Matrix fracture, cavitation

CaC03) aj) < aj. The schematic sequence is shown in Fig. 5 stress concentration localised at particle pole (micronic particle), to induce either local matrix fracture (crazing) or interface decohesion (cavitation). 

In composite propellants, a relatively strong bond between binder and filler is important for obtaining high tensile strength. However, the edges and surfaces of the inclusions serve as sites of dangerously high stress concentrations, which could cause internal failure of the softer matrix material. Most studies (I, 2, 6-9) have dealt with the problem of matrix fracture induced by the inclusions. Here, we present the mode of matrix fracture called cavitation,... 

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. 

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,... 

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 ... 

In ABS, where particle size is much smaller than in HIPS, the particles are less effective as craze initiators and the fatigue fracture surface shows evidence of considerable localized plastic deformation of the matrix polymer as well as of cavitation and/or loss of adhesion of the rubber particles. 

The post mortem observation of fracture surfaces of Izod bars confirms cavitational mode in the modified products. The close observation of the TEM pictures offers a further insight while cavities are sometimes visible inside the reactive modifier particles (R), the cavities in the blends containing non-reactive particles (NR) are always located at the modifier / matrix interface. 

Load Displacement and Fracture Behavior Over a Range of Test Speeds. As discussed at the beginning of this chapter, many authors have reported an apparent correlation between the toughening mechanisms and the development of a stress-whitened zone that diffracts light in the material. The occurrence of stress-whitening at 3-4% strain coincides with the appearance of both cavitation (6) and shear strain in the matrix (8). 

Figure 11.123. Fracture mechanism and energy dissipation the flocculated rubber network is becoming re-dispersed by impact energy the crack is propagating at the interface between free matrix and adsorbed polymer matrix layer cavitation occurs at the point where re-dispersion begins. [Reproduced from ref. 89 with kind permission of Huthig and Wepf publishers.]...

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