Rubber, cavitation damage

The alternative option for counteracting cavitation damage is the use of a resilient material such as rubber. The mechanical forces attendant on collapse of the bubbles are absorbed by elastic deformation of the resilient material. 

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.)...

These differences were correlated with the size of the rubber particles in the systems the smaller the diameter of the dispersed phase (e.g. the lower the interparticular distance), the higher the benefits of a /3-nucleation (Fig. 25b). For the grades with the smallest particle sizes, it might be attributed to an easier plastic deformation of the matrix once the damage mechanisms initiated (by particle cavitation) as a result of the smaller matrix ligaments between the rubber phase. 

Cavitation was also identified as an active mechanism in systems where a mbberlike phase (particles or interphase) is susceptible to implode under the effect of the hydrostatic stress induced by the applied tension. Fond (36) has recently revisited the critical conditions under which this form of damage becomes energetically favorable. In the case of core-shell rubber-toughened PMMA, he ascribed the extensive whitening under tension at room temperature to the profuse formation of voids in the mbber shell of the toughening particles. 

One other inhibitor type that will prevent cavitation-corrosion is the soluble oil type, which incorporates a light mineral oil plus emulsifiers and adsorption-type inhibitors, such as organic amines. Unfortunately, although effective in controlling cavitation, (possibly by cushioning effects of the adsorbed oil reinforced film) they soften and damage rubber connectors and seals, cause leakage and water loss. When emulsifiers are exhausted, the oil emulsion breaks, and allows oily films to form on heat transfer surfaces. 

A slightly different mechanism of deformation has been observed with some specific combinations of epoxy resins and core-shell particles [125,126] crazelike features were observed in the damage zone at the crack tip and shown to consist of line arrays of cavitated rubber particles. The matrix material around the cavitated particles had undergone plastic deformation, whereas the material outside the line array was left undeformed. These line arrays of cavitated particles were termed croids , derived from the words crack and void [125]. 

As rubber plantations are currently expanding to new areas under drier climatic conditions than traditional ones (north-eastern Thailand, Mato-Grosso in Brazil, north-western India), the performance of rubber trees under water stress has become a major issue. Two main traits are to be considered performance (the ability to transfer water and grow under water stress) and resistance (the ability to avoid damage and survive under water stress). Performance mainly relies on the conductivity of the hydraulic system from root to leaf, whereas resistance mainly relies on susceptibility to cavitation, leading to embolism, within xylem vessels. Stomatal closure occurs to avoid severe cavitation. Hence, differences in susceptibility to cavitation among rubber clones are likely to induce differences in resistance to severe water stress. 

It is now believed that the process of particle cavitation relieves the local buildup hydrostatic tension caused by constant volume shear banding. This allows additional enhancement of shear yielding in both thermosets and thermoplastics (16). Hence, soon after development of some initial shear yielding, the local triaxial constrains are relieved by cavitation even in relatively thick specimens. Similar explanations were proposed for PC/PE, PC/MBS (17), and rubber-modified epoxies (18-23). In polymers or under test conditions favoring crazing as a major deformation mechanism, cavitation and voiding of the rubber particles leads to a premature craze breakdown and it is damaging to the polymer. 

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. 

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