Brittleness ratio

On the commercial scale silicone resins are prepared batchwise by hydrolysis of a blend of chlorosilanes. In order that the final product shall be cross-linked, a quantity of trichlorosilanes must be incorporated into the blend. A measure of the functionality of the blend is given by the R/Si ratio (see Section 29.3). Whereas a linear polymer will have an R/Si ratio of just over 2 1, the ratio when using trichlorosilane alone will be 1 1. Since these latter materials are brittle, ratios in the range 1.2 to 1.6 1 are used in commercial practice. Since chlorophenylsilanes are also often used, the CH3/CgH5 ratio is a further convenient parameter of use in classifying the resins. 

Wernersson and Gustafsson [65] developed a nonlinear fraeture meehanies relationship based on pure shear for predicting the performance of lap joints of varying geometry and adherend properties based on the adhesive brittleness ratio ... 

Fig. 8.34 Crack brittleness ratio, Jfc/Gic versus (Jic-Gic) [27]. With kind permission of John Wiley and Sons...

Figure 8.27 Abrasion performance changes with elastic-plastic-brittle ratio. 1, Herasil (fused silica) 2, BK7 (crown) 3, SLS (float glass, Glaverbel) 4, ZKN7 (crown) 5, F2 (flint) 6, LF5...

Maalej and Ei [84] developed relations between flexural and tensile strength of strain softening ERC, based on these concepts and determined this strength ratio as a function of the brittleness ratio, B (Eq. 4.109, Figure 4.51) and the length of the fibre relative to its critical length (Figure 4.52). 

Figure 4.51 Effect of brittleness ratio, 6, on the ratio beween flexural and tensile strength (after Maalej and Li [84]).

Materials sueh as east-brittle metals and eomposites do not exhibit these uniform properties and require more eomplex failure theories. Where the properties Su. and Su of a brittle material vary greatly (approximately 4 1 ratio), the Modified Mohr Theory is preferred and is good predietor of failure under statie loading eonditions (Norton, 1996 Shigley and Misehke, 1989). 

C) 370/656X brittleness after exposure to temperatures between about 700 to 1. OSO-F. stainless steels. chromium stainless steels, over 13% Cr and any 400 Series martensitic chromium stainless steels low in carbon content (high Cr/C ratio). complex chromium compound, possibly a chromium-phosphorus compound. chromium steels at temperatures above about 700 F (370 C) keep carbon up in martensitic chromium steels and limit Cr to 13% max. 

By varying the coumarone/indene ratio and also the polymerisation conditions it is possible to obtain a range of products varying from hard and brittle to soft and sticky resins. 

Two-component systems consist of (1) polyol or polyamine, and (2) isocyanate. The hardening starts with the mixing of the two components. Due to the low viscosities of the two components, they can be used without addition of solvents. The mass ratio between the two components determines the properties of the bond line. Linear polyols and a lower surplus of isocyanates give flexible bond lines, whereas branched polyols and higher amounts of isocyanates lead to hard and brittle bond lines. The pot life of the two-component systems is determined by the reactivity of the two components, the temperature and the addition of catalysts. The pot life can vary between 0.5 and 24 h. The cure at room temperature is completed within 3 to 20 h. 

The constant value of 0.25 for Poisson s ratio versus depth reflects the geology and the rock mechanics of the mature sedimentary basin in the West Texas region. Since mature basins are well cemented, the rock columns of West Texas will act as compressible, brittle, elastic materials. 

Beryllium is a light metal (s.g. 1 -85) with a hexagonal close-packed structure (axial ratio 1 568). The most notable of its mechanical properties is its low ductility at room temperature. Deformation at room temperature is restricted to slip on the basal plane, which takes place only to a very limited extent. Consequently, at room temperature beryllium is by normal standards a brittle metal, exhibiting only about 2 to 4% tensile elongation. Mechanical deformation increases this by the development of preferred orientation, but only in the direction of working and at the expense of ductility in other directions. Ductility also increases very markedly at temperatures above about 300°C with alternative slip on the 1010 prismatic planes. In consequence, all mechanical working of beryllium is carried out at elevated temperatures. It has not yet been resolved whether the brittleness of beryllium is fundamental or results from small amounts of impurities. Beryllium is a very poor solvent for other metals and, to date, it has not been possible to overcome the brittleness problem by alloying. 

Yield stress -(MN/m ) 6 UTS (MN/m ) Modulus of eiasticity (GN/m ) Poisson s Hardness ratio (VPN) Resistance to thermal shock Workability (ductile to brittle trans. temp.)( C) Recrystallisation temperature Stress relieving temperature ... 

Polymer A with GIC = 160 J m-2 is typical for thermoset materials which are expected to be brittle [78]. At the other end of the series, polymer E and Phenoxy with G,c > 1 kJ m 2 are tougher than several wellknown thermoplastics (PMM A, PS, PES). In contrast to the more crosslinked polymers, polymer E and Phenoxy PKHJ show necking after yielding in tensile tests with draw ratios A = 1.7 and A = 2.1, respectively (Table 2.1). 

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