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The use of controlled surface flaws to examine fracture properties of brittle materials is reviewed. Surface flaws are introduced by microhardness indentation. Such flaws are semi-elliptical in shape, with dimensions dictated by the indentation load.
A single flaw of proper size placed on the tensile surface of a conventional four-point bend specimen initiates fracture because it is the “worst flaw” in the specimen. Presence of the flaw reduces both the fracture strength and its associated scatter. Because controlled surface flaw dimensions are generally visible on the fracture surface, the critical stress intensity factor, KIc , may be calculated using fracture mechanics analyses for surface flaws in bending.
Such a fracture mechanics test is experimentally convenient, adaptable to testing in severe environments, and requires a relatively simple test specimen geometry. Also of importance is the fact that the fracture flaw dimensions are close to those of the natural flaws in the material. For silicon nitride (Si3N4) and silicon carbide (SiC) the technique has been employed to determine KIc at room and elevated temperatures, as well as under dynamic loading conditions. It has also been used to study slow crack growth, environmental effects, and mixed-mode fracture in these materials. Disadvantages of the controlled surface flaw technique as well as potential new applications are discussed.
controlled surface flaws, microhardness indentation precracking, semi-elliptical surface crack, brittle materials, fracture (materials), fracture toughness, bend test, crack propagation
Materials scientist, Los Alamos Scientific Laboratory, Los Alamos, N. Mex.
Materials scientist, Systems Research Laboratories, Inc., Dayton, Ohio