Cavitation of rubber particles

Figure 13.9 Sequence of events in a croid formation, (a) Initial state at the crack tip. (b) Cavitation ofthe rubber particles dueto loading head of the crack tip. (c) Cavitation of rubber particles near the already cavitated particles due to stress-concentration effect. The croid is forming, (d) Croids are propagating ahead ofthe crack and inside the craze-like damaged zone many shear bands develop between cavitated rubber particles. (Sue, 1992 with kind permission from Kluwer Academic Publisher.)...

As discussed in the previous section, cavitation of rubber particles is practically necessary for toughening. In this section, some conditions important for cavitation to occur will be discussed. 

All these theoretical predictions are consistent with the results observed by Yee and Person [120, 131], which clearly show shear bands between cavitated particles in rubber-toughened epoxy materials. They also found that the ability of CTBN rubber to toughen epoxy is closely related to CTBN rubber cavitation, which is seen as thick dark circle within the rubber particles under optical microscopy. The need of internal cavitation of rubber particles has been questioned by others [152, 153], but these researchers have considered only the case of uniaxial tension, which have been shown to be quite different than the triaxial stress seen at the crack tip. 

The basic property of ABS resins is impact resistance. In connection with this, various theories relating to the impact property have been established. The rubber particles disperse impact energy through efficient generation and annihilation of craze. By such crazing, the energy is annihilated with deformation of rubber particles to create the cavitation of rubber particles. This phenomenon can be confirmed through transmission electron... 

Fig. 8.1. Toughening mechanisms in rubber-modified polymers (1) shear band formation near rubber particles (2) fracture of rubber particles after cavitation (3) stretching, (4) debonding and (5) tearing of rubber particles (6) transparticle fracture (7) debonding of hard particles (8) crack deflection by hard particles (9) voided/cavitated rubber particles (10) crazing (II) plastic zone at craze tip (12) diffuse shear yielding (13) shear band/craze interaction. After Garg and Mai (1988a).

Mechanical loading —> cavitation in rubber particles —> promotion of shear bands in matrix —> toughness improvement... 

As in rubber toughening, the cavitated particles induce shear yielding of the matrix, producing a plastic zone smaller than the cavitated zone. These mechanisms induce a significant toughening effect. Should matrix yielding precede the cavitation of CSR particles, the croiding mechanism would be suppressed. 

Figure 19. Schematic representation of the three different toughening mechanisms in dispersed systems, where the assumed loading direction is vertical (a) induced formation of fibrillated crazes (i.e., with microvoids in them) at the equatorial zones of rubber particles (b) induced formation of homogeneous crazes at cavitated particles and (c) induced formation of shear deformation between cavitated particles.

Toughening mechanisms due to the elastomer spheres include shear-band formation, fracture of rubber particles, stretching, debonding and tearing of rubber particles, rubber cavitation, transparticle fi acture, crazing, formation of a plastic zone at the craze tip, diffuse shear-yielding, as well as shear band/craze interaction. 

Generation of microvoids due to cavitation or debonding of rubber particles that... 

In rubber-modified plastics, under triaxial tensile stresses, voiding can be initiated inside the rubber particles. Once the rubber particles are cavitated, the hydrostatic tension in the material is relieved, with the stress state in the thin ligaments of the matrix between the voids being converted from a triaxial to a more uniaxial tensile stress state. This new stress state is favourable for the initiation of shear bands. In other words, the role of rubber particles is to cavitate internally, thereby relieving... 

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