Damage and Changes in Mechanical Properties of a Gabbro Thermally Loaded up to 1,000°C

Abstract

Thermal loading of rocks at high temperatures induces changes in their mechanical properties. In this study, a hard gabbro was tested in the laboratory. Specimens were slowly heated to a maximum temperature of 1,000°C. Subsequent to the thermal loading, specimens were subjected to uniaxial compression. A drastic decrease of both unconfined compressive strength and elastic moduli was observed. The thermal damage of the rock was also highlighted by measuring elastic wave velocities and by monitoring acoustic emissions during testing. The micromechanisms of rock degradation were investigated by analysis of thin sections after each stage of thermal loading. It was found that there is a critical temperature above which drastic changes in mechanical properties occur. Indeed, below a temperature of 600°C, microcracks start developing due to a difference in the thermal expansion coefficients of the crystals. At higher temperatures (above 600°C), oxidation of Fe 2? and Mg 2? , as well as bursting of fluid inclusions, are the principal causes of damage. Such mechanical degradation may have dramatic consequences for many geoengineering structures.

Physical property relationships of the Rotokawa Andesite, a significant geothermal reservoir rock in the Taupo Volcanic Zone, New Zealand

“…We see a clear correlation of crack area per unit volume (Sv) to the observed compressional wave velocities ( Figure 8D) and interpret this to be attenuation of the compressional wave through the cracked intracrystalline and intercrystalline boundaries that are abundant in the andesite (e.g., Figures 3 and 4). Many authors (e.g., Vinciguerra et al 2005;Keshavarz et al 2010;Blake et al 2012; Heap et al 2014) have also shown that the elastic wave velocities can be highly attenuated by the presence of microcracks.…”

“…The development of microcracks during uniaxial compression, and the coalescence of these cracks (newly formed and pre-existing), leads to the failure of the sample (Brace et al 1966;Bieniawski 1967). In samples that already show relatively high crack densities, less energy is required to coalesce existing cracks and thus they are inherently weaker (David et al 1999;Ferrero and Marini 2001;Keshavarz et al 2010). By utilizing AE monitoring during our UCS testing, we observe that fewer events occur during uniaxial compression in weaker samples than those with higher strength (Figure 10), indicating that there are far more pre-existing cracks in the weaker samples (Hardy 1981;Eberhardt et al 1998;Nicksiar and Martin 2012).…”

Investigations of Thermal Damage on the Physical and Mechanical Properties of Gneiss Rock Specimen

“…Many experimental studies have been conducted on different rocks, [1][2][3][4][5][6][7][8][9][10][11][12][13][14] with an objective to observe and explain the behavior of physical and mechanical properties of these rocks under high temperature or after thermal treatment. All these experimental works are focus on sandstone [4][5][6][7], granite [1,[8][9][10], carbonates [2] and gabbro [11].…”

“…All these experimental works are focus on sandstone [4][5][6][7], granite [1,[8][9][10], carbonates [2] and gabbro [11]. However, few studies are interested on the gneiss rock microscopic analysis of thin section of the heated specimen.…”

A Phenomenological Model of Brittle Rocks under Uniaxial Compression

“…1, the stress-strain curve of this part is usually non-linear (concave). In particular, when a specimen experiences a high degree of thermal loading, the initial stress-strain curve will be strongly non-linear (Rosengren and Jaeger 1968, Homand-Etienne and Houpert 1989, Mahmutoglu 1998, Du et al 2004, Keshavarz et al 2010. Because the nonlinearity is associated with Fig.…”