Plain Language Summary
When rocks deform, tiny cracks appear, increasing in size and number
until the rock breaks completely, often along a narrow plane of weakness
where cracks have spontaneously aligned. Sometimes, when the
microstructure is complicated, cracking accelerates quickly in a
predictable way, giving a good indication of when the rock will break.
In other cases, when the microstructure is more uniform, cracking
accelerates more slowly and the rock breaks suddenly and early. To
understand why failure is predictable in some cases but not others – a
major problem in managing risk from material failure (e.g., earthquakes)
– we used x-ray imaging to see how cracks form and interact with each
other inside deforming rocks. We found that predictable behavior only
arose when cracks aligned themselves asymmetrically. The orientation of
this damage zone was governed by the rock’s pre-existing microstructure.
We also found distinct changes in crack size and spatial arrangement
during alignment, indicating that the rock was approaching failure.
However, when cracks did not align asymmetrically, similar changes were
not observed and failure was not predictable. Our results are important
because they help explain why reliable indicators of catastrophic
failure are not always observed and why forecasting may only work in
certain cases.