Thermally activated intermittent dynamics of creeping crack fronts along disordered interfaces

We present a subcritical fracture growth model, coupled with the elastic redistribution of the acting mechanical stress along rugous rupture fronts. We show the ability of this model to quantitatively reproduce the intermittent dynamics of cracks propagating along weak disordered interfaces. To this end, we assume that the fracture energy of such interfaces (in the sense of a critical energy release rate) follows a spatially correlated normal distribution. We compare various statistical features from the obtained fracture dynamics to that from cracks propagating in sintered polymethylmethacrylate (PMMA) interfaces. In previous works, it has been demonstrated that such an approach could reproduce the mean advance of fractures and their local front velocity distribution. Here, we go further by showing that the proposed model also quantitatively accounts for the complex self-affine scaling morphology of crack fronts and their temporal evolution, for the spatial and temporal correlations of the local velocity fields and for the avalanches size distribution of the intermittent growth dynamics. We thus provide new evidence that an Arrhenius-like subcritical growth is particularly suitable for the description of creeping cracks.

Thermal influences on macroscale rock damage

<p>Fracture processes in rock have widespread implications in the geohazard, geomorphologic, and civil and mining engineering communities.  Propagation of fractures reduces overall rock mass strength, can lead to large-scale gravitational instabilities, and can cause significant hazard and damage to infrastructure.  The potential for critical fracture in the form of rock falls and rock bursts are often the primary driver for scientific investigations, civil work project planning, and mining investment outlays.  However, slower subcritical fracture from long-term monotonic and/or cyclic stress perturbations often control the eventual more rapid (and more catastrophic) response of rock.  These slower damage mechanisms may result from existing or perturbed tectonic stresses, stress relief from exhumation or excavation, or long-term environmental stressors such as thermal cycling and frost cracking.</p><p>Here we investigate the role of thermal cycling in generating subcritical stresses to which virtually all rock cliffs worldwide are exposed.  Our hypothesis – that diurnal and seasonal cycles of temperature can lead to substantial subcritical fracture propagation and eventual critical fracture – has led us to design several field and laboratory experiments to measure both the deformations and the stresses associated with environmental thermal forcing in rock.  Our studies focus on granitic exfoliation environments, common in many mountainous regions of the world, where relatively thin (centimeters to decimeters) exfoliation sheets are able to undergo a full thickness thermal response, and where exfoliation-related rock falls are common and in some places, well-documented.</p><p>In cliff environments located in Yosemite National Park (California, USA), our field studies using in-situ measurements (i.e., crackmeters and temperature sensors) have shown that diurnal and seasonal thermal cycles lead to cyclic stresses in the subcritical range, with resultant cumulative and seemingly permanent rock deformation outwards from the main cliff surface.  Additional field studies using thermal IRT (InfraRed Thermography) imaging identify the locations of rock bridges that likely serve as focal points for these thermally-induced stress concentrations.  Although we did not measure the critical fracture conditions that would result in a rock fall, we did, fortuitously, capture the deformation signals leading up to explosive fracture of a nearby granitic 100-m-diameter exfoliation dome during peak temperatures at the site (located ~60 km northwest from Yosemite), thereby proving the efficacy of thermal stresses in driving both long term – and catastrophic – rock damage.  These field studies are substantiated by analytical fracture mechanics solutions which show how rock may eventually fail under these conditions.  These studies therefore serve as proxies for understanding how some rock falls eventually occur under subcritical thermally-induced cyclic stress conditions, but also more generally for how thermal-stress conditions may affect rock damage in a multitude of environments.</p>

Double torsion testing machine to determine the subcritical fracture index in rocks

This paper discusses the design methodology applied to build a testing machine to determine the sub-critical fracture index in a rock, based on double torsion testing in order to characterize naturally fractured formations such as those located in the Colombian Llanos Foothill Basin . These formations have been subjected to cyclic loads over time, causing fractures that trend to spread at sub-critical stress intensity values. Similarly, it presents the results of testing conducted on nine specimens of the Tambor Formation from 2 different outcrops to establish the testing traceability in the equipment.