134 Characteristics Of Interest To Composite Ceramics

Ceramic matrix composite materials [42-44] present different challenges to nondestructive evaluation technologies. The complex microstructure of these materials makes definition of a "flaw" rather uncertain. The significant differences in microstructure between monolithic ceramics and composite ceramics can be seen by looking at some of the various fiber architectures that are available for composites. Figure 13.23 shows diagrams of some of the various fiber architectures available. Ko [45] has presented the details of these fiber architectures in great detail, and the reader is referred to this excellent reference for information. As opposed to monolithic ceramic materials, composites exhibit what is commonly called "graceful failure" [46]. A typical stress-strain curve for an SiC/SiC ceramic composite is shown in Fig. 13.24. The significant differences in microstructure and mechanical properties between monolithic and composites changes the approaches used for NDE to detect "flaws." Characteristics of

FIGURE 13.23 Schematic diagram of fiber architectures used for continuous fiber-reinforced ceramic matrix composites [63].

Strain

FIGURE 13.24 Typical stress-strain diagram for a two-dimensional cloth lay-up, melt-infiltrated SiC/SiC composite at room temperature.

Strain

FIGURE 13.24 Typical stress-strain diagram for a two-dimensional cloth lay-up, melt-infiltrated SiC/SiC composite at room temperature.

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s mc composites that might be classified as a flaw include: delaminations, regions of open porosity, seams at plys or through-thickness cracks and damage zones caused by factors such as foreign object damage (FOD) or accelerated oxidation. Flaws in composites might better be described as "flaw regions." The development of nondestructive evaluation methods to detect these distributed flaw regions is described in the following subsections.

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