Mechanisms of crack propagation

Usually, ceramics always contain cracks of different sizes with different orientations. The strength of the ceramic is determined by the cracks with the lowest failure strength. Under tensile loads, cracks can, depending on their orientation, be loaded in all modes, I, II, or III (cf. section 5.1.1), under compressive loads only in mode II or III, for the stress component perpendicular to the crack surface closes the crack. Because the fracture toughness is much smaller for mode I than for modes II or III, ceramics under tensile loads usually fail in this mode and are thus more sensitive to tensile than to compressive stresses. The compressive strength of ceramics is usually 10 to 15 times larger than its tensile strength.  

The fracture toughness is primarily determined by the strength of the chemical bonds within the ceramic because this determines the energy needed to create fresh surface. Beyond that, other effects within ceramics can occur that impede crack propagation because they require additional energy and thus increase the fracture toughness. The basic mechanisms are discussed in this section in section 7.5, we will see how they can be utilised to strengthen ceramics. 

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The use of a fracture mechanics approach based on the strain energy release rate to assess failure in rubber-cord laminated structures is reviewed. The mechanics of crack propagation are considered for cracking either between the plies or around individual cords, and also for crack initiation and growth near cord ends. The ability of the approach to predict the effects of various design and construction parameters on laminate failure is also discussed. 9 refs. 

Figure 6 Schematic illustration of the elements of the film induced cleavage mechanism of crack propagation [1], Note similarity to the slip dissolution model (Fig. 5) during initial stages of propagation cycle.

As observed in the above discussions, it appears that there is not a single but two or three different mechanisms which operate. The mechanism of crack propagation falls into two basic categories, the dissolution model and the mechanical fracture model. 

The mechanisms of crack propagation in poly(methyl methacrylate) eure particularly amenable to analysis. The situation is not so simple for other polymers. For example, in polystyrene there is usually multiple crazing in the vicinity of the crack tip. In tougher polymers such as polycarbonate shear yielding as well as crazing often takes place at the crack tip. In these cases crack propagation does not occur in such a well-controlled manner as in poly (methyl methacrylate) and it is more difficult to analyse. 

Describe the mechanism of crack propagation for both ductile and brittle modes of fracture. 

Schematic illustration of the elements of the film-induced cleavage mechanism of crack propagation. (From Ford, F.P., Mechanisms of environmental cracking peculiar to the power generation industry. Report NP2589, EPRI, Palo Alto, CA, September 1982 Ford, F.P., The crack tip system it s relevance to the prediction of cracking in aqueous environments, in Proceedings of First International Conference on FnvironmentaSly Assisted Cracking of Metals, Kohler, WI, October 2-7,1988, Eds. R. Gangloff and B. Ives, Published byNACE,pp. 139-165.)...

Based on the arguments made above for unirradiated stainless steels, it was hypothesized that the slip-oxidation mechanism of crack propagation was a relevant cracking model for irradiated stainless steels and that the basic Equations 18.6 and 18.10 apply, but with modifications to account for the effect of irradiation on the inputs to that model. Thus, the relevant system parameters that might be affected by irradiation and which are important to the slip-oxidation mechanism of crack propagation are... 

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