Indentation crack growth and creep rupture of Si{sub 3}N{sub 4}
- National Institute of Standards and Technology, Gaithersburg, MD (United States)
Failure mechanism maps have been useful in representing the different regimes of failure for a number of high-temperature structural ceramics, by representing the allowable stress-temperature conditions with lines of constant lifetime. Instead of merely summarizing the data, however, they augment it by attempting to assign the microstructural origin of failure. Failure maps for silicon nitride usually show two different failure regimes at elevated temperatures. At higher temperatures and lower stresses the failure is usually attributed to creep-rupture, the accumulation of damage in the form of creep cavities leading to the nucleation and growth of a fatal flaw. The material shows some extension before failure. At lower temperatures and higher stresses, the failure is usually attributed to crack growth from pre-existing flaws. No strain is observed. A break in the constant lifetime curve separates the two regimes, with allowable stress decreasing rapidly with increasing temperature in the creep-rupture regime. The exact origin of the failure in the creep-rupture regime is still open to question, since the fracture surfaces clearly show that the material failed by the growth of a crack, though this crack may have been nucleated by the creep damage. By studying the growth of cracks produced by Vickers indentation during creep of silicon nitride, we hope shed light on the nature of failure in the creep-rupture regime. Since the creep of silicon nitride is strongly asymmetric in tension and compression, we focus on the growth cracks in dog-bone shaped tensile specimens, rather than flexure specimen. By using a precision density technique, we can accurately measure the volume of cavitation damage in the specimen and correlate that with the strain and crack growth rate. We will discuss the relationship between growth rate of indentation cracks and the creep rate of the material, as well as the origin of failure in the creep-rupture regime.
- OSTI ID:
- 175209
- Report Number(s):
- CONF-950686-; TRN: 95:006111-0130
- Resource Relation:
- Conference: Joint applied mechanics and materials summer meeting, Los Angeles, CA (United States), 28-30 Jun 1995; Other Information: PBD: 1995; Related Information: Is Part Of AMD - MD `95: Summer conference; PB: 520 p.
- Country of Publication:
- United States
- Language:
- English
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Creep and slow crack growth mechanisms related to macroscopic creep behaviour of a silicon nitride ceramic at elevated temperatures
Creep and slow crack growth mechanisms related to macroscopic creep behaviour of a silicon nitride ceramic at elevated temperatures