4.2.2.2. Effect of normal stress and model brittleness on the acoustic emission phenomena
As stated by Shi et al. (2022), the phenomenon of acoustic emission takes place when a material undergoes stress concentration, leading to the rapid release of energy and the generation of transient elastic waves. In this particular section, the authors investigate the relationship between the quantity of acoustic hits and the number of horizontal bedding layers from the initiation of shear stress on the bedding layer to the ultimate failure of the rock bridge, considering two distinct normal stress levels.
a. Acoustic emission phenomena without present of normal stress
This study examines the relationship between acoustic emission events and the number of horizontal bedding layers during the entire process, from the application of shear stress on the bedding layer to the ultimate failure of the rock bridge, without considering the presence of normal stress.
a.1. Acoustic emission single soft layer
Figure 12a illustrates the acoustic emission patterns in a soft material layer, showing three stages: relative calm (stage I), crack initiation and growth (stage II), and final failure (stage III). In stage I, few Acoustic hits with high continuity indicate minimal microcrack initiation. Stage II sees more Acoustic hits, suggesting crack propagation. Stage III shows fewer, shorter Acoustic hits, indicating greater energy release. Major Acoustic hits increase with shear displacement, implying higher energy release during final failure than crack initiation.
a.2. Acoustic emission in single hard layer
Figure 12b displays acoustic emission properties of a hard layer with various fissure lengths, following a similar pattern observed in a single soft layer. Divided into three stages (quietness, crack initiation and growth, final failure), two significant acoustic event peaks mark crack initiation and unstable propagation. Compared to the soft layer (Fig. 12a), the firm layer (Fig. 12b) shows higher shear displacement for stage II initiation and stage III onset. This reflects the firm layer's ductile nature, with a lower compression-to-tensile strength ratio. Despite its deformability, the firm layer experiences brittle failure, releasing more energy during final failure. Major Acoustic hits increase with shear displacement, indicating higher energy release during final failure than crack initiation. The firm layer exhibits more Acoustic hits during rock bridge failure due to stronger particle cementation, requiring more energy for crack propagation. Increasing fissure length reduces acoustic event hits, minimizing discrepancies in major acoustic event numbers with constant mechanical properties.
a.3. Acoustic emission in two layered model
Figure 12c shows AE behavior in a two-layered model with horizontal bedding, dividing the process into four stages: silence (stage I), crack initiation and growth (stage II), initial interface breakage (stage III), and ultimate failure (stage IV). These stages saw three significant Acoustic hits. In stage II, a notable peak occurred early at 0.35 mm shear movement, originating from the lower fissure tip in the brittle material layer. Stage III showed greater continuity due to the tensile crack propagating through the hard, ductile material and the brittle interlayer material. The third major acoustic event, at 0.73 mm shear displacement in stage IV, signaled a higher energy release, indicating the initiation of a new crack in the brittle layer. The second significant acoustic event surpassed the first in energy, and the third exceeded both, reflecting the substantial energy required for complete penetration of the hard, ductile layer by the tensile crack.Top of Form
a.4. Acoustic emission in model with hard inter layer
In Fig. 12d, acoustic event properties of models with a hard interlayer are depicted, dividing the process into five stages: initial quietness, crack initiation and growth, first and second interface breakage, and final failure (I, II, III, IV, V). Four distinct Acoustic hits were observed across these stages. In stage I, a significant acoustic event peak occurred at the start of stage II, originating from a tensile crack initiation in the brittle material layer. Subsequently, a second notable acoustic event peak emerged in stage III, indicating uniform microcrack generation. Stage III showed higher continuity than stage II due to tensile crack propagation through the hard, ductile material. The third major acoustic event occurred in stage IV, with greater released energy during final failure compared to other stages. Initiation of a new crack in the hard, ductile layer required more energy for the second major acoustic event, resulting in higher released energy. The fourth major acoustic event, associated with final failure, exhibited lower energy as less energy was needed for crack propagation in brittle material.
a.5. Acoustic emission in model with soft interlayer
In Fig. 12e, acoustic emission properties of three horizontally layered models with a flexible interlayer are shown, divided into five stages: silence (I), crack initiation and growth (II), first interface breakage (III), second interface breakage (IV), and ultimate failure (V). Four distinct acoustic emission events were observed. In stage I, minimal activity with consistent continuity was noted. At the start of stage II, a significant peak was observed at a shear movement of 0.46 mm, indicating tensile crack initiation from the lower fissure tip in the ductile material layer. Subsequently, signals increased uniformly, indicating microcrack generation. In stage III, the second major peak occurred at a shear movement of 0.68 mm, with lower continuity compared to stage II due to crack propagation through the soft, brittle interlayer material. Stage IV exhibited the third major activity at 0.75 mm shear movement, with higher continuity than stage III as the crack extended through the ductile material. The fourth major event, marking final failure, occurred at the start of stage V with a shear movement of 0.86 mm, exhibiting weaker continuity. The number of events associated with shear movements of 0.46 mm, 0.68 mm, 0.75 mm, and 0.86 mm were 12, 9, 14, and 17 respectively, indicating higher energy release in the final failure stage. The initiation of a new crack in the ductile layer required more energy, resulting in a higher release during the first major event. Subsequent events showed variations in energy release depending on crack propagation conditions.
