Rubber cavitation

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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This conclusion was only partly confirmed by scanning electron microscopy micrographs of RuC>4 stained surfaces taken at the crack tip of deformed specimens at 1ms-1, where the non-nucleated and /3-nucleated materials showed, respectively, a semi-brittle and semi-ductile fracture behavior. While some limited rubber cavitation was visible for both resins, crazes—and consequently matrix shearing—could not develop to a large extent whether in the PP or in the /1-PP matrix (although these structures were somewhat more pronounced in the latter case). Therefore, a question remains open was the rubber cavitation sufficient to boost the development of dissipative mechanisms in these resins ... 

This early rubber cavitation is not necessary for an efficient toughening of amorphous PET because of its lower sensitivity to triaxial stress states (compared to semi-crystalline PET). 

Chapters 8 and 9 consider the mechanical properties of rubber- and ceramic-particle toughened-epoxy materials. The importance of rubber cavitation is highlighted in Chapter 8. It is well known that this mechanism can relieve the high degree of triaxiality at a crack tip in the material and enable subsequent plastic hole growth of the epoxy resin, which is a major toughening mechanism. We return to rigid particles in Chapter 9, which examines their use to increase the thermal shock resistance of epoxy resins. 

Crack Stability. At low test speeds, stable crack growth with an extended stress-whitened plastic zone and crack blunting occur by the same mechanisms as those involved in the kinetics of the plastic zone, namely, rubber cavitation followed by shear deformation of the matrix. The ability of the matrix to shear is controlled by its relaxation behavior, which therefore determines its plasticity and the deformation imposed on rubbery particles distant from the notch. 

Figure 8.9. SEM micrograph of block PP after Izod impact test. Contrast was obtained by ion etching. The interfacial debonding and rubber cavitation are seen [Nakagawa, 1976].

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. 

Figure 10.9 Schematic representation of rubber cavitation. Reproduced with permission from Ref. [64] 0 1994, Elsevier.

Ma et al. [Ill] also revealed that internal rubber cavitation in combination with matrix yielding are the dominant mechanisms for the PLA/ethylene-co-vinyl acetate copolymer blends, reaching 83 kJ m with a rubber content as high as 30wt% (Figure 10.13). 

Observations of rubber cavitation and growth of the voids offer an additional explanation for the enhanced shear yielding of the matrix (Donald and Kramer 1982). The presence of many closely packed particles which can cavitate enables... 

Cavitation is followed by the onset of a shear localisation process, which would not have taken place in the net resin under the same conditions. Rubber cavitation and... 

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 existence of critical MLT for effective rubber toughening can be explained [72, 177, 178] in the light of two basic mechanisms rubber cavitation followed by the formation of shear bands, and crazing. A low MLT maintains the connectivity of the yielding process, which then propagates over the entire deformation zone and makes... 

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