Probability of detection of internal voids in structural ceramics using microfocus radiography
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DonJ. Roth National Aeronautics and Space Administration, Lewis Research Center, Cleveland, Ohio 44135 (Received 30 January 1986; accepted 28 April 1986) The reliability of microfocus x radiography for detecting internal voids in structural ceramic test specimens was statistically evaluated. The microfocus system was operated in the projection mode using low x-ray photon energies (2 Before universal use of these materials is justified, methods for assessing and assuring their reliability need to be developed.3"7 Reliability of structural ceramics such as silicon nitride and silicon carbide requires fabrication process control to reduce the incidence of various flaws and to assure that any flaws that occur are small, few, and noncritical. Advanced nondestructive evaluation (NDE) techniques are needed to detect critical flaws and to assure that ceramic components operate reliably.6'8 Research is being conducted to identify and investigate appropriate NDE techniques for application to structural ceramics. When used at early stages of ceramic component fabrication, NDE techniques can help identify and avoid processing methods that lead to unreliable, nonuniform, seriously flawed ceramic parts.8"13 General porosity and distributions of discrete voids can cause wide strength variations and unacceptably low strength in monolithic ceramics. 14~17 Large voids or pores in green ceramics affect their ability to reach high density upon sintering. Therefore it is important to detect and characterize voids in structural ceramics. X radiography18'19 is particularly suitable for green ceramics because of its noncontacting and noninvasive nature. Further, x radiography is an effective NDE technique for as-fired sintered ceramics. The limitations of x radiography for detecting and characterizing internal voids in ceramics are currently a>
Abbreviated version presented at the Symposium on Defect Properties and Processing of High Technology Nonmetallic Materials sponsored by the Materials Research Society, Boston, Massachusetts, 2-6 December 1985.
J. Mater. Res. 1 (3), May/Jun 1986 http://journals.cambridge.org
not well defined. In addition, probability-of-detection (POD) statistics for critical size voids have not been established heretofore. Some prior work has investigated conventional and microfocus x radiography for evaluating structural ceramics. Kossowsky20 and Richerson et al.21 have reported limits of resolution for defects in hot-pressed and reaction-bonded silicon nitride. The sensitivity capabilities reported were of the order of 0.5% of thickness for high-density inclusions and of the order of 3% of thickness for clusters of voids. However, previous findings have not established POD statistics relative to any critically sized defects. A recent study at NASA Lewis Research Center9 established the reliability of detection of surface voids in structural ceramics by conventional and microfocus x radiography. It was reported that microfocus x radiography substantially increased the surface void detection capability. This pa
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