Porosity stress concentration

Pores are usually quite deleterious to the strength of ceramics not only because they reduce the cross-sectional area over which the load is applied, but more importantly because they act as stress concentrators. Typically the strength and porosity have been related by the following empirical relationship ... 

With impregnation by an appropriate monomer, the main effect after polymerization is the sealing of the continuous capillary pore system, which reduces the effect of stress concentrations from pores and micro-cracks, thereby increasing the strength. A reduction of porosity by 10% doubles the strength. 

The moisture content of both the wood and the adhesive affect the fracture behavior of adhesive bonded joints. Wood joints are especially sensitive to moisture effects as a result of the porosity and permeability of wood, which allows ready access by water to both the interior of the wood member and the adhesive layer. Irle and Bolton [57] showed that the superior durability of wood-based panels bonded with an alkaline PF adhesive compared to panels bonded with a UF adhesive was due to the ability of the phenolic adhesive to absorb and be plasticized by water. In the plasticized state, the phenolic adhesive is able to reduce stress concentrations that otherwise fracture the wood or the adhesive in urea-bonded panels. 

If, in contrast to the case just treated, the filler does not adhere to the matrix, then the filler particles cannot carry any of the load (Figure 12.5). In addition to the effective porosity thus introduced, stress concentrations around the particles will reduce the strength still further this effect cannot be calculated simply. Thus Nielsen proposed that the tensile strength Should be given approximately by an equation of the following general formt ... 

Modification of mechanical properties High aspect ratio glass fibers, mica, nanoclays, carbon nanotubes, carbon/graphite fibers, and aramid/synlhetic/natural fibers Low aspect ratio talc, CaC03, kaolin, wood fiour, wollastonite, and glass spheres Control of permeability Reduced permeability impermeable plate-like fillers mica, talc, nanoclays, glass flakes Enhanced permeability stress concentrators for inducing porosity CaCOj and dispersed polymers... 

An increase of strength and of fracture toughness is related to two phenomena decreased total porosity due to the application of superplasticizers and micro-fillers and reduction of micro-cracks and other discontinuities, which enhance stress concentrations and crack propagation according to basic relations proposed by Griffith. The second phenomenon was described by Kendall and Birchall (1985), among others. 

Although the nucleation of cavities does not seem to be well understood at present it is clear that cavitation depends on microstructure. Porosity and second-phase particles, which are sources of stress concentration (see Chapter 18), can act as nucleation sites for cavitation and subsequent crack growth. Remember pores can be found in most ceramics even pore-free materials such as hot-pressed alumina may contain small pores. Cavitation also occurs in ceramics with IGFs. Nucleation of the cavities will usually occur at regions where the IGF is not homogeneous, e.g., nonwetted regions, gas bubbles, and impurity particles. 

Because gels behave as brittle materials, the mechanical strength (ctr) is strongly dependant on the presence of flaws, which act as stress concentrators (Griffith, 1920). The most relevant feature of brittle materials is their toughness (JCjc), which characterizes the ability of the material to resist the propagation of flaw. Pores could be considered as flaws or as a integral parts of flaw responsible for the failure of this brittle material. If such an assumption is valid, the critical flaw size should depend on the porosity and pore size. 

The value of rj in Equation 5 is a measure of the porosity effect on the strength. A wide range of change in r resulted from its dependence on the pore geometiy an increase of the stress concentration factor created by pore increases the tj value. Shown in Fig. 11 is the value as a function of the gelatin solution concentration. The value of t] was calculated as 77 =... 

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