Acicular grains

The R-curve effect is largely responsible for the dispersion of Kj values and the influence of the size of test specimens on the results. Nevertheless, acicular grain ceramics or fiber composites partly owe their attraction to this same effect. This property is in fact put to good use now in the inspection logic of microstmctures for making the architecture of materials less brittle and therefore more attractive to the constructor [MIY 95]. Figure 8.7 is a collection of some examples of microstructures favorable to the appearance of the R-curve effect. Furthermore, it is noted that the materials for which this effect is pronoimced have a Weibull modulus higher than when the resistance curve is flat... 

Etruschi seamount. Basaltic fragments about 0.1 Ma old have been recovered from this site (Keller 1981). The rock is mostly glassy with a few small grains of olivine and acicular plagioclase crystals. 

Solubility of fillers is also important in dental cements and biomaterials. Unlike in construction cements, the cost of the end product is not a major issue in dental cements, but purity, biocompatibility, and performance of the final product is very important. Each filler component has its own role in modifying the behavior of the final product. As we shall see later in Chapter 18, wollastonite improves the flexural strength and toughness of the dental cements because the grains of wollastonite have elongated needle shape (acicular). Hydroxyapatite is added to provide biocompatibiUty to the product. Attributes of these fillers govern the final properties of the products and hence it is necessary that, not only one understands detailed properties of the binder components, but also that of the additives and filler components. For this reason, this chapter is devoted to describe the genesis and properties of the binder and important filler components. 

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