Comparison of Mechanical Behavior and Acoustic Emission Characteristics of Three Thermally-Damaged Rocks

Abstract

High temperature treatment has a significant influence on the mechanical behavior and the associated microcracking characteristic of rocks. A good understanding of the thermal damage effects on rock behavior is helpful for design and stability evaluation of engineering structures in the geothermal field. This paper studies the mechanical behavior and the acoustic emission (AE) characteristic of three typical rocks (i.e., sedimentary, metamorphic, and igneous), with an emphasis on how the difference in rock type (i.e., porosity and mineralogical composition) affects the rock behavior in response to thermal damage. Compression tests are carried out on rock specimens which are thermally damaged and AE monitoring is conducted during the compression tests. The mechanical properties including P-wave velocity, compressive strength, and Young’s modulus for the three rocks are found to generally show a decreasing trend as the temperature applied to the rock increases. However, these mechanical properties for quartz sandstone first increase to a certain extent and then decrease as the treatment temperature increases, which is mainly attributed to the high porosity of quartz sandstone. The results obtained from stress–strain curve, failure mode, and AE characteristic also show that the failure of quartz-rich rock (i.e., quartz sandstone and granite) is more brittle when compared with that of calcite-rich rock (i.e., marble). However, the ductility is enhanced to some extent as the treatment temperature increases for all the three examined rocks. Due to high brittleness of quartz sandstone and granite, more AE activities can be detected during loading and the recorded AE activities mostly accumulate when the stress approaches the peak strength, which is quite different from the results of marble.

Mineral Composition, Pore Structure, and Mechanical Characteristics of Pyroxene Granite Exposed to Heat Treatments

“…It was collected from Yanzhou mining area in the Shandong Province, China (see Figure 1). The granite is a typical igneous rock formed in the process of magmatism, and its color is slightly darker, which indicates that it may be formed in the ground deep in the earth [30]. The collected rock blocks are approximately 500 mm long, 300 mm wide, and 200 mm high.…”

“…The P-wave velocity in rock highly depends on its mineral composition and pore structure characteristics, and also can quantitatively reflects the thermally induced microcrack damage [30]. Variations of P-wave velocities of granite rocks before and after heating at different treatment temperatures are presented in Figure 12.…”

“…Almost all of the above studies have shown that thermal damage and microcracks are induced by high temperature and the rock porosity and permeability gradually increase with temperature, while the pore fractal dimension decreases. In terms of mechanical properties, most laboratory tests focus on stress-strain relationships [30], strength characteristics [9,17,19,20,[31][32][33] (such as uniaxial compressive strength (UCS), tensile strength, confined compressive strength, etc. ), deformation characteristics [17,19,[34][35][36][37] (such as elastic modulus, Poisson's ratio, peak strain, shear-slip strain, brittle stress drop coefficient, etc.…”

Scale Effect on the Anisotropy of Acoustic Emission in Coal

“…ey are primary parameters on the AE activities in both laboratory and filed work. e variation of AE counts and energy dissipation provides the precursor information of various failures during the loading process in coal and rock [41,42], and the microseismic event and energy dissipation are still the primary parameters in the dynamic disaster preventing (coal bump and rock burst) [43][44][45] and the analysis of earthquake and various microseismic activities [46,47]. e fractal features are naturally existing in the laboratory AE experiments and the field microseismic activities [12,34,35].…”

“…e RA (the rise time over the amplitude), AF (the ratio of threshold crossings over the duration of the signal), frequency, amplitude, and signal duration are also the acoustic emission parameters. Analyzing these parameters allows researchers to understand the energy release (amplitude and frequency) [41,[49][50][51] and even the crack failure patterns (RA and AF) [52,53] during the failure process. However, the specimen size has limited effect on the failure patterns of coal and rock [19,[54][55][56], and the overall failure features revealed by these parameters cannot be represented by a single value as fractal dimension and connect the laboratory experiment and the filed work.…”