The strength of many advanced ceramics reflects the flaws present in the material and the material's intrinsic fracture toughness. Monolithic ceramics and some composite ceramics (i.e., particulate- or whisker-reinforced) will fail in a brittle fashion when stressed due to unstable propagation of cracks from preexisting flaws.
The terms "flaw" and "fracture origin" are used interchangeably in this context. Scientists and engineers in the ceramics community traditionally have used these terms to describe the fracture initiation site in ceramics and other brittle materials. These terms are used in the context of fracture mechanics whereby a singularity or microstructural irregularity acts as a stress raiser from which fracture commences. It should be understood that the use of the term "flaw" does not imply that a ceramic product has been prepared improperly or is somehow defective.
Strength-test results must be interpreted in the context of these flaws, whether the strength testing has been done for quality control, materials development, or design purposes. Fractographic observations should be coupled directly to strength results. Fractographic montages and labelled Weibull graphs are an exceptionally versatile means of accomplishing this. See ASTM C1239 for more details on Weibull analysis of strength data.
American Society for Testing and Materials (ASTM) developed C1322 "Standard Practice for Fractography and Characterization of Fracture Origins in Advanced Ceramics". This practice provides an efficient and consistent methodology to locate and characterize fracture origins in advanced ceramics. The table below outlines the fracture origin characterization scheme and the linked figures show examples of the most common types of flaws observed in advanced ceramic materials. (See Fracture Origin Characterization and the Flaw Catalog.)
Any questions or comments about ASTM C1322, the fracture origin examples given here, or fractography of ceramics can be addressed to Jeffrey Swab (email@example.com) or George Quinn (firstname.lastname@example.org).
A new guide to NIST Recommended Practice Guide for Fractography of Ceramics and Glasses includes a wealth of information on fracture origins and fractography in general.
Fracture Origin Characterization and the Flaw Catalog
The fracture origin in each specimen/component shall be characterized by the following three attributes: identity, location, and size as summarized in the table below.
Please note that the descriptive terms "volume" and "surface" may have two distinctly different uses.
On one hand, the terms refer to the inherent distribution of the origin in the material. For example, pores are inherently volume-distributed origins, whereas pits or machining damage are inherently surface-distributed origins.
On the other hand, the terms may refer to the specific location of a single origin in any particular specimen or component.
For example, a pore (which is a volume-distributed origin) could be located at the surface of a particular specimen or component. A volume-distributed origin type may be in the volume (bulk), at the surface, near the surface, or at an edge. See the figure in section "Origin-Location" for clarification.
Surface-distributed origins (e.g. machining damage or pits), on the other hand, can only be found at the surface or at an edge of a specimen or component.
The word "surface" may further be differentiated. It may apply to the exterior of a test specimen cut from a bulk ceramic or component, or alternatively, the original surface of the specimen or component in the as-fired state.
To further explore the three attributes of a fracture origin, click on IDENTITY, LOCATION, and SIZE.
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