a.6. Acoustic emission in four layer
In Fig. 12f, acoustic emission patterns of a four-layered horizontally arranged model are shown, covering six stages: silence (I), crack onset and growth (II), initial, second, and third interface breakage (III, IV, V), and final failure (VI). Across these phases, four notable acoustic emission occurrences were observed. Phase I showed minimal yet continuous acoustic emission. The first significant peak appeared at the beginning of Phase II, indicating crack initiation in the soft, brittle layer's lower fissure tip. Phase III displayed increased activity, with crack propagation mainly through the firm, ductile interlayer material. Another notable event occurred at the start of Phase IV, with continuity exceeding that of Phase III as the crack go through the soft, brittle layer. The fourth significant event occurred at the beginning of Phase IV, with higher continuity due to crack propagation through the firm, ductile material layer. The number of events associated with shear movements of 0.38 mm, 0.42 mm, 0.62 mm, and 0.7 mm were 10, 12, 15, and 18, respectively, indicating escalating energy release during final failure. These acoustic emission features offer insights into failure and deformation behaviors of layered models, aiding in assessing their mechanical response and stability.
b. Acoustic emission phenomena with present of normal stress of 2 MPa
This segment explores the connection between acoustic emission occurrences and the number of horizontal bedding strata throughout the entire sequence. The procedure begins with the imposition of shear strain on the bedding stratum and ends with the collapse of the rock span under a perpendicular stress of 2 MPa.
b.1. Acoustic emission in single soft layer
As shown in Fig. 13a, the Acoustic Emission process in a soft, brittle layer follows three stages: initial silence (stage I), crack initiation and growth (stage II), and final failure (stage III). This acoustic event behavior mirrors that of a single soft, brittle layer without a normal load (Fig. 13a). Comparing Figs. 12a and 13a reveals an increase in shear movement associated with Acoustic hits as normal stress rises. Additionally, higher normal stress correlates with more Acoustic hits, indicating greater released energy.
b.2. Acoustic emission in single hard layer
Figure 13b depicts the acoustic emission process in a firm layer. Like the acoustic event behavior observed without a vertical load (Fig. 12b), this process unfolds in three stages: initial silence (stage I), crack initiation and steady growth (stage II), and eventual failure (stage III). Comparing Fig. 12b and Fig. 13b suggests that as the vertical stress increases, the shear movement associated with Acoustic hits also rises. Moreover, higher vertical stress corresponds to more Acoustic hits, indicating greater energy release. Comparing Fig. 13a and Fig. 13b, acoustic event intensity in the pliable model is lower than in the firm model. For instance, the number of events related to the first major acoustic event is 18 in the pliable model (at a shear movement of 0.618 mm in Fig. 13a) and 34 in the firm model (at a shear movement of 0.7 mm in Fig. 13b). Similarly, for the second major acoustic event, there are 27 events in the pliable model (at a shear movement of 0.88 mm in Fig. 13a) and 40 events in the firm model (at a shear movement of 1.35 mm in Fig. 13b). This indicates that acoustic event during significant crack propagation is lower in the pliable layer due to its increased flexibility. Additionally, the difference in shear movements associated with the two major acoustic event peaks is smaller in the pliable model (0.262 mm in Fig. 13a) than in the firm model (0.65 mm in Fig. 13b), suggesting rapid failure after crack initiation in the pliable model compared to delayed failure in the firm model. Comparing Fig. 12b and Fig. 13b also reveals that as the vertical stress increases, the magnitude of shear movement linked to Acoustic hits rises, along with an increase in the number of Acoustic hits, indicating higher energy release.
b.3. Acoustic emission in two layered model
Figure 13c shows the acoustic event process in a two-layered model, split into four stages: initial quiet (I), crack initiation (II), interface rupture (III), and final failure (IV). Three notable Acoustic hits occurred. Stage I had few Acoustic hits but high continuity. The first major acoustic event peak appeared in stage II at 0.7 mm shear movement, with uniform signals. The second peak was in early stage III at 0.84 mm, and the third in early stage IV at 1.1 mm. Stage III showed higher continuity than II. Failure started with vertical tensile fractures from the upper soft layer fissure, propagating to the interface at 0.14 mm. Then, a crack from the interface extended parallel to the shear direction until the model's lower edge, and mixed-mode fractures developed in the hard layer at 0.26 mm. Due to the soft layer's lower deformability, stage III had higher continuity than stage II. The number of Acoustic hits for shear movements of 0.7 mm, 0.84 mm, and 1.1 mm were 17, 32, and 40, respectively, indicating higher energy release at final failure. Comparing Fig. 12c and Fig. 13c, higher normal stress led to increased acoustic event occurrences, suggesting greater energy release.
