. 1998, 199 (8), 1485-1488. 

The selection of the dominant deformation mechanism in the matrix depends not only on the properties of this matrix material but also on the test temperature, strain rate, as well as the size, shape, and internal morphology of the rubber particles (BucknaU 1977, 1997, 2000 Michler 2005 Michler and Balta-Calleja 2012 Michler and Starke 1996). The properties of the matrix material, defined by its chemical structure and composition, determine not rally the type of the local yield zones and plastic deformation mechanisms active but also the critical parameters for toughening. In amorphous polymers which tend to form fibrillated crazes upon deformation, the particle diameter, D, is of primary importance. Several authors postulated that in some other amorphous and semiciystalline polymers with the dominant formation of dUatational shear bands or extensive shear yielding, the other critical parameter can be the interparticle distance (ID) (the thickness of the matrix ligaments between particles) rather than the particle diameter. 

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