Acoustic Emission Characteristics on the UCS of a Synthetic Rock
Authors: Gautam P. K. , Gutierrez Marte 
Published: August 2024

ABSTRACT:
This paper studied the acoustic emission (AE) characteristics to show that there is a connection between AE dominant frequency information and crack propagation patterns in identifying synthetic rock specimens during loading, like when new cracks start to form or when existing cracks propagate. The study examined how brittleness influenced acoustic emission (AE) properties, including cumulative AE energy, average frequency (AF), and rise time to peak amplitude (RA). The results show the ratio of macrocracks to microcracks formed during rock failure. Rock brittleness impacts the AF value, but not the RA value. The more brittle the rock is, the higher the proportion of tensile cracks in the fracture process. The brittleness of synthetic rocks appeared to have an apparent effect on the AF value, but it did not significantly affect the RA value. The main frequency ranges for AF values in mix A1, mix A2, mix B1, and mix B2 samples were 219–3231 kHz, 206-330 kHz, 258–327 kHz, and 236-350 kHz, respectively. Analysis of the dominant frequency revealed that mix A2 and mix B2 are more distinct and useful for estimating the brittleness of the highly brittle rock compared to mix A1 and mix B1. The research results could further enrich the evaluation of rock brittleness and contribute to the deformation and failure to understand rock damage better.

1. INTRODUCTION
In recent years, the social economy has rapidly expanded, leading to a notable rise in the development of transportation and hydraulic tunnels, underground natural resource exploration, urban underground space utilization, and other underground excavation projects (Huang et al., 2021; Li et al., 2021; Gao et al., 2020). Excavation operations damage the rock mass, leading to significant alterations in stress distribution because of the mass's intricate structure, including faults, joints, defects, heterogeneity, and other preexisting natural discontinuities. This leads to the release of internal stress fields and stress concentration, which causes fractures to develop, advance, and expand inside the rock mass (Xu et al., 2021; Zhang et al., 2012; Li et al., 2017). Moreover, the overall load-bearing capacity of the rock mass diminishes, resulting in geological catastrophes, including subterranean building collapses, substantial deformation of adjacent rock, and sudden rock bursts. These factors significantly influence the design and operation of subterranean construction and excavation. Therefore, understanding rock deformation and its failure characteristics is crucial in the domains of rock mechanics and rock engineering. Stress-induced brittle fracturing weakens rock mass strength in man-made structures, which is significant for both theoretical and site investigation purposes (Wang et al., 2019; Lin et al., 2020; Wang et al., 2014; Mondal et al., 2019; Zhang et al., 2016; Wang et al., 2018; Lee and Jeon, 2011). Several researchers have found that when brittle rocks are compressed and found several stages of failure: failure and post-peak deformation, crack closure, linear elastic deformation, crack initiation, crack damage, and crack coalescence (Bieniawski, 1967; Wawersik and Fairhurst, 1970; Zhao et al., 2015). The stresses observed at different phases-crack closure stress, crack initiation stress, crack damage stress, and peak stress—are critical stress thresholds that influence the deformation and failure of brittle rocks under compression. Internal fissures in the rock release strain energy as transient elastic waves, leading to acoustic emission (Lockner, 1993; Turcotte et al., 2003; Wang et al., 2021; Sun et al., 2021). There is a significant link between the development of internal damage in rocks and acoustic emissions (AE). This relationship forms the foundation for using AE technology in rock mechanics and rock engineering (Stierle et al., 2016; Meng et al., 2018). Several studies have found at how rocks react to acoustic emission by using different types of compression tests, like tension, splitting, three-point bending, and uniaxial (Wang et al., 2022). Feng et al. (2019) used in-situ acoustic emission (AE) to observe cracks in rocks within the mining and geotechnical discipline. Thus, in this work, we chose acoustic emission (AE) technology to observe the progressive development of damage in rocks. The AE hit, which detects and quantifies an AE signal on a specific channel, is called the AE hit (PAC 2014). The quantity of AE hits detected by AE sensors is directly related to the number of microfractures formed inside the rock, as stated by Eberhardt et al. (1999) and Backers et al. (2005). AE parametric analysis, a standard indicator used to study rock deformation and failure processes, is determined by integrating the rectified waveform during the AE hits. Researchers frequently use AE parameters such as amplitude, counts, rise-time, duration, and AE energy to study rock failure mechanisms, measuring it by integrating the rectified waveform over the duration of the AE event (Chang and Lee 2004; Wang and Ge 2008; Meng et al. 2015). The AE energy magnitude can describe the scale and magnitude of a rock fracturing event (Hetenyi, 1996; Meng et al., 2015). The AE b-value has become known as a precursor indicator in rock engineering disasters, as a decrease in this value precedes the rock's ultimate failure (Colombo et al., 2003; Kurz et al., 2006; Meng et al., 2016; Meng et al., 2019; Liu et al., 2020; Dong et al., 2021). As a result, this technique is difficult to use in engineering practice and laboratory research because it requires more than six AE sensors. Using statistical analysis to identify dominant frequencies in AE is a new way of studying synthetic rock failure processes, as well as the micro- and macro-failure processes of synthetic rocks. There is a notable lack of observations on the relationship between the dominant AE frequency characteristics and the real-time cracking process of synthetic rock. Zhang et al. (2023) used the AE technique to investigate the macro-scale failure pattern and variations in sandstone's mechanical characteristics. It compacted the sandstone with varying levels of moisture content. Researchers observed that the AE spectrum exhibits four distinct responses during sandstone fracture. There are different types of signals that show cracks in minerals and crystals. Low-frequency low amplitude (LF-LA) signals show cracks that move between or within minerals or crystal particles; intermediate frequency low amplitude (IF-LA) signals show friction in microcracks; high frequency low amplitude (HF-LA) signals show fracture in microcracks; and low frequency high amplitude (LF-HA) signals show large-scale cracks. Therefore, the aim of this study is to examine how the main frequency and amplitude of acoustic emission (AE) waveforms change in real time as synthetic rocks undergo uniform compression, with a focus on how cracks form in the rocks. This study utilizes acoustic emission methods to investigate the brittle characteristics of a synthetic rock.