CONSTRAINING THE PHYSICS OF SUBCRITICAL CRACK GROWTH IN CARRARA MARBLE USING NEUTRON DIFFRACTION TECHNIQUES

In order to understand the physics of subcritical crack growth in metamorphic rock it is important that we better resolve uncertainties regarding the means by which external subcritical stress magnitudes combine with internal residual elastic strains to gradually degrade rock strength. Here we present two experiments utilizing neutron diffraction techniques to evaluate changes in intergranular residual strain magnitudes as a result of subcritical loads applied to a near-pure Carrara marble (CaCO₃ < 98%vol). Firstly, we evaluate the change in intergranular strains in unfailed wet and dry sample in an inverted single edge notch three-point bending creep test (iSENB). In a second experiment, we explore the variations in residual strain in-situ during staged Brazilian tests.

Results from the first experiment indicate that the initial contractional strain state is reduced in magnitude along cracks developed during iSENB tests, while extensional residual strains develop at and ahead of the crack tips. These additional residual strains can reduce both physical, and chemical barriers to crack growth ahead of the crack tip.

During the Brazilian tests we observed contractional strains at loads of just ~10-15% of the ultimate strength both along (x-direction), and perpendicular (y-direction) to, the loading axis. Strain in y-direction changes to an extensional state upon unloading from ~10-15%. Subsequent loading to ~30%, ~50% and ~75% generates additional contractional strains in x-direction, while strains in y-direction become extensional with external loads above ~50%. The unloaded residual strains remain at ~100µstrain in reference to the untested sample state.

These two experiments demonstrate that a) extensional residual strains accumulate ahead of propagating fracture, b) strain is relieved by cracks, and c) strains accumulate at stress magnitudes well below the assumed crack initiation limit. These results provide first insights into the interaction of subcritical crack growth and residual strains in metamorphic rocks. Along with a testing program aimed at evaluating the effect of water on subcritical fracture propagation, these tests provide important controls on the physics of crack propagation in intact bedrock on natural rock slopes.