Critical cracking thickness

CCT, critical cracking thickness Boltzmann constant (1.381x10 local permeability [m ] fracture resistance [N m ] average permeability in/of compact [m ] particle shape factor compact thickness [m] initial particle number concentration [m refractive index of particle material refractive index of dispersion material number density of ion i dimensionless number dimensionless number Stokes number Peclet number capillary pressure [N-m ] dynamic pressure [N m ] local liquid pressure in the compact [N-m local solid pressure in the compact [N-m ] superficial fluid velocity [m-s q gas constant [J K ] centre to centre distance [m]... 

This section reviews properties of dispersions and characteristics, which are important to the formulation and application of dispersions. They include solids content, pH, stability, and critical cracking thickness. 

Critical Cracking Thickness - The maximum thickness which can be coated in a single layer (pass) of polytetrafluoroethylene dispersion without crack formation. This thickness is measured after sintering has been completed. 

Fig. 6.12 Critical PS thickness above which cracking of the PS film during evaporation of water is expected, according to Eq. (6.9). Note that drying in pentane or ethanol increases the critical thickness by a factor of about 26 or 11, respectively. Inset Cracking can be avoided by freeze drying (1) or critical point drying (2), which removes the solvent without crossing the fluid-gas boundary in the phase diagram (3).

Fig. 17 Temperature distributions a at the onset of craze fibril breakdown and b during crack propagation for = 3000MPa in/s, when a temperature-dependent critical craze thickness is considered...

The cohesive surface formulation for crazing implemented within a thermomechanical framework provides insight into the heat generation during crack propagation, and indicates that the temperature-dependent critical craze thickness can result in an increase in toughness, but the marked rise reported in [60,62] probably has another origin. Here, we believe that dynamic effects need to be considered. 

Multiple dipping finally increases the maximum allowable thickness of the (calcined) crack-free membrane with respect to that obtainable in a single step [13,14,18]. At each given set of experimental conditions there is a certain critical layer thickness which should not be exceeded if cracking during drying and/or calcination is to be avoided. 

The experimental observation that there exists a critical thickness above which cracking occurs cannot easily be explained. Brinker [1] discusses a theory which explains that very thin layers can bear much larger stresses because critical cracks carmot be formed unless a certain critical thickness is surpassed. This thickness is estimated to be equal to or less than 1 pm and Brinker comes to the conclusion that thicker films will always crack. This is certainly not the case for alumina, titania and zirconia films for which much larger (alumina) to larger (titania) thicknesses are observed. As shown in Table 8.2 critical thicknesses of a few pm in single-step dip-coated films occur and critical flaws are smaller than this thickness and so can be present. 

Fig. 14.10. Critical film thickness for crack propagation in a (110)YBCO film on (110)SrTiO3 according to eq. (14.2). The fracture toughness Kf is 0.8 MPa /2 and Mis 102 GPa [14.42].

A notched beam of yttria-stabilized ZrOj containing a through-thickness surface crack is broken in four-point bending (mode I), beam depth 10 mm. The fracture stress was 100 MPa and the critical crack size (c) was 1 mm. The Young s modulus and Poisson s ratio of the ZrOj are 200 GPa and 0.30, respectively. 

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