Laboratory measurement of subcritical crack growth and healing in calcite using Double-Torsion tests

Subcritical crack growth accelerates weathering of rocks and minerals, reduces the strength of rock slope, and affects the stability of subsurface faults. Under special circumstances, the produced cracks can also heal spontaneously (Self healing) regaining a part of the original tensile strength.  These crack behaviors are a manifestation of molecular-scale surface forces acting between the surfaces near the crack tip. As a part of the effort to understand how these forces control the subcritical crack growth and healing of geological materials, we examine the tensile crack behavior of calcite single crystals. A miniature Double-Torsion (DT) test system was developed for testing small plate samples (40 mm x 20 mm x 1.5 mm) cut out of optical-quality calcite single crystals (Iceland Spar crystals). These samples are oriented in such a way that the induced crack is along the (1014) plane (the primary cleavage plane). The main output of the experiment is the crack velocity (vc) vs the magnitude of applied driving force (stress intensity factor K or strain energy release rate G), which is a typical way to summarize the rate-dependent crack behavior. From the experiment, we have learned that (1) calcite exhibits strong healing behavior compared to materials such as glass or (amorphous) quartz in humid air and water, (2) healing is time dependent (the strength of a healed crack increases over time), (3) liquid water (rather than vapor) introduces strong hysteresis in the recracking vs healing behavior.   The obtained laboratory data are used to develop a mechanistic model for predicting macroscale crack behavior in rock, particularly in a water and electrolyte-rich environment.