PSI - Issue 45

Ali et al. / Structural Integrity Procedia 00 (2023) 000 – 000 Ali et al. / Structural Integrity Procedia 00 (2023) 000 – 000 Ali et al. / Structural Integrity Procedia 00 (2023) 000 – 000 Ali et al. / Structural Integrity Procedia 00 (2023) 000 – 000

Zulfiqar Ali et al. / Procedia Structural Integrity 45 (2023) 60 – 65 1. Introduction During uniaxial cyclic compression, potential cracks with lower strength than the applied stress initiate and propagate in the rock specimen. Upon unloading and reloading, new cracks do not develop until the maximum stress previously applied is surpassed (Yamamoto et al. 1999; Ali et al. 2021). This phenomenon, called the Kaiser effect or stress memory in rocks, results in a surge in acoustic emission (AE) activity and strain rate change in the stress-strain curve (Karakus et al. 2015; Ali et al. 2022). The change in the strain rate can be determined using several method, like deformation rate analysis (DRA) and secant modulus method (SMM) (Ali et al. 2022). Fujii et al. (2018) proposed the tangent modulus method (TMM) to investigate the stress memory in rocks under uniaxial cyclic compression. The method involves preloading and unloading of the rock specimen to a specific stress level, followed by two cycles of uniaxial compression at a higher stress level. Stress-tangent modulus curve of the two loading cycles is obtained which yields a bending or a separation between the two curves. The separation point is assumed to be the maximum stress previously applied, as shown in the Fig. 1. 1. Introduction During uniaxial cyclic compression, potential cracks with lower strength than the applied stress initiate and propagate in the rock specimen. Upon unloading and reloading, new cracks do not develop until the maximum stress previously applied is surpassed (Yamamoto et al. 1999; Ali et al. 2021). This phenomenon, called the Kaiser effect or stress memory in rocks, results in a surge in acoustic emission (AE) activity and strain rate change in the stress-strain curve (Karakus et al. 2015; Ali et al. 2022). The change in the strain rate can be determined using several method, like deformation rate analysis (DRA) and secant modulus method (SMM) (Ali et al. 2022). Fujii et al. (2018) proposed the tangent modulus method (TMM) to investigate the stress memory in rocks under uniaxial cyclic compression. The method involves preloading and unloading of the rock specimen to a specific stress level, followed by two cycles of uniaxial compression at a higher stress level. Stress-tangent modulus curve of the two loading cycles is obtained which yields a bending or a separation between the two curves. The separation point is assumed to be the maximum stress previously applied, as shown in the Fig. 1. 1. Introduction During uniaxial cyclic compression, potential cracks with lower strength than the applied stress initiate and propagate in the rock specimen. Upon unloading and reloading, new cracks do not develop until the maximum stress previously applied is surpassed (Yamamoto et al. 1999; Ali et al. 2021). This phenomenon, called the Kaiser effect or stress memory in rocks, results in a surge in acoustic emission (AE) activity and strain rate change in the stress-strain curve (Karakus et al. 2015; Ali et al. 2022). The change in the strain rate can be determined using several method, like deformation rate analysis (DRA) and secant modulus method (SMM) (Ali et al. 2022). Fujii et al. (2018) proposed the tangent modulus method (TMM) to investigate the stress memory in rocks under uniaxial cyclic compression. The method involves preloading and unloading of the rock specimen to a specific stress level, followed by two cycles of uniaxial compression at a higher stress level. Stress-tangent modulus curve of the two loading cycles is obtained which yields a bending or a separation between the two curves. The separation point is assumed to be the maximum stress previously applied, as shown in the Fig. 1. 1. Introduction During uniaxial cyclic compression, potential cracks with lower strength than the applied stress initiate and propagate in the rock specimen. Upon unloading and reloading, new cracks do not develop until the maximum stress previously applied is surpassed (Yamamoto et al. 1999; Ali et al. 2021). This phenomenon, called the Kaiser effect or stress memory in rocks, results in a surge in acoustic emission (AE) activity and strain rate change in the stress-strain curve (Karakus et al. 2015; Ali et al. 2022). The change in the strain rate can be determined using several method, like deformation rate analysis (DRA) and secant modulus method (SMM) (Ali et al. 2022). Fujii et al. (2018) proposed the tangent modulus method (TMM) to investigate the stress memory in rocks under uniaxial cyclic compression. The method involves preloading and unloading of the rock specimen to a specific stress level, followed by two cycles of uniaxial compression at a higher stress level. Stress-tangent modulus curve of the two loading cycles is obtained which yields a bending or a separation between the two curves. The separation point is assumed to be the maximum stress previously applied, as shown in the Fig. 1.

