Microstructural damage accumulation

Recent experiments by Dr. Bar-Cohen el al. ha e shown that ultrasonic oblique insonification can be used to characterize thermal damage to composites [156]. Using an inversion technique based on a micromechanical model, the reflected ultrasonic signals arc analyzed to determine the overall laminate stiffness constant before and after loading. Another technique developed by the NASA to encompass the limitation of pulse-echo ultrasonic and photomicroscopic methods is diffuse-field acoustoultrasonic coupled vibration damping [157]. Both NASA techniques are complementary and arc used to assess microstructural damage accumulation in ceramic matrix composites. 

Fig. 8.13 Microstructural damage during creep of Al203/SiC composite at 1400°C in air. Cavitation occurs within glass phase accumulated at a GBI junction. Glassy ligaments bridge the separated interface.27...

The mechanisms responsible for fracture in structural ceramics at elevated temperatures have been reviewed [154]. Sensitivity to flaws or microstructural inhomogeneities which nucleate microcracks are among the failure mechanisms. The flaws which control failure under creep conditions are different from those responsible for fast fracture at room temperature. A common feature is the development of cracks through gradual damage accumulation, depend on the microstructure. The role of cracks in the deformation and fracture behavior of polycrystalline structural ceramics have been reviewed [155]. 

Cavitation is the formation of cavities. This phenomenon has been found to take place in ceramics containing a glass phase. The final creep fracture in this case results from the accumulation of cavities. The factors controlling this kind of creep are the microstructure, volume of glass phase, temperature, and applied stress. These factors give rise to bulk and localized damage. 

---