Brittle Fracture of Ceramics

At room temperature, both crystalline and noncrystalline ceramics almost always fracture before any plastic deformation can occur in response to an applied tensile load. The topics of brittle fracture and fracture mechanics, as discussed previously in Sections 8.4 and 8.5, also relate to the fracture of ceramic materials they will be reviewed briefly in this context. 

The measme of a ceramic material s ability to resist fractme when a crack is present is specified in terms of fractme toughness. The plane strain fractme toughness Kj as discussed in Section 8.5, is defined according to the expression 

For compressive stresses, there is no stress amplification associated with any existent flaws. For this reason, brittle ceramics display much higher strengths in compression than in tension (on the order of a factor of 10), and they are generally used when load conditions are compressive. Also, the fracture strength of a brittle ceramic may be enhanced dramatically by imposing residual compressive stresses at its surface. One way this may be accomplished is by thermal tempering (see Section 13.10). 

Statistical theories have been developed that in conjunction with experimental data are used to determine the risk of fracture for a given material a discussion of these is beyond the scope of the present treatment. However, because of the dispersion in the measured fracture strengths of brittle ceramic materials, average values and factors of safety as discussed in Sections 6.11 and 6.12 typically are not used for design purposes. 

It is sometimes necessary to acquire information regarding the cause of a ceramic fracture so that measures may be taken to reduce the likelihood of future incidents. A failure analysis normally focuses on determination of the location, type, and source of the 

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Mecholsky [86] has proposed an equation of this sort to represent the brittle fracture of ceramics it would be of interest to investigate its applicability to the fracture of adhesive bonds. 

Such features are very common in brittle fracture of ceramic materials the fracture origin is a critical flaw which behaves like a crack the crack path is perpendicular to... 

The general principles involved in the failure analysis of ceramic materials are similar to the failure analysis of metals and alloys. Because of the brittle behavior of ceramic materials, failure may result in many pieces of the sample, which have to be reassembled in order to obtain information on the form of loading and the point of fracture initiation. The utmost care should be exercised in the reassembly of the fractured pieces so that the features of the fracture are preserved. 

ANTHONY G. EVANS is the Gordon McKay Professor of Materials Engineering at Harvard University. His research interests include the mechanical properties of brittle materials particularly the fracture of ceramics under conditions of impact thermal and mechanical stress and failure prediction based on nondestructive evaluation. He is a recipient of the American Ceramic Society s Ross ColEn Purdy Award and has authored and co-authored several publications. Dr. Evans is a member of the National Materials Advisory Board and has served on several National Research Council committees. Dr. Evans was elected to the National Academy of Engineering in 1997 for his contributions to the development and understanding of structural materials. 

For obvious reasons, all structures should be designed to avoid brittle fracture of any kind however, under particular conditions, such fracture may occur in practically all materials. For metals or plastics, if brittle fracture is conditioned by low temperature, fatigue, rate of loading, etc., it is a natural and common way of fracture for several kinds of matrices used in building and civil engineering materials. Matrices based on various cements and all kinds of ceramic materials are considered as brittle. Brittleness is the principal disadvantage, which should be controlled in all structural and even non-structural applications of these materials. 

The major drawback of ceramics is their intrinsic brittleness. For example, most metals have a fracture toughness forty times greater than... 

So why aren t today s engines made of ceramics The short answer is that, unlike metals, ceramics cannot bend and deform to absorb impacts. Intense research is currently under way to solve the problem of ceramic brittleness, with some success. Improved resistance to fracturing, for example, can be attained by careful quality control of starting materials and processing. As we shall see in the next section, brittleness can also be combated by compositing ceramics with other materials. 

The program also addressed the need to develop lough ceramic-matrix composites (CMCs) with much greater resistance to brittle fracture. Early in the program, researchers round that the chemical structure that imparts superior thermal and mechanical properties to ceramics also results in negative altribuies. panicularly of brittleness, which easily can lead 10 catastrophic failure. 

Kuebler, J., Fracture toughness of ceramics using the SEVNB method from a preliminary study to a standard test method, in Fracture Resistance Testing of Monolithic and Composite Brittle Materials, ASTM STP 1409, ed. J.A. Salem, M.G. Jenkins and G.D. Quinn, ASTM, West Conshohocken, PA, pp. 93-106, January 2002. 

Hencke, H., Thomas, J.R., Hasselman, D.P.H. (1984), Role of material properties in the thermal stress fracture of brittle ceramics subjected to conductive heat transfer , J. Am. Ceram. Soc., 67, 393-398. 

Brittle fracture is accompanied by very little plastic deformation and is predominant in ceramics and inorganic material. Mostly, a very rapid propagation of the crack is observed. 

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