To investigate the failure behavior of foliated or layered rock materials, many scholars derived a number of constitutive laws and failure criteria for transversely isotropic rocks [11,12,13,14]. These constitutive laws and failure criteria agreed well with the experimental data of many investigators in terms of prediction of strength and deformation, but they were difficult to be used to characterize crack evolution during the loading process. Accordingly, substantial studies were carried out on the strength, deformation characteristics, and failure behavior of transversely isotropic rocks by both experimental and numerical methods [15,16,17,18]. Tien et al. [19] investigated 3D macroscopic-fractured surfaces of simulated transversely isotropic rocks by reconstructing unrolled images from a rotary scanner and divided the failure modes into four kinds: sliding failure along discontinuities, tensile fracture across the discontinuities, tensile-split along the discontinuities, and sliding failure across the discontinuities. Yin and Yang [20] studied three kinds of transversely isotropic rock-like specimens with different bedding thickness ratios under conventional triaxial compression and divided the failure modes into three kinds: tensile-split along the core axes (0, 15, and 30), shear-sliding along the bedding plane (45, 60, and 75), and split along the vertical bedding plane (90). Furthermore, the development of the Digital Image Correlation method (DIC) [21,22,23], Acoustic Emission techniques (AE) [24,25,26,27], and Scanning Electron Microscopic observation (SEM) was found to be suitable for capturing the progressive failure process and understanding the failure mechanism of the rock. The aforementioned studies show that the failure mode transforms from tensile splitting across discontinuities to shear slip along discontinuities, and finally to tensile splitting along discontinuities with increasing foliation angle.
In addition to foliation angle, water content has a significant impact on the mechanical properties and failure behavior of phyllite. Previous studies indicate that the presence of water significantly weakens the mechanical properties of the rock such as its strength, rigidity, and brittleness [28]. In the saturation process, microcracks are filled with water, and crack tips are the most active areas of the water-rock reaction, which may decrease the critical stress intensity factor [29,30]. Furthermore, phyllite is a compact lustrous metamorphic rock. The rock is always rich in mica, chlorite, and quartz and it possesses the lepido granoblastic texture. Some changes may happen when phyllite is saturated such as clay mineral expansion, pore filling, non-uniform deformation, and calcite dissolution, respectively [31,32]. All of these changes result in the weakness of physical-mechanical properties and the generation of new defects like microcracks.
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Figure 6 demonstrates the crack evolution process of dry phyllite with 0 foliation angle at different loading times. A shear sliding crack initiates from the end of specimen and extends obliquely downward to the middle area of the specimen. Then, a tensile splitting crack, connected to the initial shear sliding crack, vertically propagates downwards to the inside specimen. Meantime, another shear sliding crack initiates from the connection position of shear sliding crack and tensile splitting crack and extends obliquely upward to the end of specimen. Afterwards, an approximately inverted conical failure surface appears and further promotes the initial tensile splitting crack vertically downward and finally causes the instable failure of the specimen. Significantly, the initiation and extension of cracks are accompanied by the evident variations of AE events rate and accumulated AE energy. Though AE count peaks appear several times and the cumulative counts rise step by step, the largest peak of AE events rate and the maximum accumulated AE energy occurs at the failure moment of the specimen. The crack evolution process of water-saturated phyllite at α = 0 and 30 was similar to that of dry phyllite α = 0, but there are also significant differences between them. For example, the extension of the initial shear sliding crack is steeper and the propagation of the tensile splitting crack easily bends and turns to a foliation plane. In addition, the value of AE events rate and accumulated AE energy of water-saturated phyllite are over one order of magnitude smaller than those of water-saturated phyllite. These results show that water has a significant influence on both a reduction in the intensity of the AE signal generated by rock failure and a weakening of the signal reception by hindering signal propagation.
