Electric discharge

Source: Office of Technology Assessment.

Source: Office of Technology Assessment.

developments have been the focus of recent topical conferences [11, 12], and the reader is referred to these excellent sources. It will not be the intent of this chapter to thoroughly review the historical developments of the several NDE technologies, but it is instructive to look at some of the primary developments and closely related technologies, such as high-speed, large-capacity desk-top computers. Figure 13.1 shows schematically one historical perspective on the recent development of ceramic materials together with one perspective on parallel developments in NDE technology. To be noted is the fact that composite ceramic materials really were not available until very recently—after 1990. It is essential to highlight the personal impact computers have had on the development of NDE because this has allowed developments related to sensors, digital signal processing, and image processing.

Prior to the early 1970s, there was little worldwide development activity in the area of structural ceramics and certainly very little research directed toward NDE for ceramics. A great deal of the very earliest efforts directed toward NDE developments for ceramic materials for gas turbines occurred in the late 1970s and the early 1980s in Germany [13]. Efforts began in the late 1970s in the United States [14], and subsequently in the early 1980s efforts began in Japan [15]. It is interesting to note that in the summary of their 1983 report, Goebbels and Reiter [13] noted the following, which was directed toward monolithic materials:

TABLE 13.2 Fracture Toughness and Critical Flaw Sizes of Monolithic and Composite Ceramic Materials Compared with Metals"

Fracture Toughness Critical Flaw Size

Conventional microstructure
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