ASTM C1465-08

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 with the exception that provisions for testing at elevated temperatures are given. Furthermore, this test method may establish the influences of processing variables and composition on slow crack growth as well as on strength behavior of newly developed or existing materials, thus allowing tailoring and optimizing material processing for further modification. In summary, this test method may be used for material development, quality control, characterization, and limited design data generation purposes.

Note 3—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, which provides a balance between practical configurations and resulting errors, as discussed in Refs (7, 8). Only the four-point test configuration is used in this test method.

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 σ_f = [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[K_I/K_IC]ⁿ (see Appendix X1).

Note 4—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 noted that the existence of such behavior cannot be determined from this test method. The analysis further assumes that the same flaw types control strength over the entire test range. That is, no new flaws are created, and the flaws that control the strength at the highest stress rate control the strength at the lowest stress rate.

Slow crack growth behavior of ceramic materials can vary as a function of mechanical, material, thermal, and environmental variables. Therefore, it is essential that test results accurately reflect the effects of specific variables under study. Only then can data be compared from one investigation to another on a valid basis, or serve as a valid basis for characterizing materials and assessing structural behavior.

The strength of advanced ceramics is probabilistic in nature. Therefore, slow crack growth that is determined from the flexural strengths of a ceramic material is also a probabilistic phenomenon. Hence, a proper range and number of test rates in conjunction with an appropriate number of specimens at each test rate are required for statistical reproducibility and design (4). Guidance is provided in this test method.

Note 5—For a given ceramic material/environment system, the SCG parameter n is independent of specimen size although its reproducibility is dependent on the variables previously mentioned. By contrast, the SCG parameter D depends significantly on strength, and thus on specimen size (see Eq X1.7).

The elevated-temperature strength of a ceramic material for a given test specimen and test fixture configuration is dependent on its inherent resistance to fracture, the presence of flaws, test rate, and environmental effects. Analysis of a fracture surface, fractography, though beyond the scope of this test method, is highly recommended for all purposes, especially to verify the mechanism(s) associated with failure (refer to Practice C 1322).