Standard Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics by Constant Stress Flexural Testing (Stress Rupture) at Elevated Temperatures

Importancia y uso:

4.1 The service life of many structural ceramic components is often limited by the subcritical growth of cracks over time, under stress at a defined temperature, and in a defined chemical environment (Refs 1-3). When one or more cracks grow to a critical size, brittle catastrophic failure may occur in the component. Slow crack growth in ceramics is commonly accelerated at elevated temperatures. 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 elevated temperatures as a function of time, temperature, and environment. This test method is based on Test Method C1576 with the addition of provisions for elevated temperature testing.

4.2 This test method is also used to determine the influences of processing variables and composition on slow crack growth at elevated temperatures, 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, time-to-failure, and limited design data generation purposes.

Note 2: 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, depending on the range and magnitude of applied stresses used, may entail extrapolation and uncertainty.

4.4 This test method and Test Method C1576 are similar and related to Test Methods C1368 and C1465; however, C1368 and C1465 use constant stress-rates (linearly increasing stress over time) to determine corresponding flexural strengths, whereas this test method and C1576 employ a constant stress (fixed stress levels over time) to determine corresponding times-to-failure. In general, the data generated by this test method may be more representative of actual service conditions as compared with data from constant stress-rate testing. However, in terms of test time, 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 elastic beam theory, with the following assumptions: the material is isotropic and homogeneous; the moduli of elasticity in tension and compression are identical; and the material is linearly elastic. These assumptions are based on small grain size in the ceramic specimens. The grain size should be no greater than ¹/₅₀ of the beam depth as measured by the mean linear intercept method (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 test specimen sizes and test fixtures have been selected in accordance with Test Method C1211 which provides a balance between practical configurations and resulting errors, as discussed in Refs 4 and 5. Test Method C1211 also specifies fixture material requirements for elevated test temperature stability and functionality.

4.7 The SCG data are evaluated by regression of log applied-stress vs. log time-to-failure to the experimental data. 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 3: A variety of crack velocity functions exist in the literature. A comparison of the functions for the prediction of long-term constant stress (static fatigue) data from short-term constant stress rate (dynamic fatigue) data (Ref 6) indicates that the exponential forms better predict the data than the power-law form. Further, the exponential form has a theoretical basis (Refs 7-10); however, the power law form is simpler mathematically. Both forms have been shown to fit short-term test data well.

4.8 The approach used in this test method assumes that the ceramic material displays no rising R-curve behavior, that is, no increasing fracture resistance (or crack-extension resistance) with increasing crack length for a given test temperature. The existence of such R-curve behavior cannot be determined from this test method. The analysis further assumes that the same flaw type controls all times-to-failure for a given test temperature.

4.9 Slow crack growth behavior of ceramic materials can vary as a function of material properties, thermal conditions, 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 compared from one investigation to another on a valid basis, or serve as a valid basis for characterizing materials and assessing structural behavior.

4.10 Like mechanical strength, the SCG time-to-failure of advanced ceramics is probabilistic in nature. Therefore, slow crack growth that is determined from times-to-failure under given constant applied stresses is also a probabilistic phenomenon. 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 (Refs 1, 11-13; see Appendix X2). Hence, a proper range and number of constant applied stress levels, in conjunction with an appropriate number of test specimens, are required for statistical reproducibility and reliable design data generation (Ref 1-3). This test method provides guidance in this regard.

4.11 The time-to-failure of a ceramic material for a given test specimen and test fixture configuration is dependent on the ceramic material’s inherent resistance to fracture, the presence of flaws, the applied stress, and the temperature and environmental effects. 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 (Refs 14, 15), and fractography is recommended in this test method (refer to Practice C1322).

Subcomité:

C28.01

Volúmen:

15.01

Número ICS:

81.060.30 (Advanced ceramics)

Palabras clave:

advanced ceramics; constant stress testing; elevated temperature; flexural testing; four-point flexure; slow crack growth; slow crack growth parameters; time-to-failure;