4.1 The service life of many advanced ceramic components is often limited by the subcritical growth (SCG) of cracks over time. In SCG conditions, small subcritical cracks in the ceramic may grow over time under stress in a defined chemical environment at a given temperature (1-3). When one or more cracks grow to a critical size, brittle, catastrophic failure may occur in the component. This test method provides a procedure for measuring the long-term load-carrying ability and appraising the relative slow crack growth susceptibility of ceramic materials at ambient temperatures as a function of time and environment.

4.2 This test method is also used to determine 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.

4.3 This test method may be used for material development, quality control, characterization, design code or model verification, and limited design data generation purposes.

Note 4: Data generated by this test method may not necessarily correspond to crack growth velocities that may be encountered in service conditions. The use of data generated by this test method for design purposes, depending on the range and magnitude of applied stresses used, may entail extrapolation and uncertainty.

4.4 Test Method C1576 is related to Test Method C1368 ("constant stress-rate flexural testing"). Test Method C1576 employs a series of tests with different constant stress levels to determine corresponding times-to-failure. Test Method C1368 uses a series of tests with different continuously increasing stress levels (at a defined constant rate of increase) to determine times-to-failure. In general, the data generated by the constant stress test may be more representative of actual service conditions as compared with data from constant stress-rate testing. In contrast, constant stress testing is inherently and significantly more time-consuming than constant stress-rate testing.

4.5 The flexural stress computation in this test method is based on simple linear elastic beam theory, with the assumptions that the material is linearly elastic with no creep deformation, is isotropic and homogeneous, and the moduli of elasticity in tension and compression are identical.

4.5.1 The grain size should be no greater than one fiftieth (¹/₅₀ ) of the beam depth as measured by the mean linear intercept method (Test Methods E112). In cases where the material grain size is bimodal or the grain size distribution is wide, the limit should apply to the larger grains.

4.6 The flexure test specimen sizes and test fixtures have been selected to correspond with Test Methods C1161 and C1368, which provides a balance between practical configurations and resulting errors, as discussed in Refs. (4, 5).

4.7 The SCG test data are evaluated by regression analysis of log applied stress (S) versus log time-to-failure (t_f) for the experimental data to produce a linear SCG curve. The recommendation is to determine the slow crack growth parameters by applying the power law crack velocity function. For derivation of this, and for alternative crack velocity functions, see Appendix X1.

Note 5: A variety of crack velocity functions exist in the literature. A comparison of the functions for the prediction of long-term constant stress SCG (static fatigue) data from short-term SCG constant stress rate (dynamic fatigue) data (6) indicates that the exponential forms better predict the data than the power-law form. Further, the exponential form has a theoretical basis (7-10), while the power law form is simpler mathematically. Both have been shown to fit short-term test data well.

4.8 The approach used in this method assumes that the material displays no rising R-curve behavior, that is, no increasing fracture resistance (or crack-extension resistance) with increasing crack length. The existence of R-curve behavior cannot be determined from this test method.

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

4.10 Like mechanical strength, the slow-crack growth phenomenon and the time-to-failure of advanced ceramics are probabilistic in nature. The scatter in time-to-failure in constant stress testing is much greater than the scatter in strength in constant stress-rate (or any strength) testing (1, 11-13); see Appendix X2. Hence, a proper range and number of constant applied stresses, in conjunction with an appropriate number of test specimens, are required for statistical reproducibility and reliable design data generation (1-3). This test method provides guidance in this regard (8.1.1).

4.11 The time-to-failure of a ceramic material for a given test specimen and test fixture configuration is dependent on the ceramic's inherent resistance to fracture and the population, initial size, and character of the cracks/flaws. Fractographic analysis to verify the failure mechanisms has proven to be a valuable tool in the analysis of SCG data to verify that the same flaw type is dominant over the entire test range (14, 15), and fractography is recommended in this test method (refer to Practice C1322).