In order to provide a life prediction methodology, three principal phenomena must be addressed: (i) the reduced strength of the reinforcements with an oxide reaction layer, (ii) the stress concentration on the reinforcements at the perimeters of unbridged crack segments, and (iii) the oxygen concentration within the matrix cracks, which is coupled with the thickness of the oxide reaction product on the reinforcements. Analysis of these effects leads to an expression for the failure time, ∆tc [74]:
The features emphasized by (15) are the explicit role of the applied stress and the friction stress as well as the fiber strength. But the major issues are related to the chemistry, which determines the reference time to. The basic kinetic factors are currently unresolved.
The crack growth rates found in composites reinforced with available SiC-based fibers have been inexplicably high, especially at intermediate temperatures, such as 500 to 800°C. This problem is related to the fiber chemistry. Improved chemistry diminishes their sensitivity to degradation but does not eliminate the effect. It has yet to be established that the kinetic limitation of stress oxidation in nonoxide CMCs can be an effective life-enhancing strategy. This difficulty does not exist in all-oxide composites, suggesting that these materials be given preference in oxidizing situations. The life assurance problem then becomes management of creep and rupture. The creep susceptibility of the available polycrystalline oxide fibers limits oxide CMCs to temperatures below 1200°C. Exploitation of some newly discovered effects would be needed to overcome this limitation. These effects include creep enhancements enabled by yttria doping or by nanoparticle incorporation.
CONCLUDING COMMENTS
An implementation strategy for TMCs is well-developed. It recognizes that these materials provide stiffness benefits when used as unidirectional reinforcements in selected areas of stiffness-critical components. Redesigns that enhance performance while minimizing the amount of TMC address cost objectives. However, more widespread implementation is limited by two problems. (i) The interior transverse properties constrain the design by requiring minimal transverse loads in the TMC sections. (ii) The use of SiC fibers limits higher-temperature applications because of thermal fatigue, caused by the thermal expansion mismatch. New developments in fibers and fiber coatings are needed to obviate these limitations. There are also some concerns brought about by the notch sensitivity of TMCs. In particular, fatigue cracks that penetrate the TMC from the surrounding Ti alloy result in severe LCF degradation. There are no obvious solutions to this problem, because the notch sensitivity is inherent to the anisotropy. It must be taken into account in the design and the life prediction methodology.
The implementation of CMCs is dictated by material shortcomings as well as manufacturing cost issues, having commonality with polymer matrix composites (PMCs). Delamination at transitions and around hot spots presents major challenges. Unlike PMCs, the feasibility of suppressing delamination by substantially enhancing the matrix toughness and by “eliminating” manufacturing flaws is restricted by processing requirements for the matrix. The only reasonable approach appears to be the use of braiding or cross-stitching. This technology has yet to be demonstrated because of the problems associated with the weaving of stiff ceramic fibers.
There are additional materials problems. For nonoxide CMCs, it has yet to be demonstrated that stress oxidation can be suppressed to an extent that allows acceptable thermomechanical fatigue life. Fiber –fiber coating combinations having chemistries that inhibit stress-enhanced oxidation are needed.