The crack evolution processes of dry phyllite at α = 30 and water-saturated phyllite at α = 45 were similar. Figure 7 illustrates the crack evolution process of water-saturated phyllite with 45 foliation angle at different loading times. Several shear sliding cracks initiate from the end of specimen and extend obliquely downward to the position around the middle area of specimen. Then another shear sliding crack, connected to the initial shear sliding crack, extends obliquely downward to one side of specimen. Afterwards, a triangular sliding block appears in one side of the specimen and finally causes the instable failure. Moreover, a sharp increase in AE events rate and an accumulated AE energy would occur at the moment when the stress suddenly drops and the larger the stress drops, the greater the AE events rate and accumulated AE energy increment are. The crack evolution process of dry phyllite at α = 30 was similar to that of water-saturated phyllite α = 45.
Figure 8 shows the crack evolution process of water-saturated phyllite with 60 foliation angle at different loading times. A shear sliding crack initiates from the end of the specimen and extends obliquely downward. Then, several shear sliding cracks propagate from the end of the specimen, which is near to and parallel to initial shear sliding crack. Moreover, the propagation path of the shear sliding crack is located in one of foliation planes and causes the ultimate instable failure of specimen. The crack evolution processes of water-saturated phyllite α = 60 and dry phyllite at α = 45 and 60 are similar, but two differences still exist in them: (1) the crack propagation along foliation planes is affected by the foliation angle; (2) the water content contributes to decrease the resistance of crack propagation so that few shear sliding crack initiates from the end of the specimen and extends obliquely downward along foliation planes in the water-saturated specimen.
Classification of tensile and shear cracks based on AE parameter analysis: (a) Graphical representations of AE characteristic parameters; (b) Comparison between typical waveform of tensile event and shear event; (c) Crack classification methods in AE parameter analysis.
Figure 11 shows typical scatter diagrams between AF and RA values for foliated phyllite with different foliation angles under dry and water-saturated conditions. The AF-RA points are divided into two groups by the transition line. The upper groups (red) are located in the domain where AF value is higher and RA value is lower, which is consistent with the feature of tensile crack. Similarly, the lower groups (black) are located in the domain where AF value is lower and RA value is higher, which is consistent with the feature of shear/mixed crack. Furthermore, the intensity degree of AF-RA points reflects the crack scale during rock deformation. Among the specimens with different foliation angles, the specimen with 30 foliation angle yields the minimum cracking scale and the one of 0 foliation angle does the maximum cracking scale. Compared with the dry specimens, the saturated specimens yield relatively lower cracking scale.
Figure 15 shows that the SEM scanning characteristics of four typical shear fracture surfaces of phyllite. When the foliation angle is close to 0, the component of maximum principal stress (σ1) vertical to the foliation plane is larger than that of maximum principal stress (σ1) horizontal to the foliation plane. In this condition, the foliation plane hardly affects the initial failure process. As illustrated in Figure 15a, the whole shear fracture surface is relatively complete but partially broken with the flaky mineral particles that mostly present alternating foliations contact, and the surface appears with irregular jagged characteristics because of the shear failure in intergranular and trans-granular cracks. Overall, the whole surface is rough, but the area between two adjacent foliations is much smoother than that in other areas. Furthermore, the irregular jagged fracture surfaces are also flat but the orientation of them is different. Nevertheless, there is also a slight difference between foliated phyllite and intact rock materials, and the difference lies in the crack propagation path transforming from shearing slide along maximum shear stress to tensile-split along maximum principal stress.
Figure 16 shows that the characteristics of two typical tensile splitting fracture surfaces of phyllite from SEM observation. When the foliation angle is close to 90, the maximum principal stress (σ1) is approximately parallel to the foliation plane, which makes the tension at the foliation plane large enough to develop splitting tensile cracks along the foliation plane and lead to failure of the specimen. As shown in Figure 16, the fracture surface across the foliation plane is relatively rough but that along the foliation plane is flat. There are many broken flaky mineral particles in the fracture surface across the foliation plane, having the characteristics of intergranular or trans-granular tensile failure. Meanwhile, these areas between two adjacent foliations are much smoother than other areas. For the fracture surface along the foliation plane, the tensile failure between adjacent foliation planes brought about the appearance of numerous debris in the cleavage. The results show that the foliation plane in phyllite plays a vital role in changing failure behavior and affecting the microscopic morphology of the fracture surface. 2ff7e9595c
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