Abstract Acoustic emission (AE) and digital image correlation (DIC) were combined to characterize the fracture process zone (FPZ) in Berea sandstone, a quasi-brittle material with a maximum grain size of 1 mm. The complimentary techniques of AE and DIC provide information on (i) damage throughout the volume, including AE locations and relative energy, and (ii) detailed DIC measurements of opening displacements associated with the FPZ. Three types of specimens were tested by three-point bending: center notch, smooth boundary, and large radius center notch. Experimental results indicate the following: (1) The length of FPZ is about 10 times longer than the maximum grain size. (2) The evaluation of energy based on the two different measurements, AE and DIC, is consistent with a linear softening law for the FPZ. (3) The fracture energy is estimated to be 120 – 140 J/m 2 , about seven times greater than the fracture energy based on linear fracture mechanics.
#1Qing Lin(China University of Petroleum)H-Index: 11
#2Wei-Wei Ji(CAS: Chinese Academy of Sciences)H-Index: 4
Last. Yunhu Lu(China University of Petroleum)H-Index: 11
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Abstract Laboratory determination of the mode II fracture toughness KIIC for rocks and rock-type materials remains a challenge task. One of the testing approaches is the compression of disk-type specimens, which is simple and convenient. However, although the notch in disk-type specimen is under mode II loading, the detailed studies reveal the drawbacks about interpretations of the disk-type specimens: (i) mode II fracture is only assumed without careful identifications about the actual fracture...
Abstract Natural rock joints with infilling materials such as clay, broken rock fragments, or even cement mortar infusion material are common in the field of rock engineering. The fracture behavior of a rock with a filled flaw is related to both the flaw inclination and mechanical properties of the infilling materials. In this study, uniaxial compression experiments were performed for sandstone specimens with different flaw inclinations varying from 0° to 90°, and four kinds of infilling conditi...
Last. Shiyuan Li(China University of Petroleum)H-Index: 2
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Abstract The influences of mode II loading on fracture process in Berea sandstone were investigated by comparing mode I and mixed-mode fractures that were created by center and eccentric notch beams under three-point bending. Fracture process was characterized by Acoustic Emission (AE) and related energy. Experimental results show mode II loading does not affect the length of fully-developed fracture process zone and maximum value of AE energy. However, it affects the zone from reaching fully-de...
Active seismic monitoring was used to detect and characterize crack initiation, crack propagation and crack coalescence in pre-cracked rock specimens. Uniaxial compression tests were conducted on Indiana limestone specimens with two parallel pre-existing cracks. During the experiments, the mechanically induced cracks around the flaw tips were monitored by measuring surface displacements using digital image correlation (DIC). Transmitted and reflected compressional and shear waves through the spe...
#1Bing Q. Li(MIT: Massachusetts Institute of Technology)H-Index: 4
#2Herbert H. Einstein(MIT: Massachusetts Institute of Technology)H-Index: 33
We present an experimental study in which a pre-notched specimen of Barre Granite was subjected to four point bending under crack mouth opening displacement control. The experimental observations consisted of load–displacement measurements, acoustic emissions, and photography on a macroscopic (~cm) as well as microscopic (~μm) scale. These observations were compared and analysed to better understand process zone development and crack propagation. Load–displacement data showed that the load reach...
#1Ali Tarokh(UMN: University of Minnesota)H-Index: 8
#2Roman Y. Makhnenko(UIUC: University of Illinois at Urbana–Champaign)H-Index: 12
Last. Joseph F. Labuz(UMN: University of Minnesota)H-Index: 32
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The zone of microcracks surrounding a notch tip—the process zone—is a phenomenon observed in fracture of quasi-brittle materials, and the characterization of the process zone is the topic of the paper. Specimens of different sizes with a center notch fabricated from a granite of large grain (Rockville granite, average grain size of 10 mm), were tested in three-point bending. Acoustic emissions were recorded and locations of microcracks were determined up to peak load. The results show that both ...
#2Ahmed Loukili(École centrale de Nantes)H-Index: 33
Fracture in quasi-brittle materials like concrete is mainly governed by two processes: energy dissipation and discontinuous displacement fields or discrete cracks. In this study we discuss the simultaneous development of both types of fracture. Energy dissipation and crack openings are analyzed during the fracture development by acoustic emission and digital image correlation (DIC) respectively. DIC results are processed using nonlinear damage modelling and lattice element modelling to reveal th...
Tensile cracking in asphalt pavements due to vehicular and thermal loads has become an experimental and numerical research focus in the asphalt materials community. Previous studies have used the discrete element method (DEM) to study asphalt concrete fracture. These studies used trial-and-error to obtain local fracture properties such that the DEM models approximate the experimental load-crack mouth opening displacement response. In the current study, we identify the cohesive fracture propertie...
This paper reports the results of an extensive study of the deformation and fracturing of Bukit Timah granite in Singapore under uniaxial and triaxial loading conditions. In the investigation, the stress fracturing thresholds (crack initiation, crack coalescence and crack damage) were determined using stress–strain and stiffness curves and the acoustic emission (AE) detection technique. Crack initiation was found to be best determined by the volumetric strain curve in both uniaxial and triaxial ...
