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The micromechanisms of tensile fracture are reviewed, with particular emphasis on the influence of chemical effects on fracture controlled by pre-existing cracks (stress corrosion). A fracture mechanism map for quartz is constructed using a combination of theoretical insights and experimental data. The manner in which stress corrosion will modify the predictions of fracture mechanism maps is discussed by reviewing the numerous theories of stress corrosion. Experimental data are presented on stress corrosion in tensile deformation of quartz, quartz rocks, calcite rocks, basaltic rocks, granitic rocks and other geological materials. Although the experimental evidence for stress corrosion is overwhelming, very few data were obtained under conditions that simulate those in the bulk of the earth's crust and so the extent of its geophysical significance is yet to be fully established. Examples are given, however, of how invoking stress corrosion as a rate-controlling deformation mechanism sheds new light on extremely diverse geophysical phenomena, such as: predicting the strength and sliding friction properties of rocks, modelling earthquake rupture, the stability of hot, dry rock geothermal reservoirs, stimulation of oil and gas reservoirs, the crack-seal mechanism of rock deformation and low stress dilatancy, fracture mechanics of lunar rocks, magmatic intrusions and the relaxation of internal stresses in rock. apparatus are designed to be used in measurements of crack propagation parameters for mode I deformation (tensile). Acoustic emission can be monitored simultaneously with other parameters relevant to the test. 2. Additional apparatus development has been done to enable the measurement of mode II (in-plane shear) crack propagation parameters. Two methods have been chosen: (a) one based on a double direct shear specimen for tests at ambient pressure, and (b) one based on a modification to the analysis of post-failure stress/ displacement data from triaxial tests suggested by Rice (Proc. Int. School of Physics 'Enrico Fermi 1 , LXXVIII, 1979). 3. Double torsion experiments to study stress corrosion and acoustic emission of Whin Sill dolerite gave the following results for n (stress corrosion index, V = «K,. n (event rate index, dN /dt = 6KT E ), and nR (ringdSwn rate index, dNR/dt = yK-^R). Crack velocity is V, stress intensity factor is Kj, «, 6 and y are constants, NR and NE are number of ring-down counts and events, respectively, and t is time. n "E nR Air, 20 C, 30%RH 31.2 (0.990) 31.1 (0.984) 32.9 (0.981) H20, 20WC 29.0 (0.992) 29.1 (0.977) 29.9 (0.973) H20, 75UC 28.4 (0.995) Figures in brackets are correlation coefficients. The activation enthalpy for crack propagation determined by two different methods gave the following results: 30.4±1.9 kJ.mole" 1 and 34 to 47.6 kJ.mole~l. Kj c for this dolerite was 3.28±0.1 MN.nT 3/2. As we have found for other materials the rate of acoustic emission is a good guide to the crack velocity. 4. Short rod tests have been run on a variety of rock types at 20°C and ambient humidity to check out the validity of this test for rock and to get an idea of Kjc values for materials on which no data existed before this study. Some results are given here. Where possible they are compared with results for double torsion experiments. -3/2 KIc (MN.m /Z ) Material SR DT Westerley granite 1.62±.08 1.74 Whin Sill dolerite 2.96±.19 3.28 Black gabbro 2.73±.40 2.88 Pink granite 1.53±.17 1.66 Icelandic Tholeiite 0.87±.06 Serpentinized dunite 1.39±.38 Arkansas novaculite 1.77±.25 1.34 Oughtibridge gannister 1.39±.27 Penant sandstone 1.97±.06 Tennessee sandstone 0.79±.05 0.45 Carrara marble 0.82±.04 0.64 Solnhofen limestone 1.09±.06 1.06 5. Acoustic emission was monitored from thermal and stress cycled Westerley granite. Also stress intensity factor /crack velocity diagrams were determined for heat treated granite. On increasing the maximum temperature of heat treatment the microcrack density increases and K_ decreases. The most marked change in these properties occurs between 200QC and 30QOC. During stress cycling the Kaiser effect is only observed up to a specific fraction of K, , thereafter there is an anomalous increase in the acoustic emission that suggests the release of locked-in, residual strain energy. With increasing heat-treatment there is a reduction in the stress intensity factor required to obtain a given crack velocity. 