Toughening effect whisker

The fiber or whisker pull out has the potential for the greatest toughening effect and consists in a debonding of fiber from the adjacent matrix consuming large energy. This mechanism depends strongly on the aspect ratio of the second phase and is favored by weak interface bonds. 

In this section, limits were imposed on the improvement of toughness in ceramics by the incorporation of britde, discontinuous reinforcing phases. Observations indicate that such additions bridge cracks in the regions behind the crack tips. Whiskers act in the same manner as discontinuous reinforcing phases. Bridging models may be used to optimize toughening effects and to allow for the choice and modification of pertinent material characteristics, such as physical properties and microstructure. 

Experimentally it has been shown that both frictional bridging and whisker pullout play an important role in toughening industrially manufactured composites. Such investigations confirm that to maximize toughness via both mechanisms requires a high volume fraction of whiskers and a high composite modulus to whisker modulus ratio. For example, consider the effect of 20 vol % SiC whisker E = 500 GPa) reinforcement of various matrices on the toughness as presented in Table 7 (53). 

Figure 11.17 (a) The effect of SiC whisker content on toughness enhancement in different matrices.(b) Toughening is associated with crack bridging and grain pullout of elongated matrix grains. 

As can be deduced from the discussion above, the characteristics of the whiskers, such as diameter and strength, have a direct effect on the toughening behavior and mechanical behavior, The surface chemistry of the whiskers influences the nature of the interface bond between the whiskers and the matrix, which is addressed in the y /y/ term in Eq. 2. It has been found that whiskers from various manufacturers have different characteristics, which affect their performance as reinforcements. 

Other toughening mechanisms are related to the debonding and pull-out of the inclusions during fracture. These mechanisms are especially effective in whisker and short-fibre reinforced glass composite materials [35]. 

The reinforcing effects of nanofibers in an epoxy matrix and in a rubber matrix using electrospun nanofibers of polybenzimidazole (PBl) was observed. The average diameter of the electrospun fibers was around 300 nm. The nanofibers toughened the brittle epoxy resin. The fracture toughness and modulus of the nanofiber (15 wt.%)-reinforced epoxy composite were both higher than for an epoxy composite made with PBl fibrils (17 wt.%], which were whisker-like particles. 

Becher, P. F., Heueh, C., Angellita, P., and Tiegs, T. N. (1988). Toughening behavior in whisker-reinforced ceramic matrix composites. / Am. Ceram Soc. 71 1051-1061. Homeny,J., Vaughn, W. L., and Ferber, M. K. (1990). Silicon carbide whisker/alumina matrix composites effect of whisker surface treatment on fracture toughness./ Am Ceram. Soc. 73 394-402. 

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