Debonded particles

The rate of debonding decreases as the number of debonded particles increases and as the stress increases. The debonding constants characterize the interaction and the influence of neighboring particles. Their values depend on the filler concentration and on the adhesion of the filler to the matrix. The volume increase due to debonding is given by the equation ... 

Finally, it should be noted that stress-whitening arises because these voids, created by cavitation and debonded particles, scatter light. Hence, it is observed that the most intense stress-whitening is present in the recipe that undergoes the greatest extent of particle cavitation and debonding. 

Figure 7. Yield stress of the compatibilized blends compared with calculations. Key —, a yielding angle of 70° and Hashins upper modulus bound —, Nielsens approach for debonded particles and, Nielsens approach with a debond-...

In poorly bonded NT- Al O /epoxy composites, particles are clearly visible and the crack seems to have propagated aroimd their equator. Ihe fracture surface of nanocomposites consists of hemispherical holes (A), top surface of the debonded particles (B) and particles covered by epoxy matrix (C). The crack may propagate above or below the poles of the particles through the matrix. Interfacial debonding seen in the mirror zone is not seen in the hackle zone for treated alumina-epoxy nanocomposites. Another toughening mechanism noticed in the hackle zone is particle pullout, which is seen in both NT- Al O /epoxy nanocomposites and APTES-Al Oj/epoxy nanocomposites, whereas micro-cracking is noticed only in APTES-Al Oj/epoxy nanocomposites. 

In conclusion, it can be stated that the effect of fillers on modulus is relatively well understood and may be predicted. The other properties are less easy to predict as they are measured under conditions where the composite is deformed to a greater extent. This means that other factors such as particle debonding, particle re-orientation and polymer orientation, must be considered. 

Figure 6 illustrates the dependence of the engineering draw stress on filler content Like the peld stress, draw stress depends on adhesion between the polymer and the particles. The upper limit of the draw stress, equal to the draw stress of unfilled polymer, describes polymers filled with well-bonded particles. The lower border of the fork, corresponding to completely debonded particles in the )deld zone, is a straight line with the slope approximately equal to minus one. If particles are debonded partially, the draw stress is given by ... 

The volume concentration of debonded particles or the fraction of the debonded filler, ( )o = ( )xAj)f can be determined by measuring the Young s modulus of a filled elastomer before and after preloading that breaks adhesion contacts. When studying the filler separation in the model samples, it was noted that not all particles debond simultaneously, and that the number of debonded particles depends not only on the magnitude of the disturbing stress, but also on the time... 

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