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Fracture Mechanics of Functionally Graded Materials.
Final rept. 1 Apr 93-31 Aug 96,
LEHIGH UNIV BETHLEHEM PA
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Functionally graded materials are generally two-phase composites with continuously varying volume fractions. Used as coatings and interfacial zones, they help to reduce mechanically and thermally induced stresses caused by the material property mismatch and to improve the bonding strength. In this project some basic problems concerning fracture mechanics of graded materials are identified, general analytical methods for solving the related crack problems are developed, the singular behavior of the solutions for typical material nonhomogeneities is examined, and solutions of some benchmark problems are obtained. The results are intended to provide technical support for material scientists and engineers who are trying to develop methods for processing these materials and for design engineers who are interested in using them in technological applications. Typical applications of functionally graded materials include thermal barrier coatings of high temperature components in gas turbines, surface hardening for tribological protection, and as interlayers in microelectronic and optoelectronic components. The results found show that by eliminating the discontinuities in material property distributions the mathematical anomalies regarding the crack tip stress oscillations for the interface cracks and the nonsquare root singularities for cracks intersecting the interfaces are also eliminated. From the viewpoint of fracture mechanics the importance of this result lies in the fact that in analyzing the components involving functionally graded materials one can use the existing crack tip finite element modeling developed for ordinary square root singularities and apply the energy balance based theories of conventional fracture mechanics.
APPROVED FOR PUBLIC RELEASE