Micromechanisms of subcritical crack growth in ceramic-matrix composites
- Stanford Univ., CA (United States)
- Rockwell Science Center, Thousand Oaks, CA (United States)
Significant advances have been achieved in increasing the strength and toughness of inherently brittle ceramic materials by reinforcing with brittle second phases. The versatile set of properties offered by ceramic composites such as low density, corrosion and wear resistance, and high temperature performance are currently being exploited in a wide range of applications as diverse as components in gas turbines, materials for chemical handling, cutting tools, and even bioprosthetic implants. Paradoxically, however, the micromechanisms employed to enhance toughness have in many instances inadvertently increased the susceptibility of advanced ceramics and their composites to other forms of longer term degradation, most particularly, those associated with mechanically-induced degradation from cyclic fatigue loading and degradation at elevated temperature from creep-assisted processes. The dominant source of toughening in a wide range of ceramic composites is derived from crack bridging. Bridging may arise from intact grains as, for example, in in-situ reinforced Si{sub 3}N{sub 4} ceramics, or from the incorporation of brittle reinforcement phases in ceramic composites. Over the last decade, significant advances have been made with regard to the mechanics description of bridging. Crack bridging effects are usually modeled as distributed tractions on the crack surfaces, p, that are a function of the crack opening displacement, u. Theoretically, many forms of p(u) can be assumed leading to a wide range of predicted toughening behavior. In this presentation, the microstructural mechanisms associated with subcritical crack growth, which determine the long term performance, are discussed. Emphasis is given to the role of cyclic loading in suppressing the salient toughening mechanism, for example, through progressive wear of sliding reinforcement/matrix interfaces, or through increasing the near-tip crack-driving force on the unloading cycle through crack wedging effects.
- OSTI ID:
- 175382
- Report Number(s):
- CONF-950686--
- Country of Publication:
- United States
- Language:
- English
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