Matrix cavitation

This study of stress-whitening in rubber-modified epoxies showed that the size of the whitened zone at the root of a notch decreases with increasing rubber content. Stress-whitening has been shown to be caused by hydrostatic stress. Two different species of stress whitening were found. One is reversible by heating and is deduced to be due to matrix cavitation the other is irreversible by heating and is due to highly cavitated rubber particles and shear bands. 

With a manufacturing method (or MEH method), deformation mechanisms in the presence of compressive residual stress around particles in relation with the toughening conditions, relative stress components, and fracture morphology have been analyzed using Mohr circles. A major difference in deformation between MEH and non-MEH methods was found to be in the location of plastic deformation under plane-strain. The plastic deformation of non-MEH was dominantly in matrix and appeared in the form of matrix cavitation. In the case of MEH, it was dominantly in microspheres. It was suggested that compressive residual stress promotes plastic deformation of microspheres caused by extrusion effect. 

A series of events can take place in response to the thermal stresses (/) plastic deformation of the ductile metal matrix (sHp, twinning, cavitation, grain boundary sliding, and/or migration) (2) cracking and failure of the brittle fiber (5) an adverse reaction at the interface and (4) failure of the fiber—matrix interface (17—20). 

A proposed mechanism for toughening of mbber-modifted epoxies based on the microstmcture and fracture characteristics (310—312) involves mbber cavitation and matrix shear-yielding. A quantitative expression describes the fracture toughness values over a wide range of temperatures and rates. 

The abrasion resistance of cobalt-base alloys generally depends on the hardness of the carbide phases and/or the metal matrix. For the complex mechanisms of soHd-particle and slurry erosion, however, generalizations cannot be made, although for the soHd-particle erosion, ductihty may be a factor. For hquid-droplet or cavitation erosion the performance of a material is largely dependent on abiUty to absorb the shock (stress) waves without microscopic fracture occurring. In cobalt-base wear alloys, it has been found that carbide volume fraction, hence, bulk hardness, has Httie effect on resistance to Hquid-droplet and cavitation erosion (32). Much more important are the properties of the matrix. 

On a laboratory scale, generally an ultrasonic probe (horn) and an ultrasonic cleaner are used. The ultrasonic field in an ultrasonic cleaner is not homogeneous. Sonication extraction uses ultrasonic frequencies to disrupt or detach the target analyte from the matrix. Horn type sonic probes operate at pulsed powers of 400-600 W in the sample solvent container. Ultrasonic extraction works by agitating the solution and producing cavitation in the... 

While the surface modification is not effective to suppress cavitation, Yee and coworkers performed an experiment to suppress the cavitation mechanically in a rubber-modified epoxy network. They applied hydrostatic pressure during mechanical testing of rubber toughened epoxies [160]. At pressures above BOSS MPa the rubber particles are unable to cavitate and consequently no massive shear yielding is observed, resulting in poor mechanical properties just like with the unmodified matrix. These experiments proved that cavitation is a necessary condition for effective toughening. 

In addition to the cavitation process related to the presence of a dispersed phase, the formation of voids in the plastic zone has been observed to occur also in the matrix phase. Kinloch and Huang stated that the plastic void growth succeeding cavitation also contributes to energy absorption and might become as important as the shear banding, especially at fairly elevated temperatures [161]. 

Clarkson et al. (1986) conclude that proteolytic enzymes contribute to root lesion formation. Accordingly, Katz et al. (1987) found root cavitation with loss of matrix to occur in mild acidic solutions only in the presence of proteases. It is conceivable that the degradation of the matrix promotes the formation of a root lesion in two ways. First, the matrix forms a barrier to ionic diffusion, which is removed by degradation. Second, the degradation of the matrix yields nutrients, which may sustain the growth of cariogenic bacteria (Hojo et al., 1991). 

When ultrasound is used as energy carrier, a sound intensity in the range from 5-10 W cm-2 is employed. This energy is sufficient to heat the material up to or even above its melting point. As a result, the diffusion velocity of the free radicals in turn increased. In addition, in the fluid phase of the matrix, sonochemical reactions are possible, based on cavitation. Such cavitation is associated with... 

If the adhesion between particles and matrix is good, rubber particles internally cavitate when a load is applied. 

In the case of thermoplastic particles, because the bulk modulus is equivalent to that of the matrix, no cavitation is observed. 

A considerable amount of energy is stored in the sample before the appearance of the first crack. In this step, the rubbery particles act - after cavitation or debonding - as triggers for the generation of shear bands in the matrix (Huang and Kinloch, 1992b). 

As discussed in the previous section, the toughening effect depends both on the matrix, where the shear bands are propagating, and the rubbery phase, which induces cavitation and crack bridging. 

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

A threshold of interfacial adhesion between both phases is needed to (a) promote the cavitation mechanism and (b) activate the crack-bridging mechanism. For rubbery particles, the former contributes much more than the latter to the total fracture energy. Adhesion is achieved by the use of functionalized rubbers that become covalently bonded to the matrix. Higher toughness values have been reported by the use of functionalized rubbers (Kinloch, 1989 Huang et al., 1993b). However, these experimental results also reflect the effect of other changes (particle size distribution,... 

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