b.4. Acoustic emission in model with hard interlayer
Figure 13d shows the acoustic event process in a three-layered horizontal model, divided into four stages: initial silence (stage I), crack initiation and growth (stage II), first interface rupture (stage III), and final failure (stage IV). Three significant Acoustic hits occurred. In stage I, Acoustic hits were sparse yet continuous. The first major acoustic event peak occurred at the start of stage II (shear movement: 0.54 mm), showing consistent growth with strong continuity. The second major acoustic event peak occurred early in stage III (shear movement: 0.59 mm), followed by the third at the onset of stage IV (shear movement: 1 mm). Stage III exhibited higher continuity than stage II. Failure initiated with vertical tensile fractures from notches in the soft material layer (shear movement: 0.05 mm). In the subsequent fracturing stage, a tensile crack originated from layer interfaces and propagated vertically within the middle hard layer, merging with other fractures. Mixed-mode fractures developed diagonally within the layers (shear movement: 0.41 mm). Stage II displayed lower continuity than stage III due to the brittle material layer's lower deformability compared to the ductile material layer. The number of Acoustic hits associated with shear movements of 0.54 mm (first major acoustic event), 0.59 mm (second major acoustic event), and 1 mm (third major acoustic event) were 15, 23, and 27, respectively. This indicates higher energy release during final failure, characterized by oriented shear band propagation. The energy required for crack initiation (first major acoustic event) was relatively low, resulting in a lower energy release. The third major acoustic event during final failure exhibited a higher energy release compared to the other major Acoustic hits. Comparing Fig. 12d and Fig. 13d reveals an increase in shear movement associated with acoustic event occurrences with higher normal stress. Additionally, Acoustic hits increased with elevated normal stress, indicating a larger energy release.
b.5. Acoustic emission in model with soft interlayer
Figure 13e displays the acoustic emission process in a three-layered horizontal model, divided into five stages: initial quiet phase (stage I), crack initiation (stage II), first interface breakage (stage III), second interface breakage (stage IV), and final failure (stage V). Three significant Acoustic hits occurred during these stages. In stage I, Acoustic hits were sparse but continuous. The first major acoustic event peak appeared early in stage II, correlating with a shear movement of 0.8 mm, displaying uniform intensity. The second major acoustic event peak occurred at the onset of stage III (shear movement: 1.06 mm), followed by the third major acoustic event at the beginning of stage IV (shear movement: 1.16 mm). The failure mechanism involved vertical tensile fractures originating from notches in the hard material layer, merging with layer interfaces, and subsequent propagation. This was followed by fractures originating from layer interfaces, propagating vertically within the middle soft layer until merging with other fractures, along with diagonal fractures within layers. Stage III exhibited lower continuity than stage II due to the brittle material layer's lower deformability compared to the ductile material layer. The number of Acoustic hits associated with shear movements of 0.8 mm, 1.06 mm, and 1.16 mm were 30, 35, and 42, respectively, indicating higher energy release during final breakage. Comparing Fig. 12e and Fig. 13e, it's evident that acoustic event occurrences increase with normal stress, indicating a larger released energy under elevated normal stress conditions.
b.6. Acoustic emission in four layer
Figure 13f displays the acoustic event process within a four-layered horizontal model, divided into six stages. Minimal Acoustic hits but continuous activity characterized stage I, while the first major acoustic event peak emerged early in stage II at 0.697 mm shear movement. Stage III saw the second major acoustic event peak at 0.74 mm shear movement, surpassing stage II's continuity due to differences in layer deformability. The third major acoustic event occurred at the start of stage IV, at 0.93 mm shear movement. Stage III's continuity exceeded that of stage IV. The fourth major acoustic event emerged at the beginning of stage IV, with a shear movement of 1 mm. Stage IV saw tensile fractures originating from the first lower interface and propagating diagonally within the soft material layer, while stage III saw propagation through the ductile layer. The fifth major acoustic event occurred at the onset of stage V, linked to a shear movement of 1.13 mm. The number of Acoustic hits associated with shear movements of 0.697 mm, 0.74 mm, 0.93 mm, 1 mm, and 1.13 mm were 17, 29, 19, 32, and 38, respectively. The thicker ductile layer in the three-layered model led to more Acoustic hits due to higher energy requirements for crack growth. Comparing Fig. 12f and Fig. 13f, acoustic event occurrences increased with higher normal stress, indicating a larger energy release.