For many structural ceramic components in service, their use is often limited by lifetimes that are controlled by a process of slow crack growth. This test method provides the empirical parameters for appraising the relative slow crack growth susceptibility of ceramic materials under specified environments at elevated temperatures. This test method is similar to Test Method C 1368
Note 38212;Data generated by this test method do not necessarily correspond to crack velocities that may be encountered in service conditions. The use of data generated by this test method for design purposes may entail considerable extrapolation and loss of accuracy.
In this test method, the flexural stress computation is based on simple beam theory, with the assumptions that the material is isotropic and homogeneous, the moduli of elasticity in tension and compression are identical, and the material is linearly elastic. The average grain size should be no greater than one fiftieth (1/50) of the beam thickness.
In this test method, the test specimen sizes and test fixtures were chosen in accordance with Test Method C 1211
In this test method, the slow crack growth parameters (n and D) are determined based on the mathematical relationship between flexural strength and applied stress rate, log σf = [1/(n + 1)] log ˙σ + log D, together with the measured experimental data. The basic underlying assumption on the derivation of this relationship is that slow crack growth is governed by an empirical power-law crack velocity, v = A[KI/KIC]n (see Appendix X1).
Note 48212;There are various other forms of crack velocity laws which are usually more complex or less convenient mathematically, or both, but may be physically more realistic (9). The mathematical analysis in this test method does not cover such alternative crack velocity formulations.
In this test method, the mathematical relationship between flexural strength and stress rate was derived based on the assumption that the slow crack growth parameter is at least n ≥ 5 (3, 10). Therefore, if a material exhibits a very high susceptibility to slow crack growth, that is, n < 5, special care should be taken when interpreting the results.
The mathematical analysis of test results according to the method in 4.4 assumes that the material displays no rising R-curve behavior, that is, no increasing fracture resistance (or crack-extension resistance) with increasing crack length. It should be......<......
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