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(a) (a) (a) (a)

(b) (b) (b) (b)

Fig. 1. Schematic of the TMM showing loading regime and bending points in the TMM curve. Fig. 1. Schematic of the TMM showing loading regime and bending points in the TMM curve. Fig. 1. Schematic of the TMM showing loading regime and bending points in the TMM curve. Fig. 1. Schematic of the TMM showing loading regime and bending points in the TMM curve. According to Fuji et al . (2018) point 1 and 2 in Fig. 1(a) denotes the in-situ stress condition under which a rock exhibits a few voids that are tabular and sufficiently large such that they are partially closed. Once the rock is unloaded and reloaded again under uniaxial cyclic compression, the rock is stiff during the first cyclic loading up to the in-situ stress level (3 to 4) because no further void closure take place. However, the stiffness decreases under further compression (4 to 5) due to the crack closure, sliding and new crack generation. Hence, a bending point appears at point 4 as shown in Fig. 1(b). Since the specimen is reloaded to the same maximum load in the second cycle, it is assumed that no further crack or void closure and generation takes place, thereby resulting in high stiffness throughout the second loading cycle (6 to 7). Hence, once the tangent modulus curves for the two cycles are plotted in a stress-tangent modulus graph, a clear separation in the two curves are observed. However, like many other researchers who reported the effect of time delay on stress memory (Lavrov 2003; Fuji et al . (2018); Ban et al. 2020), Ali et al. 2022 found that this method is highly sensitive to time delay which impedes its use for in-situ stress measurements. This paper introduces a modified method that utilizes gradient change to reconcile the discrepancy in TMM and establish it as a feasible technique for measuring in-situ stress. 2. Experimental Procedures The experiments were performed on various rock types, including soft rocks such as sandstone and limestone, as well as harder crystalline rocks such as bluestone and granite. All specimens were prepared according to the ISRM standards for uniaxial compressive strength tests, as outlined by Fairhurst and Hudson (1999). The cylindrical specimens had an aspect ratio ranging between 2 and 2.5, and the surfaces were ground parallel to minimize end friction effects and ensure a uniform stress state during the tests. MTS 300kN closed loop servo controlled testing machine, consisting of an axial dynamic loading frame and a data acquisition system, was used to perform the tests. The deformation process was monitored using linear variable differential transformers (LVDTs), axial strain gauges, and an AE monitoring system. The specimens were initially prestressed to simulate the in-situ stresses and subsequently two-cycle compression were applied at approximately 20% higher stress levels to measure the applied stress (Fig. 2). The effect of time delay was studied by removing the specimen from According to Fuji et al . (2018) point 1 and 2 in Fig. 1(a) denotes the in-situ stress condition under which a rock exhibits a few voids that are tabular and sufficiently large such that they are partially closed. Once the rock is unloaded and reloaded again under uniaxial cyclic compression, the rock is stiff during the first cyclic loading up to the in-situ stress level (3 to 4) because no further void closure take place. However, the stiffness decreases under further compression (4 to 5) due to the crack closure, sliding and new crack generation. Hence, a bending point appears at point 4 as shown in Fig. 1(b). Since the specimen is reloaded to the same maximum load in the second cycle, it is assumed that no further crack or void closure and generation takes place, thereby resulting in high stiffness throughout the second loading cycle (6 to 7). Hence, once the tangent modulus curves for the two cycles are plotted in a stress-tangent modulus graph, a clear separation in the two curves are observed. However, like many other researchers who reported the effect of time delay on stress memory (Lavrov 2003; Fuji et al . (2018); Ban et al. 2020), Ali et al. 2022 found that this method is highly sensitive to time delay which impedes its use for in-situ stress measurements. This paper introduces a modified method that utilizes gradient change to reconcile the discrepancy in TMM and establish it as a feasible technique for measuring in-situ stress. 2. Experimental Procedures The experiments were performed on various rock types, including soft rocks such as sandstone and limestone, as well as harder crystalline rocks such as bluestone and granite. All specimens were prepared according to the ISRM standards for uniaxial compressive strength tests, as outlined by Fairhurst and Hudson (1999). The cylindrical specimens had an aspect ratio ranging between 2 and 2.5, and the surfaces were ground parallel to minimize end friction effects and ensure a uniform stress state during the tests. MTS 300kN closed loop servo controlled testing machine, consisting of an axial dynamic loading frame and a data acquisition system, was used to perform the tests. The deformation process was monitored using linear variable differential transformers (LVDTs), axial strain gauges, and an AE monitoring system. The specimens were initially prestressed to simulate