Multiple-crazing mechanism toughening

The section Toughening by the Multiple-Crazing Mechanism mentioned the effect of superposition of the local fields of stress concentration, which must also be considered. As shown in Figure 8, there is a remarkable increase of the local stress between particles by superposition if the interparticle distance is smaller than the particle diameter (A/D < 1, which corresponds to particle volume contents above 5%). 

It is well known that the main mechanisms of inelastic deformation are shear yielding and multiple crazing in the rigid matrix phase, as well as cavitation in the soft dispersed phase in rubber-toughened plastics and multiphase polymers [42]. Eor many years, these mechanisms have been studied using microscopy techniques. 

Crazing is a mechanism of plastic deformation that is extremely localized. Even when the number of crazes in the sample is substantially increased, as in the case of multiple crazing in rubber-toughened blends, their early stages of development... 

The major theories used to interpret the toughening mechanisms of rubber-modified PP are multiple-crazing, damage competition, shear yielding theories, and microvoid and cavitation mechanisms. The rubber particles in a PP matrix have to be uniform in size, and the interfacial adhesion and morphological structure between the matrix and filler have to be excellent. 

The effects of rubber particles upon toughness are due largely to their ability to reduce the yield stress of the polymer. By acting as stress concentrators, the particles promote shear yielding and/or multiple crazing at lower applied stresses than in the unmodified matrix (see 5.2 and 5.3). Fracture mechanics tests on rubber-toughened plastics at low temperatures show that if specimen... 

Figure 5.10 Three-stage mechanism of toughening multiple crazing) [2, 6] ...

The role of elastomeric tougheners is intimately related to the fracture mechanism in operation. In matrices that are inherently shear yielding, impact modifiers act as stress concentrators where shear bands are initiated. When the polymeric matrix tends to craze, the toughening particles induce multiple craze formation and their elastomeric nature prevents the growth of large crazes that could develop into unstable cracks. 

Fig. 9. Schematic presentation of the three-stage mechanism of toughening with multiple initiation of local yield events (eg, crazes as in case C in Fig. 8).

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