Ceramics crack growth rate

Early work (e.g., Refs. 44 and 45) on silicon nitride ceramics for a limited range of high temperature cyclic loading conditions led to the hypothesis that the mechanisms of cyclic and static fracture at elevated temperature are identical, and that the cyclic crack growth rates can be predicted on the basis of static fracture data. One of the techniques commonly used to derive cyclic crack growth rates solely on the basis of static load fracture data involves integration of the relationship in Eqn. (13) over the duration of the fatigue cycle such that... 

Figure 7.5 shows a comparison of the static and cyclic crack velocities for the alumina/SiC composite, similar to that illustrated in Fig. 7.2 for alumina. As seen for monolithic alumina, the ceramic composite undergoes slower rates of fracture when (1) fluctuations are introduced in the tensile loads and (2) the cyclic frequency is raised. The differences between static and cyclic crack growth rates are maximum at the lower Kmax levels. As in the case of unreinforced alumina, crack velocities estimated for cyclic loads on the basis of sustained load crack growth data (assuming identical failure mechanisms) are higher than those measured experimentally. 

In this chapter, we have sought to provide a state-of-the-art review of the mechanics and micromechanisms of high temperature crack growth in ceramics and discontinuously reinforced ceramic composites. Because of the limited amount of experimental data available in the literature which pertains primarily to oxide cermics and SiC reinforcements, the discussions of crack growth rates and fracture mechanisms have centered around alumina ceramics, with and without SiC reinforcements. However, the generality of the mechan-... 

Further development of theoretical models is limited by the lack of availability of information in ceramic composites pertaining to crack shape, crack growth rates, bridging, and process zone sizes associated with growing creep cracks in ceramic composites. 

FIGURE 18.11 (a) Log-log plot of crack growth rate versus K, which is defined in the inset, (b) Cyclic-fatigue crack growth rates for several ceramics and metal alloys as a function of applied stress intensity range, AK. 

The subcritical crack growth rate in silicon nitride materials is relatively low in comparison to other ceramic materials. The growth exponent is in the range of... 

Ceramics normally show about + 25% of the variance. Index [ ] Crack Growth Rate... 

Stress corrosion cracking (SCC) of ceramics is generally recognized as a type of subcritical crack growth. Relative humidity is an important factor in SCC crack growth for many ceramics. Water generally increases SCC crack growth rates. 

It is accepted that ceramics under load fail as a result of subcritical crack growth from preexisting flaws, and as shown in Sections 5.9.2 and 5.2 and Table 5.1 these are relatively easily generated. Hence ceramic lifetime predictions require evaluation of crack growth rates when subjected to an applied stress just as much as estimations of the initial flaw sizes. 

ASTM C 1368-06 Standard test method for determination of slow crack growth parameters of advanced ceramics by constant stress-rate flexural testing at ambient temperature. West Conshohocken ASTM International 2006. 

C1368-01 Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics by Constant Stress-Rate... 

S. R. Choi and J. P. Gyekenyesi, Specimen geometry effect on the determination of slow crack growth parameters of advanced ceramics in constant flexural stress-rate testing at elevated temperatures, Ceram. Eng. Sci. Proc., 20 [3] 525-534 (1999). 

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