the in-situ stresses and subsequently two-cycle compression were applied at approximately 20% higher stress levels to measure the applied stress (Fig. 2). The effect of time delay was studied by removing the specimen from According to Fuji et al . (2018) point 1 and 2 in Fig. 1(a) denotes the in-situ stress condition under which a rock exhibits a few voids that are tabular and sufficiently large such that they are partially closed. Once the rock is unloaded and reloaded again under uniaxial cyclic compression, the rock is stiff during the first cyclic loading up to the in-situ stress level (3 to 4) because no further void closure take place. However, the stiffness decreases under further compression (4 to 5) due to the crack closure, sliding and new crack generation. Hence, a bending point appears at point 4 as shown in Fig. 1(b). Since the specimen is reloaded to the same maximum load in the second cycle, it is assumed that no further crack or void closure and generation takes place, thereby resulting in high stiffness throughout the second loading cycle (6 to 7). Hence, once the tangent modulus curves for the two cycles are plotted in a stress-tangent modulus graph, a clear separation in the two curves are observed. However, like many other researchers who reported the effect of time delay on stress memory (Lavrov 2003; Fuji et al . (2018); Ban et al. 2020), Ali et al. 2022 found that this method is highly sensitive to time delay which impedes its use for in-situ stress measurements. This paper introduces a modified method that utilizes gradient change to reconcile the discrepancy in TMM and establish it as a feasible technique for measuring in-situ stress. 2. Experimental Procedures The experiments were performed on various rock types, including soft rocks such as sandstone and limestone, as well as harder crystalline rocks such as bluestone and granite. All specimens were prepared according to the ISRM standards for uniaxial compressive strength tests, as outlined by Fairhurst and Hudson (1999). The cylindrical specimens had an aspect ratio ranging between 2 and 2.5, and the surfaces were ground parallel to minimize end friction effects and ensure a uniform stress state during the tests. MTS 300kN closed loop servo controlled testing machine, consisting of an axial dynamic loading frame and a data acquisition system, was used to perform the tests. The deformation process was monitored using linear variable differential transformers (LVDTs), axial strain gauges, and an AE monitoring system. The specimens were initially prestressed to simulate the in-situ stresses and subsequently two-cycle compression were applied at approximately 20% higher stress levels to measure the applied stress (Fig. 2). The effect of time delay was studied by removing the specimen from According to Fuji et al . (2018) point 1 and 2 in Fig. 1(a) denotes the in-situ stress condition under which a rock exhibits a few voids that are tabular and sufficiently large such that they are partially closed. Once the rock is unloaded and reloaded again under uniaxial cyclic compression, the rock is stiff during the first cyclic loading up to the in-situ stress level (3 to 4) because no further void closure take place. However, the stiffness decreases under further compression (4 to 5) due to the crack closure, sliding and new crack generation. Hence, a bending point appears at point 4 as shown in Fig. 1(b). Since the specimen is reloaded to the same maximum load in the second cycle, it is assumed that no further crack or void closure and generation takes place, thereby resulting in high stiffness throughout the second loading cycle (6 to 7). Hence, once the tangent modulus curves for the two cycles are plotted in a stress-tangent modulus graph, a clear separation in the two curves are observed. However, like many other researchers who reported the effect of time delay on stress memory (Lavrov 2003; Fuji et al . (2018); Ban et al. 2020), Ali et al. 2022 found that this method is highly sensitive to time delay which impedes its use for in-situ stress measurements. This paper introduces a modified method that utilizes gradient change to reconcile the discrepancy in TMM and establish it as a feasible technique for measuring in-situ stress. 2. Experimental Procedures The experiments were performed on various rock types, including soft rocks such as sandstone and limestone, as well as harder crystalline rocks such as bluestone and granite. All specimens were prepared according to the ISRM standards for uniaxial compressive strength tests, as outlined by Fairhurst and Hudson (1999). The cylindrical specimens had an aspect ratio ranging between 2 and 2.5, and the surfaces were ground parallel to minimize end friction effects and ensure a uniform stress state during the tests. MTS 300kN closed loop servo controlled testing machine, consisting of an axial dynamic loading frame and a data acquisition system, was used to perform the tests. The deformation process was monitored using linear variable differential transformers (LVDTs), axial strain gauges, and an AE monitoring system. The specimens were initially prestressed to simulate the in-situ stresses and subsequently two-cycle compression were applied at approximately 20% higher stress levels to measure the applied stress (Fig. 2). The effect of time delay was studied by removing the specimen from