Microvoid formation

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. 

The stress bias criterion [24,25] refers implicitly to two mechanisms of microvoid formation in a dilatational stress field and stabilization of the microvoids through a deviatoric stress component and local plasticity. Its definition is... 

Microvoid Formation during Sample Preparation of Polymer Glasses. 

In further studies Zhurkov etal measured microvoid formation and bond rupture on the same specimens. Bond fracture was assessed using the infra-red technique described in Section 2. They obtained identical behaviour for the kinetics of... 

Compared with BD13, at low strain of 0.2, the matrix appearance is similar, but less particles are debonded from the matrix. At 0.4 true strain, the matrix deformation and microvoids formation are less important than BD13 as well. Large matrix deformation and large quantity of microvoids appear when the tme strain reaches 0.6. The microfilaments across the interface can still be seen for some small size particles. Even though the microvoid formation and growth are developed greatly, the horizontal... 

Density measurements were done on different composite samples before the tensile test and immediately after they reached the yield point. In each case we observed density decrement, which was of the order of 0.5 t i% and which could be attributed to microvoid formation during the test the decrement was higher in the case of composites containing SIC treated beads. In the case of yield stress Oy, the effects of the filler should depend on the filler surface area and therefore related to 02/3, Smith and Nicolais and Narkis theoretically suggested lower limit value of Oy, attributing the loading capacity to the polymer only. If n spherical particles of radius r are dispersed in the unit cube the cross section of the continuous phase is A = 1 - IT-(nr) 2 and the volume fraction is 0 = jn(nr). After substitution we get A = 1 - (3/4n) / 0 / =... 

In any medium, cavities, voids, and density fluctuations exist. It is believed that these induce cavitation, leading to molecular rupture. In solid polymers, the microvoids present intrinsically are responsible for cavitation when they are subjected to a hydrostatic pressure in the manner of an impulse. One of the main causes of microvoid generation in polymer materials is the interatomic bond rupture when they are subjected to mechanical and thermal stresses. Extensive studies showing microvoid formation in stressed polymers have been carried out (Zhurkov et al., 1972). 

There is evidence to suggest that exposure to humid environments, at temperatures above about 60°C can produce permanent damage in epoxy adhesives.Apicella and coworkers have explained this in terms of the formation of microcavities produced by clusters of water molecules. This mechanism of damage is expected to be evident in thermoset resins in which the rigid crosslinked structure does not allow the matrix to relax after microvoid formation. 

One could also address micro flow using Darcy s law for flow within the fiber bundles but characterized by a different permeability as will be shown later, but to rigorously describe the flow within fiber bundles, one must also account for surface tension and couple this flow with the macro flow in between the fiber bundles. More details can be found in the references that model the microvoid formation in the fiber bundles by adding a sink term to the equation of mass conservation. 

In Eq. [9.6], the fiber bundles are assumed to be completely impregnated instantaneously as the flow front passes through. If one would like to investigate the microvoid formation in the bundles, usually a sink term is added to Eq. [9.6]. " In that approach, the microvoid dimensions and strength of the sink are related to the viscosity, pressures of the void-free surface and capillary, and the permeability of the fiber bundle. 

Besides the visible dry spots, microvoids (see Fig. 10.6) are another type of air entrapment in VARTM processes. The cause of microvoids is different from that of dry spots. The microvoids are formed due to the incompatible dual scale flow behavior of the wetting process inside a fiber tow (or fiber bundle), which is used to form the liber mat, and the resin flow process in the gap between fiber tows. The resin flow in the gap between two liber tows is governed by the Darcy s law (i.e., Eq. [10.1]). However, the resin filling inside a fiber tow is driven by the capillary effect if air/resin interface is involved. The relationship between the microvoid formation in RTM processes and the dual scale effects of the Darcy s flow and the capillary flow has been investigated by many researchers. - Generally, at the resin flow front, small amounts of air could be trapped and form microvoids if the... 

The cavitation mechanism within the modifier particles begins with microvoid formation and fibrillation in the butyl acrylate shell, as shown in higher magnification in Figure 10b. The fibrillated shells with the internal PMMA cores resemble spiders. The fibrils are connected by the PMMA cores, and the cavities in... 

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