A review is presented of the experimental data on subcritical crack growth in geological materials. The main parameters describing subcritical crack growth are the critical stress intensity factor Kc, the subcritical crack growth limit Ko, and the stress intensity factor-crack velocity (K-v) relationship between Ko and Kc. The K-v data are presented in terms of an equation in which the crack velocity depends on stress intensity factor raised to a power n because this is common practice in experimental studies. These data are presented as tables and in synoptic diagrams. For silicates the value of n increases as the environment becomes depleted in hyroxyl species and with increase in the microstructural complexity of the solid. Values of n as low as 9.5 have been found for tensile cracking of quartz in basic environments and as high as 170 for tensile cracking of basalt in moist air. Insufficient experimental data are available to predict subcritical crack growth behavior at depth in the earth's crust without major extrapolations of the data base. Schematic outlines are presented, therefore, of the probable influence on subcritical crack growth of some key parameters in the crustal environment. These include stress intensity factor, temperature, pressure, activity of corrosive environmental agent, microstructure, and residual strains. In addition, a discussion is presented of the likely magnitude of the subcritical crack growth limit. For stress corrosion tensile crack growth of quartz a limit of approximately 0.2 of the critical stress intensity factor is inferred from theoretical calculations. Further problems discussed with regard to the extrapolation of experimental data to crustal conditions include the choice of a suitable equation to describe crack growth and the magnitude of parameters in these equations. A brief discussion of the double torsion testing method is presented in order to aid the interpretation of experimental results because it is almost the sole method used to study subcritical cracking in rocks.

Subcritical crack growth in geological materials

11 – experimental fracture mechanics data for rocks and minerals

Analysis of fracture initiation angle in some cracked ceramics using the generalized maximum tangential stress criterion

The double-torsion testing technique for fracture toughness and slow crack growth determination has been critically reviewed. The analytical compliance and finite element stress analyses of the double-torsion test specimen are summarized. The fracture toughness and crack growth testing procedure using this test configuration is described along with the applicable relationships. The strengths and limitations of this testing technique vis-a-vis other standardized techniques have been critically evaluated. While the double-torsion test method has some limiting features it has been demonstrated that its applicability is not limited as long as these are addressed correctly. Recommendations for conducting double-torsion experiments have been provided and potential avenues for improvement of this test method have been identified. It is concluded based on the review that standardization of this test method is required in order to make it more practicable.

The double-torsion testing technique for determination of fracture toughness and slow crack growth behavior of materials: A review

Mixed mode fracture resistance was analyzed for some previously tested ceramics using a three-parameter fracture criterion called the generalized maximum tangential stress (GMTS) criterion. For using this fracture criterion, the size of a critical distance from the crack tip was required. Therefore, an analytical model was suggested for determining the size of critical distance in terms of mode I fracture toughness and the tensile strength of ceramics. For some ceramics with reported grain size, the size of critical distance was found to be approximately 100 times the average grain size. The experimental results obtained from the cracked Brazilian disk specimen and reported in the literature for sialon, mullite, SiC, glass, sintered carbide, porcelain, zirconia, and alumina could be estimated very well when the GMTS fracture criterion was used.

A. Fakhr

Papers

The fracture toughness testing of ceramics and acoustic emission