Micro shear bands

EPDM particles) are clearly visible on the fracture surface. The ribbon-like structures visible on the fractured surfaces are probably the micro-shear bands in the blends. 

The EPDM particles were formd either to initiate crazes or to terminate them, depending on the interfacial bonding, the particle size, the concentration, and the interparticle distances. The variation of notched Izod impact strength of PPBC blends with different EPDM concentrations is shown in Fig. 10.16. The fracture surface of a blend with 10 wt% EPDM, as examined under SEM, is shown in Fig. 10.20. The hemispherical embeddings and hoUows (representing the removed EPDM particles) are clearly visible on the fracture surface. The ribbonlike structures visible on the fractured surfaces are probably the micro-shear bands in the blends. 

Both principal fracture mechanisms, shear yielding and crazing, are influenced by the particle size. In PPBC matrix, where spherical elastomeric particles are chemically bonded, the energy absorption takes place mainly by deformation of the matrix. In such systems, a large amount of shear yielding is to be expected. The shear yielding becomes more prominent upon increasing the concentration of EPDM as well as reduction of their particle size. The micro-shear bands in the fracture surface (Pig. 10.23e) clearly support these expectations. 

On the other hand, the shear bands and deformation zones are the result of shear processes and do not contain voids so that deformation advances at nearly constant volume. Shear bands can be either localized or diffuse, but even for very localized bands, their interface with bulk material is much thicker than for crazes. Thick bands and deformation zones are usually made of coalescing micro-shear bands. 

Mahmood, Z. et al. 2011. A simulation study of micro-structure evolution inside the shear band in biaxial compression test. International Journal for Numerical and Analytical Methods in Geomechanics 35 652-667. 

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. 

Shear yielding in the form of a quasi-homogeneous,bulk process can contribute substantially to the crack resistance of a polymeric solid. On the other hand, however, localized shear yielding in the form of shear micro-bands is befleved to be a precursor of brittle fracture in many semicrystalline and glassy thermoplas-... 

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