Microstructural damage

Influence of Creep Rate Mismatch Ratio on Microstructural Damage Mode... 

Fig. 5.7 Macroscopic damage modes that occur during the tensile and flexural creep of fiber-reinforced ceramics. It is assumed that matrix or fiber damage is avoided during initial application of the creep load (see discussion of loading rate effects in the next section). Periodic fiber fracture can occur if the creep rate of the matrix exceeds that of the fibers. Periodic matrix fracture is common when the matrix has a higher creep resistance than the fibers. In this figure, it is assumed that initial microstructural damage is avoided during application of the creep load.

Fig. 5.9 Influence of initial loading rate on fiber and matrix stress and microstructural damage mode in materials where CMR< 1. Under rapid loading, the matrix stress may achieve a level sufficient to initiate matrix fracture.

There are a number of indicators of fatigue damage that have attracted interest in the literature. During the life of a component subjected to fatigue, the material can exhibit changes in modulus, permanent offset strain, shape of the hysteresis loops, and temperature rise of the specimen surface. Direct evidence of matrix crack density can be obtained by surface replication, while a more detailed analysis of microstructural damage requires scanning electron microscopy (SEM). 

What are the changes that occur in the microstructure of a material in the first cycle The earlier discussion of the microstructural damage that occurs during monotonic loading (Section 6.2.2) provides the insight needed to answer... 

The model proposed by Rouby and Reynaud46 represents the first systematic approach for understanding how the microstructural damage governs the fatigue life of continuous fiber-reinforced ceramic matrix composites. This model will be used to explain various aspects of fatigue failure in the remaining portion of this chapter. 

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...

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. 

Figure 23. Microstructural damage in the surface region of the Si3N4 materials with Y2O3/AI2O3 (3 in Table 4) after lOOOh (A) and Y2O3 (5 in Table 4) after 2500h (B) oxidation at 1500°C.

Therefore, characterizing the macromechanical properties of particulate polymer composites without studying the microstructural damage mechanisms and their effects on macromechanical properties is not possible. 

Figure 1-6. Idealized creep curve and corresponding microstructural damage.

Two aspects related to failure are observed in fatigue crack growth at the tips in materials. The first, which promotes crack advance, is the microstructural damage... 

Transient stresses due to differential sintering between the inclusions and the matrix may arise, and if large enough, they can reduce the densification rate and cause microstructural damage. 

---