Last. Kaiwen Xia(U of T: University of Toronto)H-Index: 30
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Abstract The semi-circular bend (SCB) method suggested by International Society for Rock Mechanics (ISRM) is demonstrated to yield much conservative mode I fracture toughness. A large fracture process zone observed by both acoustic emission (AE) monitoring and numerical modeling is analyzed to be responsible for that phenomenon. Using numerically simulated effective crack lengths at the peak-load stage in the experiments, the measured toughness values are close to the level I fracture toughness....
#1Wang Xiaoran(CUMT: China University of Mining and Technology)H-Index: 1
#2Enyuan Wang(CUMT: China University of Mining and Technology)H-Index: 35
Last. Xin Zhou(CUMT: China University of Mining and Technology)
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Abstract Three-dimensional location and characterization of microcracks in controlled fracture tests of hard coal beams were experimentally conducted through the quantitative acoustic emission (AE) technique. The recorded AE event were physically interpreted as a displacement discontinuity (DD), i.e. microcrack, involving only opening/closing and shear-sliding mechanisms to describe the fracture modes directly anticipated within a mechanically loaded material. Based on the sensor calibration usi...
Abstract null null Three-point bending experiments were performed to observe mode-I fracture in a series of geometrically similar, center-notched rectangular beams composed of Sioux quartzite, a stiff, hard rock with flexural tensile strength σt = 32 MPa. Acoustic emission (AE) were recorded and microcrack locations were determined to track crack growth. Digital image correlation (DIC) was used to augment the acoustic emission data. Detailed observations indicate that (i) subsize specimens of Si...
#1Xiaoran Wang(CUMT: China University of Mining and Technology)
#2Pouyan Asem(UMN: University of Minnesota)H-Index: 6
Last. Joseph F. Labuz(UMN: University of Minnesota)H-Index: 32
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Abstract Acoustic emission (AE) and digital image correlation were used to monitor mode I fracture in center notch and smooth boundary specimens. The AE source was characterized as a microcrack and represented by the moment tensor, which was obtained by minimizing the error between displacements calculated from an elastodynamic solution and those measured, subjected to the constraint of a microcrack source. Results showed that microcracking was locally mixed-mode dominant because of oblique micr...
#1Qiang Li(CSU: Central South University)H-Index: 3
#2Xibing Li(CSU: Central South University)H-Index: 44
Last. Tubing Yin(CSU: Central South University)H-Index: 14
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Abstract Hard rock breaking is a great challenge for deep drilling and mining engineering, and the microwave-assisted rock breaking technique offers a promising solution. However, the lack of knowledge of the effects of microwave irradiation on hard rocks limits the further development of this technology. Laboratory tests have been conducted to investigate the variations of fracture properties of granite subjected to microwave heating. The semi-circular bend (SCB) specimens were prepared, and mo...
#1Qing Lin(China University of Petroleum)H-Index: 11
#2Xin Bian(China University of Petroleum)H-Index: 1
Last. Yunhu Lu(China University of Petroleum)H-Index: 11
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Abstract Fracture criterion of local symmetry has been widely used to predict the fracture kink and simulate the fracture trajectory in rocks under the mixed-mode loading. Although its prediction is a tension dominated fracture, under small scale yielding the criterion must include two aspects: the local mode I stress intensity factor (SIF) kI is maximum and the local mode II SIF kII is zero. Two aspects are not identical when fracture just initiates, but they will be equivalent after the crack ...
Abstract This paper investigates the mechanical and fracture properties of basalt fiber reinforced fly ash geopolymer concrete with different length (3 mm, 6 mm, 12 mm and 18 mm), which were tested by means of compressive strength test, splitting tensile strength test and three point bending test. The fiber reinforced mechanism was characterized by SEM. Simultaneously, the strain field and displacement field of FAGC were explored by using of digital image correlation. The results reveal that the...
Abstract During the hydraulic fracturing, fractures are often kinked, blunted, or offset owing to the alternating, interbedded interfaces of diverse properties that render fracturing performance unsatisfactory. In this study, two cases (the artificial fracture perpendicular to and parallel to the bedding interfaces) were examined through three-point bending experiments on semi-circular bending (SCB) using shale specimens. The fracture process zone (FPZ) was studied based on the Digital Image Cor...
Abstract It is well known that the measured values of the fracture toughness of rocks are influenced by material heterogeneity, dimensions, boundary conditions, and asymmetric mechanical behavior. Consequently, the results obtained by standard testing methods developed primarily for homogenous materials with symmetric mechanical behavior, can significantly differ. The standard methods take global approach. Thus, they suppose that the material tested will follow a specific physical model and that...
Abstract In order to investigate the development characteristics and size effect of rock fracture process zone (FPZ), the GCTS RTX-3000 rock mechanics testing machine was used to carry out on semi-circular bend (SCB) sandstone specimens with diameters of 50 mm, 100 mm, 150 mm and 200 mm, respectively. Combined with the digital image correlation (DIC) method, the variations of the displacement field at the crack tip of specimens with different sizes were analyzed. The results show that the crack ...