Abstract
Thermo-mechanical loading can occur in numerous engineering geological environments, from both natural and anthropogenic sources. Different minerals and micro-defects in rock cause heterogeneity at a grain scale, affecting the mechanical and thermal properties of the material. Changes in strength and stiffness can occur from exposure to elevated temperatures, with the accumulation of localised stresses resulting in thermally induced micro-cracking within the rock. In this study we investigated thermal micro-cracking at a grain scale through both laboratory experiments and their numerical simulations. We performed laboratory triaxial experiments on specimens of fine-grained sandstone at a confining pressure of 5 MPa and room temperature (20^{circ }hbox {C}), as well as heating to 50^{circ }hbox {C}, 75^{circ }hbox {C} and 100^{circ }hbox {C} prior to mechanical loading. The laboratory experiments were then replicated using discrete element method simulations. The geometry and granular structure of the sandstone was replicated using a Voronoi tessellation scheme to produce a grain based model. Strength and stiffness properties of the Voronoi contacts were calibrated to the laboratory specimens. Grain scale thermal properties were applied to the grain based models according to mineral percentages obtained from quantitative X-ray diffraction analysis on laboratory specimens. Thermo-mechanically coupled modelling was then undertaken to reproduce the thermal loading rates used in the laboratory, before applying a mechanical load in the models until failure. Laboratory results show a reduction of up to 15% peak strength with increasing thermal loading between room temperature and 100^{circ }hbox {C}, and micro-structural analysis shows the development of thermally induced micro-cracking in laboratory specimens. The mechanical numerical simulations calibrate well with the laboratory results, and introducing coupled thermal loading to the simulations shows the development of localised stresses within the models, leading to the formation of thermally induced micro-cracks and strength reduction upon mechanical loading.
Highlights
Coupled thermo-mechanical processes present engineering challenges in a number of geological environments
For the thermal loading of all specimens, a lag is observed in the first 1000 s between the heating command at 1.5C minÀ1 and the heating of the specimen measured from the thermocouple within the triaxial pressure vessel
The crack initiation (CI) and crack damage (CD) thresholds are seen to reduce with increasing thermal loading, proportionally with the peak strength, confirming that the weakening is occurring prior to mechanical loading due to thermal micro-cracking during the thermal loading phase of the experiments
Summary
Coupled thermo-mechanical processes present engineering challenges in a number of geological environments. Research into the thermo-mechanical loading of rock began in the 1970s, with studies focussed on the Earth’s crustal behaviour These early studies primarily investigated crustal lithologies such as granite, gabbro, dolerite and rhyolite and found that thermally loading specimens from room temperature up to temperatures as high as 800C induced micro-cracking and caused irrecoverable thermal expansion. This thermal micro-cracking was attributed to result from localised stress concentrations induced by the thermal expansion heterogeneity of adjacent minerals with different thermal expansion coefficients, and occurred in the absence of a thermal gradient across a rock specimen (Richter and Simmons 1977). Adding mechanical and physical measurements to testing protocols showed that thermally induced micro-cracking was found to be significant enough to affect Young’s modulus (Simmons and Cooper 1978; Bruner 1979; Heard and Page 1982), ultrasonic velocities (Johnson et al 1978), seismic attenuation (Johnson et al 1978; Clark et al 1981), fracture toughness (Meredith and Atkinson 1985), permeability (Bauer and Johnson 1979) and produce
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