Influence of residual stresses on ceramic-metal interfacial toughness. Ph.D. Thesis
The critical property determining the extent to which ductile metal reinforcing particles can toughen a brittle ceramic matrix is the toughness of the ceramic-metal interface between particle and matrix. Unfortunately, due to the lack of understanding of the mechanics of bimaterial interfaces in the presence of residual stresses, experimental data on the fracture toughness of ceramic-metal interfaces is scarce. Consequently, the purpose of the present work is to account for the influence of residual stresses on the measured fracture toughness of a representative nickel/alumina system and, in conjunction with a maximum hoop stress criterion, to explain the observed increase in toughness with increasing mixity of loading. A finite element model incorporating the rate-dependent inelastic behavior of nickel at high temperatures was implemented in order to provide a realistic estimate of the residual stresses that develop during the bonding process. The calculated Mises equivalent stresses in the nickel averaged between 110 and 115 MPa, well above the room temperature yield strength of 82 MPa. For the sandwich specimens adopted in the current study, a simple argument yields a critical foil thickness below which residual stress effects are expected to be minimal. The influence of differing foil thicknesses below this threshold is shown to be insignificant. For sandwich specimens a general result is demonstrated: compared to the same loading applied without residual stresses present, the effect of residual stresses is to decrease the magnitude of the phase angle that will develop along the interface. Microprobe measurements verified that by controlling the bonding atmosphere the formation of a reaction product at the interface was prevented, and that the fracture mechanism was pure cleavage of the interface. The measured interfacial toughness values increased from 11 J/sq m at psi-caret approximately equals 0 deg to 22 J/sq m at psi-caret approximately equals +/-23 deg.
- Research Organization:
- Cornell Univ., Ithaca, NY (United States)
- OSTI ID:
- 147172
- Country of Publication:
- United States
- Language:
- English
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