Subcritical crack growth measurement

Figure 4.33 shows the rate of slow, time-dependent subcritical crack growth, measured using the S5rmmetric four-point flexure specimen geometry, along the TiN-Si02 interface in the multilayer structure as a function... 

Both AC and DC potential-drop methods are well-established techniques for monitoring subcritical crack growth. A combined AC/DC potential-drop measuring technique can, in some cases, help in obtaining more information from a single test, in particular for the onset of stable crack growth.172... 

Experimental Details Measuring Subcritical Crack Growth... 

The techniques and test geometries that have been used to measure subcritical crack growth in ceramics are several, but they share a common principle, namely, the subjection of a well-defined crack to a well-defined stress intensity Ki, and a measurement of its velocity v. The technique considered here, the advantages of which are elaborated upon below, is the double torsion geometry shown in Fig. 12.8. For the double torsion specimen, K is given by... 

Hatton CG, Main IG, Meredith PG (1990) A comparison and structural measurements in scahng exponents during tensile subcritical crack growth. J Strac Geol, 15 1485-1495... 

Load-displacement data were monitored during many of the bend tests and fracture toughness tests, A high accuracy LVDT-type gage, calibrated to a maximum displacement of 25 to 50-microns, was used to measure displacement of the center of the bend bar with respect to the upper loading pins. Non-linearities were observed in the load-displacement records, particularly for the notched bend bars used in the fracture toughness tests. Such non-linearities may have been the result of subcritical crack growth. 

There is no consistent explanation for the generation of these features at particular values of. Nevertheless, the fact that a value of ATj can be inferred from a measurement of Rj is useful in analyzing failure. Mecholsky and Freiman have shown how to use these features to quantify the stress state in the part, including localized residual stresses, to determine whether subcritical crack growth occurred, and to quantify the severity of the flaw. 

The measured V — Ki diagrams of alumina and the composites prepared with the colloidal route and different volume fraction of unstabilized zirconia are plotted in Fig.l4 AI2O3, A7.5Z, AIOZ and A15Z correspond to nanocomposites with 0, 7.5, 10 and 15 vol % ZrOi respectively and A7.5Z-2Y is a composite with 7.5 vol % Zr02, stabilized by 2 mol % Y2O3. Subcritical crack growth laws were measured using the double torsion technique and the relaxation method. 

The two major drawbacks to the Kj c measurement through the use of iT-decreasing specimens are, as said in Sect. 15.5, the long waiting times due to incubation and the subcritical crack growth that may occur immediately during the... 

Results of subcritical crack growth analysis show that for hard metals it is not easy to find out the parameters n and A. The different strain rate method is probably not applicable for these materials because of a large scatter of strength values, Fig. 12. An approximate calculation based on these results gives a value of n cca 200. From results achieved in static bending tests on specimens with artificial cracks (Fig. 13) using equations (8) and (9) we have achieved for n a value of 180 and 60, resp. According to our opinion for determination of n and A the most suitable method is the lifetime measurement and the determination of the v vs. K relation... 

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