6. Preliminary estimates of critical strain energy release rate in mode II deformation for granite are of the order 10 4J.m~2. 7. A fracture mechanism map for quartz has been constructed. It may be inferred from this diagram that the propagation of pre-existing cracks by stress corrosion will be the most important mechanism of tensile failure in the upper 15-20 km of the earth's crust. 8. A study of the influence of pore water on the fracture and sliding friction strength of Westerley granite shows that at 20oc the presence of water has little effect, i.e. the so-called Rehbinder effects are not very important. The following results were obtained from stress relaxation experiments on dry and wet, intact and initially prefaulted specimens at 300QC and 400°C and under a pore water pressure of 200 bars or 1 kbar at a fixed effective confining pressure of 1.5 kbar. (a) Dry granite shows no reduction in sliding stress at strain rates down to 10~12s~l. (b) The sliding stress on wetting is reduced at strain rates below ca. 10~"7s""l, but not by as much as Tennessee sandstone or Mojave quartzite. (c) Increasing pore fluid pressure at constant effective pressure substantially increases the rate of stress relaxation. (d) Values of the stress exponent, n, where strain rate * (stress) n are as follows: PH20 n 200 bars 25 1000 bars 6 (e) The activation enthalpy for frictional sliding of wet specimens of Westerley granite from 300°C to 40QOC, varied from 20 45 kJ.mole"!. These results do not support a model in which the rate of sliding of wet specimens is controlled by pressure solution. An alternative model based on stress corrosion has been developed which is a more satisfactory fit to these data. Similar work is now under way on a Tholeiitic basalt. Additionally, textural studies of specimens deformed in constant strain rate mode to total strains of ca. 3-5% are being performed. Reports ATKINSON, B.K. 1980. An outline proposal of some aims, strategies and objectives in earthquake prediction. In Proceedings 2nd Workshop on European Earthquake Prediction Programme jointly organised by European Space Agency and Parliamentary Assembly of the Council of Europe, Strasbourg, 1980, 135-155. ATKINSON, B.K. 1980. Fracture Mechanics modelling of earthquake generating processes. In Proceedings of an Interdisciplinary Conference on Earthquake Prediction Research in the N. Anatolian Fault Zone, Istanbul, 1980 (in press) ATKINSON, B.K. and Avdis, V. 1980. Fracture Mechanics parameters of some rock-forming minerals determined with an indentation technique. Int. J. Rock Mech. Min. Sci. and Geomech. Abstr. (in press). Norton, M.G. and ATKINSON, B.K. 1980. Stress-dependent morphological features on fracture surfaces of quartz and glass. Tectonophysics (in press). ATKINSON, B.K. 1980. Review of subcritical crack propagation in rock. Proc. 26th International Geological Congress, Paris, 1980. (To be published in J. Struct. Geol.) ATKINSON, B.K. and Rawlings, R.D. 1980. Acoustic emission during stress corrosion cracking in rocks. Proc. 3rd Maurice Ewing Symposium on Earthquake Prediction, New York, 1980. Geophysical Union (in press). ATKINSON, B.K. and Meredith, P.G. 1980. Stress corrosion of quartz: Influence of chemical environment. Tectonophysics (in press). Meredith, P.G. and ATKINSON, B.K. 1980. Stress corrosion and acoustic emission of Whin Sill dolerite (in preparation) Dennis, S.M. and ATKINSON, B.K. 1980. The influence of pore fluids on the sliding of faulted surfaces of Westerley granite under simulated geologic environments (in prep.) Dennis, P.F. and ATKINSON, B.K. 1980. Flow and fracture deformation mechanism maps for quartz (in preparation) ATKINSON, B.K. 1981. Earthquake precursors. Physics in Technology 12. ATKINSON, B.K. 1980. How to take the shock out of earthquakes. The Guardian, 25 September. |
