Creep Rupture in Ceramics

In Fig. 6.94, a combination of flexural and tensile tests was used thus, two transitions in creep failure behavior are observed. At high stress (above about 175 MPa in flexure, performed by 4-point test), failure occurs very rapidly (52 h) and at low strain (51 %, not shown). Thus, failure occurs before steady-state creep is established. For a given stress, the scatter in time-to-failure is large (2 orders of magnitude). Thus, it is difficult to determine a stress exponent for stress rupture. However, it is quite high, about 40. At stresses below 175 MPa in flexure a transition occurs. The failure strain is much greater (12-16 % for AVCO 18 % for ARCO) and the stress exponent is between 2 and 3. 

The failure of ceramic polycrystals may generally be related to preexisting flaws (or in cases of high-temperature failure, to flaws generated during service). 

Creep life is limited by the rate of crack propagation. Critical crack size development depends on the stress dependence of the failure time. The time-to-failure at stresses under tension below a certain limit (82 MPa for alumina) is a consequence of an increase in failtrre strain at the crack tip. Basically, failure is 

40 MPa, 1250 °C [93]. With kind permission of John Wiley and Sons 

Developing appropriate ceramic—matrix composites [henceforth CMCs] for possible aero-engine applications started more than a decade ago. CMCs continue to be developed for improved toughness, to overcome the inherently brittle nature of most of the monolithic ceramics. Many experiments have shown that long-term loading of CMCs (for thousands of hours) produce improved high-temperature properties in monolithic ceramics by causing the dispersion of ceramic whiskers or 

---

The emphasis to this point has been on steady-state creep but clearly information is also needed on creep rupture. For ceramics, an empirical approach suggested by Monkman and Grant is often used (see Wiederhorn, 1992). In this approach the failure time is given as a power function of the steady-state creep rate e, i.e.,... 

In Chap. 6, creep was discussed in general, but a considerable portion of the chapter was devoted to creep fracture and creep rupture, so these will not be reviewed here. However, since various additives do influence the basic properties of materials (e.g., second phases, fibers, whiskers), to complete this discussion on creep phenomena in ceramics, an example follows of the effect of whiskers in a... 

Ohji T (1994) Tensile Creep Rupture And Subcritical Crack Growth of Silicon Nitride. In Hoffmann MJ, Petzow G (eds) Tailoring of Mechanical Properties of Si3N4 Ceramics. Kluwer Academic Publishers, Netherlands, p 339... 

In this paper, the importance of particle and whisker reinforcement to creep and creep rupture behavior of ceramics is discussed. Particle and whisker additions generally increase both the fracture toughness and creep resistance of structural ceramics. These additions also act as nucleation sites for cavities. Cavities form preferentially in tensile specimens. This results in a creep asymmetry, in which composites creep faster in tension than in compression. As a consequence of cavitation, the stress exponent for creep in tension 6-10,... 

T.-J. Chuang, D. F. Carroll, and S. M. Wiederhom Creep Rupture of a Metal-Ceramic Particulate Composite, Seventh International Conference on Fracture, in Advances in Fracture Research, Vol. 4, eds., K. Salama, K. Ravi-Chandler, D. M. R. Taplin, and P. Rama Rao, Pergamon Press, New York, NY, 1989, pp. 2965-2976. 

S. M. Wiederhom, B. J. Hockey, and T.-J. Chuang, Creep and Creep Rupture of Structural Ceramics, in Toughening Mechanisms in Quasi-Brittle Materials, ed. S. P. Shah, Kluwer Academic Publishers, The Netherlands, 1991, pp. 555-576. 

Fundamental questions about factors that control the creep rates of ceramic materials have not been answered. The effects of carbon and solid solution dopants on the creep rate of SiC materials need to be better understood. The role of intragranular stacking faults on P-SiC creep rates should also be determined. Furthermore, a determination must be made as to whether the microstructure of a-SiC is intrinsically more creep resistant than the microstrueture of p-SiC. For oxide ceramics, the role of microstructure in controlling creep rate and creep rupture strength must he determined, partieularly for multiphase microstructures. 

Tressler, R.E., and J.A. DiCarlo. 1995. Creep and rupture of advanced ceramic fiber reinforcements. Pp. 141-155 in High Temperature Ceramic Matrix Composites 1, Design, Durability, and Performance. Vol. 57 in Ceramic Transactions, A.G. Evans and R. Naslain (eds.). Westerville, Ohio American Ceramic Society. 

Yun, H.M., J.C. Goldsby, and J.A. DiCarlo. 1994. Tensile creep and stress-rupture behavior of polymer derived SiC fibers. Pp. 17-28 in Advances in Ceramic-Matrix Composites 11. Vol. 46 in Ceramic Transactions, J.P. Singh and N.P. Bansal (eds.). Westerville, Ohio ... 

TABLE 3. 1000-hr upper use-temperatures for SiC ceramic fibers as estimated from single fiber creep-rupture results in air (and argon)... 

Table II. Summary of creep-rupture results for the N720/A ceramic composite at various temperatures in laboratory air and in steam. Results at 1200 C from Ruggles-Wrenn et al ...

To improve the performance of SiC and to increase its resistance against creep failure, generally various constituents are added to monolithic SiC ceramics. Additives in various shapes and sizes are usually added to SiC to achieve a better material for structural use and to extend its service lifetime. An evaluation of creep failure, commonly referred to as creep rupture or stress rupture , is a critical step in evaluating the suitability of a certain ceramic for use in the desired application. The stress rupture and creep properties of a SiC matrix reinforced with SiC fiber (i.e., a SiC/SiC composite) has been evaluated by tests conducted in order to assess the propensity of SiC/SiC for high-temperature appfications over an extended lifetime. In Fig. 6.105, plots of stress versus time-to-rupture are shown for several temperatures. As commonly done, these plots are on a log-log scale. Each curve can be fitted by means of an empirical relation, similar to the earlier exponential equation expressing the time-to-rupture, h, to a stress exponent for stress rupture as ... 

C. E. Smith, G. N. Morscher, and Z. H. Xia, Monitoring Damage Accumulation in Ceramic Matrix Composites Using Electrical Resistivity, Scripta Materialia, 59 [4] 463-466 (2008). Smith, C. E., Morscher, G. N. and Xia, Z., Electrical Resistance as a Nondestructive Evaluation Technique for SiC/SiC Ceramic Matrix Composites Under Creep-Rupture Loading, International Journal of Applied Ceramic Technology, 8 298-307 (2011). 

---