The theory of stress corrosion for slow crack propagation is reviewed in the light of classical Griffith theory of fracture. Experimental data for stress corrosion cracking for glasses, ceramics, and metals are reviewed. We suggest that stress corrosion cracking plays an important role in the intrusion of magmas and in the transport of magmas upward through the lithosphere. It is shown that the effect of decreasing temperature (at progressively shallower levels along the geotherm) would be to decrease the crack velocity by several orders of magnitude if other factors were equal. We also propose that stress corrosion may be an important process in time-dependent earthquake phenomena such as premonitory behavior and earthquake aftershocks. We suggest that slow cracking in the earth is not seismically detectable but may nevertheless precede the terminal (catastrophic) phase of the fracture that is discerned as an earthquake. The seismically quiet periods before some earthquakes and the seismically quiet regions beneath some volcanoes may in fact be regimes of slow crack propagation. Slow crack propagation in a lithospheric plate may provide access routes for magmas which give rise to prominent linear volcanic chains.

Stress corrosion theory of crack propagation with applications to geophysics

Load relaxation and cross-head displacement rate-change experiments have been used to establish log10 stress intensity factor (K) versus log10 crack velocity (v) diagrams for double torsion specimens, of synthetic quartz cracked on thea plane in liquid water and moist air. For crack propagation normal toz and normal tor at 20°C,K
Ic (the critical stress intensity factor) was found to be 0.852±0.045 MN·m−3/2 and 1.002±0.048 MN·m−3/2, respectively. Subcritical crack growth at velocities from 10−3 m·s−1 to 10−9 m·s−1 at temperatures from 20°C to 80°C is believed to be facilitated by chemical reaction between the siloxane bonds of the quartz and the water or water vapour of the environment (stress corrosion). The slopes, of isotherms in theK-v diagrams are dependent upon crystallographic orientation. The isotherms have a slope of 12±0.6 for cracking normal tor and 19.9±1.7 for cracking normal toz. The activation enthalpy for crack propagation in the former orientation in liquid water at temperatures from 20°C to 80°C is 52.5±3.8 kJ·mole−1. A discussion is presented of the characteristics of theK-v diagrams for quartz.

A fracture mechanics study of subcritical tensile cracking of quartz in wet environments

Fracture toughness of Tennessee Sandstone and Carrara Marble using the double torsion testing method

Stress corrosion and the rate-dependent tensile failure of a fine-grained quartz rock

Fracture mechanics parameters of some rock-forming minerals determined using an indentation technique

Numerous theoretical calculations have indicated that the tensile strength of materials should lie close to ∿ 1/10 of the material’s elastic modulus, E (see Kelley(l) for a compendium). On the other hand, due to the presence of cracks and flaws, measured strengths lie between E/103 and E/102. By eliminating pre-existing cracks, tensile strengths can approach theoretical estimates. This has been demonstrated, for example, by Ernsberger (2) for the case of glasses.

F. F. Lange

Papers

Origin and Use of Fracture Mechanics