Abstract
The aim of this study is to characterise the changes in mechanical properties and to provide a comprehensive micro-structural analysis of Harcourt granite over different pre-heating temperatures under two cooling treatments (1) rapid and (2) slow cooling. A series of uniaxial compression tests was conducted to evaluate the mechanical properties of granite specimens subjected to pre-heating to temperatures ranging from 25–1000 °C under both cooling conditions. An acoustic emission (AE) system was incorporated to identify the fracture propagation stress thresholds. Furthermore, the effect of loading and unloading behaviour on the elastic properties of Harcourt granite was evaluated at two locations prior to failure: (1) crack initiation and (2) crack damage. Scanning electron microscopy (SEM) analyses were conducted on heat-treated thin rock slices to observe the crack/fracture patterns and to quantify the extent of micro-cracking during intense heating followed by cooling. The results revealed that the thermal field induced in the Harcourt granite pore structure during heating up to 100 °C followed by cooling causes cracks to close, resulting in increased mechanical characteristics, in particular, material stiffness and strength. Thereafter, a decline in mechanical properties occurs with the increase of pre-heating temperatures from 100 °C to 800 °C. However, the thermal deterioration under rapid cooling is much higher than that under slow cooling, because rapid cooling appears to produce a significant amount of micro-cracking due to the irreversible thermal shock induced. Multiple stages of loading and unloading prior to failure degrade the elastic properties of Harcourt granite due to the damage accumulated through the coalescence of micro-cracks induced during compression loading. However, this degradation is insignificant for pre-heating temperatures over 400 °C, since the specimens are already damaged due to excessive thermal deterioration. Moreover, unloading after crack initiation tends to cause insignificant irreversible strains, whereas significant permanent strains occur during unloading after crack damage, and this appears to increase with the increase of pre-heating temperature over 400 °C.
Highlights
The statistical review of world energy by the International Energy Agency (IEA) in 2017 shows that the global energy market is being affected by the emerging energy demand from developing countries, in Asia, and this resulted in an increase of 1% in global primary energy consumption in2016
Micro-cracking in thin slices subjected to heating followed by cooling treatments was observed from Scanning electron microscopy (SEM) imaging, and no propagation of thermal cracking was observed in slices pre-heated up to 200 ◦ C followed by either slow or rapid cooling
Significant micro-cracking was observed in the thin slices which were heated beyond 200 ◦ C followed by either rapid or slow cooling
Summary
The statistical review of world energy by the International Energy Agency (IEA) in 2017 shows that the global energy market is being affected by the emerging energy demand from developing countries, in Asia, and this resulted in an increase of 1% in global primary energy consumption in2016. The statistical review of world energy by the International Energy Agency (IEA) in 2017 shows that the global energy market is being affected by the emerging energy demand from developing countries, in Asia, and this resulted in an increase of 1% in global primary energy consumption in. Most Asian countries are increasing their reliance on geothermal energy, creating pathways to large-scale projects and addressing challenges in geothermal energy production [3,4,5,6], including a process of extracting heat via a fluid circulated through an underground fracture network (3–5 km deep) with high temperatures up to 400 ◦ C [5]. A number of studies have been conducted in the field of geothermal energy exploitation, and optimum conditions with enhanced energy extraction and reduced cost of investment were identified for geothermal reservoirs [7,8]
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