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IMPLEMENTATION CHALLENGES FOR HIGH-TEMPERATURE COMPOSITES
FIGURE 15 Delamination caused by a through-thickness thermal gradient in a CMC. Also shown are the steady-state energy release rates. Here ∆To is the temperature difference across the CMC, and α is its thermal expansion coefficient.
ity would become a factor . The implications become apparent upon equating the monolith thickness, h, to the notch size, and the peak stress, σmax, to that from the combined residual and cyclic stresses. When h and σmax are small, the stresses on the fibers in the TMC never reach their UTS and the cracks extend benignly, only in the matrix. Conversely, at larger stresses, the outer fibers in the TMC fail. This causes the entire bridging zone to rupture, resulting in rapid crack acceleration. This fiber failure transition thus represents a design limit, or threshold. Life prediction methods require that the parameters governing the fatigue limit are manufacturing-insensitive and well-controlled.
For CMCs, there is no mechanism for cyclic growth of matrix cracks. A more important factor is the increase in tip, which causes cracks to extend upon reaching the matrix toughness [68,69]. However, this is a benign phenomenon, since matrix cracking is stable and, moreover, is the source of the inelastic strain that imparts stress redistribution capacity to the material.
High-temperature forms of stress corrosion with attendant embrittlement pose major materials development and life prediction challenges [73,74]. When one or more material constituent is susceptible to oxidation, a rapid degradation in performance may ensue, particularly when the process is accelerated by stress. The clearest illustration is oxidation embrittlement in nonoxide CMCs. In this case, oxidation decreases the fiber strength and increases the interface friction stress. These effects act synergistically to cause premature fiber failure at small cracks in the matrix. The consequence is rapid crack growth, with diminished crack “blunting,” resulting in low rupture life. The behavior has all of the characteristics exhibited by stress corrosion, with the active species now being oxygen. The phenomenon is illustrated in Figure 17 .
Matrix cracks created upon loading become pathways for the relatively rapid ingress of oxygen. The oxygen reacts to form solid and gaseous products. There is a threshold stress below which the phenomenon does not occur, given by