Microvoiding

Under compression or shear most polymers show qualitatively similar behaviour. However, under the application of tensile stress, two different defonnation processes after the yield point are known. Ductile polymers elongate in an irreversible process similar to flow, while brittle systems whiten due the fonnation of microvoids. These voids rapidly grow and lead to sample failure [50, 51]- The reason for these conspicuously different defonnation mechanisms are thought to be related to the local dynamics of the polymer chains and to the entanglement network density. 

Methylene chloiide formulas are the most common organic chemical removers. The low molar volume of methylene chloride allows it to rapidly penetrate the finish by entering the microvoids of the finish. When the solvent teaches the substrate, the remover releases the adhesive bond between the finish and the substrate and causes the finish to swell. The result is a bhstering effect and an efficient rapid lifting action. Larger molecule solvents generally cannot... 

It is important to note that we assume the random fracture approximation (RPA) is applicable. This assumption has certain implications, the most important of which is that it bypasses the real evolutionary details of the highly complex process of the lattice bond stress distribution a) creating bond rupture events, which influence other bond rupture events, redistribution of 0(microvoid formation, propagation, coalescence, etc., and finally, macroscopic failure. We have made real lattice fracture calculations by computer simulations but typically, the lattice size is not large enough to be within percolation criteria before the calculations become excessive. However, the fractal nature of the distributed damage clusters is always evident and the RPA, while providing an easy solution to an extremely complex process, remains physically realistic. 

In investigations of the failure of fiber compositions (PETP — short glass fibers) [251] it was found that the main process responsible for composite failure under load is the rupture at the matrix-fiber interface. The author of [251] observed formation of microvoids in loaded samples, both at the interphases and in the bulk. The microvoids, or cavities) grow in size and become interconnected by microcracks, and this results in fiber separation from the binder. However, when the matrix-fiber bond is strong enough, the cavities appear mostly in the bulk of matrix, the failure of the specimen does not over-power cohesion and traces of polymer remain on the fibers. 

Microindentation anisotropy 143, 145 Microspherulitic structure 139 Microvoiding 193 Microvoids 194, 205 Model membranes 49, 53, 55 Modulus 135 Moisture sensitivity 199... 

In the case of a-Si H, it turns out that Qab is positive [439]. This can be inferred from the bond energies of Si—Si (2.35 eV). Si—H (3.3 eV), and H—H (4.5 eV). A Si—H network is thus unstable compared to pure Si and H2. A mixture of Si—Si and Si—H bonds will be driven towards their two stable mixture phases, which leads to the solubility limit of H in Si. Acco et al. [69,70] have determined this limit to be about 4%. Excess hydrogen in -Si H forms SiH2 or SiH around microvoids. 

Cavitational effects leading to an increase in the temperatures and pressure at the localized microvoid cavity implosion sites. 

The inhomogeneous structure of the a-Si H films most likely leads to H2 motion that cannot be described as uniformly diffusive throughout the film. It has been suggested that H2 motion occurs along microvoids, present even in device quality films, that are aligned preferentially perpendicular to the films surface (Vanderheiden et al., 1987). These oriented microvoids can be considered as remnant micro-columnar structure in the films. The fact that the amount of H2 that can be reintroduced can never... 

Robson, R. M. (1999). Microvoids in Bombyx rnori silk. An electron microscope study. Int. J. Biol. Macromol. 24, 145—150. 

Chapter 14 - It was shown, that the conception of reactive medium heterogeneity is connected with free volume representations, that it was to be expected for diffusion-controlled solid phase reactions. If free volume microvoids were not connected with one... 

Further from geometrical considerations in assumption of three-dimensional microvoid of... 

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