Deformation cavitation-controlled

Cessna (9) carried out similar experiments as those reported here on several commercial impact thermoplastics over a range of strain rates. His work suggested that the classes of impact plastics studied exhibit a similar transition from volume-conserving to cavitation-controlled deformation processes as deformation rates are increased or temperature decreased. The present work supports those findings, as do the predictions of Bucknall and Drinkwater. Unlike Cessna, in no case did we find evidence of closure of cavities by shear yielding after cavitation. 

In most two-phase ceramics, particle concentrations exceed the rheological threshold discussed above. Therefore, deformation is controlled not by fluid flow, but by direct interactions between the particles that make up the solid. Above the packing threshold, many particles are either in direct contact, or are sufficiently close that particle interactions are important. Processes that occur close to the particle surface dominate the deformation behavior. These processes include solution-precipitation, matrix flow (or matrix percolation) between the particles, and cavitation. 

Osborne Reynolds was the first to show that tightly packed granular solids expand their volume when deformed.84 This phenomenon is called dilatancy. It is well understood and is discussed in some detail in the literature on soil mechanics.85-87 In vitreous-bonded structural materials such as silicon nitride, dilatancy has been suggested as a contributing factor in the formation of cavities,88 and may be an important factor in the cavitation of ceramic matrix composites.64 Dilatancy has also been suggested as an important factor in controlling the creep and creep relaxation of glass-ceramics.89... 

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. 

The hrst point concerns the intrinsic properties of the materials constituting the blends. It is evident that, for the blends under consideration, we have mixed PP that suffers very high cavitation with increasing amounts of PA6 that deforms plastically with particularly low volume strain. Although the PA6 phase does not support the same amount of strain in the blends as the PP matrix (PA6 particles has shown little deformation in the micrographs), it is presumable that the lower tendency to cavitation of the blend is partly controlled by the more isochoric nature of the PA6 component. 

This balance between interfacial propagation and bulk deformation has been described for linear elastic materials [56] and results from the competition between two mechanisms the velocity of propagation of an interfacial crack, which is controlled by the critical energy release rate Gc, and the bulk deformation, which is controlled by the cavitation stress and hence essentially by the elastic modulus E or G. In the hnear elastic model, the key parameter is the ratio GJE, which represents the distance over which an elastic layer needs to be deformed before being fuUy detached from the hard surface. This model has been verified experimentally for elastic gels [57]. 

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