Showing papers on "Crack closure published in 2021"

Experimental Study on Evolution of Fracture Network and Permeability Characteristics of Bituminous Coal Under Repeated Mining Effect

Abstract: In a group of deep coal seams, mining of upper multilayers generates cyclic loading–unloading stresses in bottom layers, improving their permeability and enhancing coalbed methane gas drainage. In this study, the respective repeated mining stress path was introduced into the seepage test. A high-resolution 3D X-ray microscopy imaging system was used for computed tomography scanning of coal samples. The crack characteristics and stress–permeability relationships were analyzed based on the experimental results. It was found that the permeability increased approximately by 5.8 times after the loading–unloading cyclic tests. The fracture volume and rate grew obviously, and the complexity degree of the fracture network increased. The permeability evolution during the test was closely related to stress–strain state of the coal sample with variable response characteristics of axial deviatoric stress, confining pressure, and axial pressure. The realization of different numbers of loading–unloading cycles in the tests revealed similar fracturing effects of the initial and later cycles, whereas the largest fracturing effect was obtained within the effective stress range from 4 to 11 MPa. The mechanical deformation was found to play an important role in the stress–permeability relationship. The crack closure and elastic deformation at low stress-state caused the permeability loss, while crack growth and plastic deformation at high-stress state increased the permeability. The plastic strain significantly grew with the number of loading–unloading cycles, but the fracturing effect of different numbers of cycles in the critical fissure point was nearly the same. The results obtained are considered instrumental in studying the pressure relief and extraction of coalbed methane during upper protective layer mining in coal seam groups.

Effect of Filling on Failure Characteristics of Diorite with Double Rectangular Holes Under Coupled Static–Dynamic Loads

Abstract: The effect of infilling on mechanical response and crack behavior of pre-stressed rock with double rectangular holes under dynamic load was investigated through a series of coupled static–dynamic loading tests on diorite specimens in three different filling states, i.e., no infilling, single infilling and double infilling, using a modified split Hopkinson pressure bar. The deformation, damage and fracture process of specimens were recorded and analyzed by low-speed and high-speed cameras with digital image correlation method. Test results reveal that under the same dynamic load, the strength of specimens increases first and then decreases with the increase of axial static pre-stress, and reaches the maximum under 25% UCS due to crack closure. The strengthening effect of double infilling on strength is much more significant than that of single infilling and increases with the pre-stress. Observations show that with the increase of pre-stress, the degrees of damage and strain localization of specimens increase, and the superficial damage on the free surface of sidewalls are more severe. The effect of infilling is significant on the crack initiation and propagation, especially on the inhibition of sidewall spalling, rock ejection and the axial displacement failure of rock septum between holes. With the increase of pre-stress, the failure pattern and the crack coalescence mode change from obvious tensile to shear, and the coalescence position changes from the inside to outside of the septum area. However, the change of filling state has little effect on the coalescence mode, only on the coalescence position.

Experimental Investigation of Thermal Effect on Fracability Index of Geothermal Reservoirs

Abstract: The thermal effect is not considered in the current fracability index model. In this paper, we investigated experimentally the formation capacity of fracture networks in hot dry rocks at different temperatures, by using rock mechanics triaxial testing system and acoustic emission monitoring. A new brittleness index with consideration of thermal effect is proposed on the basis of crack volumetric strain calculated at different stress levels, which captures the following two characteristics: (1) thermal micro-crack density in the crack closure stage and (2) crack propagation, nucleation and coalescence by external stress. Then, a comprehensive fracability index model is established, which integrates the thermal effect and fracture toughness at different temperatures and confining pressures. The fracability index calculated by the new model is consistent with acoustic emission characteristic, microstructure analysis and b-value analysis, which verify that the proposed model is reliable. The results show that both temperature and confining pressure have a significant effect of formation capacity of fracture networks in hot dry rocks. This study provides new insight into improving the thermal efficiency in geothermal reservoirs.

Autogenous Self-Healing Capacity of Early-Age Ultra-High-Performance Fiber-Reinforced Concrete

Abstract: This paper analyzes the autogenous self-healing capacity of early-age Ultra-High-Performance Fiber-Reinforced concretes (UHPFRCs) by measuring the crack closure and the possible mechanical recovery on healed specimens. The main parameters considered in this research were the healing exposure conditions (humidity chamber, immersion in tap water, immersion in seawater and heat curing) and the precracking levels (microcracks and macrocracks). For the microcrack level, four-point bending tests were performed on prismatic specimens (100 × 100 × 500 mm3) obtaining a multiple cracking pattern characterized by crack widths ranged from 10 to 20 µm. Whereas for the macrocrack level (behavior after crack localization), splitting tests were carried out on notched cubic specimens (100 × 100 × 100 mm3) obtaining crack widths of up to 0.4 mm. For both precracking levels, specimens were precracked at two days and were cured for one month in the mentioned exposure conditions. Healing products were analyzed on the specimen surface and also inside the cracks; to this purpose, their microstructure was analyzed by means of SEM and EDS analyses. The results have shown that the highest crack closure values were obtained for the heat-cured specimens and for the specimens immersed in water (tap water and seawater) whereas the less efficient condition was the humidity chamber.

Measurement of fracture parameters based upon digital image correlation and virtual crack closure techniques

Abstract: This study aims to develop a novel approach to characterizing fracture parameters of cracked structures made of composite laminates and brittle adhesive joints by using digital image correlation (DIC) and virtual crack closure technique (VCCT). The parameters of mixed-mode fracture can be identified properly using the present method, which allows characterizing the important fracture properties of composite laminates, such as mixed parameter η in the Benzeggagh-Kenane fracture criterion. The energy release rates (ERR) of cracked structure is formulated in a context of linear elastic fracture mechanics (LEFE) based upon DIC measurement, finite element analysis and VCCT. Explicit formulae of energy release rates are derived in terms of the full-field displacements measured from DIC. To verify the effectiveness of the proposed method, the joint specimens are prepared and tested subject to the double cantilever beam, end-notched flexure and single-leg bending conditions, respectively. Energy release rates and fracture toughness for each specimen are obtained by using the proposed approach, which are compared with those obtained from the analytical solutions and finite element analysis (FEA). For showcasing the potential of further applications, the present method is also employed to acquire the fracture parameters of composite laminate, which contains both inter-laminar and intra-laminar fracture modes. The new parametric identification approach exhibits excellent agreement with the other methods. The proposed DIC - VCCT based method demonstrates the effectiveness for determining the energy release rates of various cracked structures.

Influence of interface ply orientation on delamination growth in composite laminates

Abstract: The standard experimental procedures for determining the interlaminar fracture toughness are designed for delamination propagation in unidirectional specimens. However, in aerospace structural components, delamination usually occurs between plies at different orientations resulting in different damage mechanisms which can increase the value of the fracture toughness as the delamination propagates. Generally, numerical analyses employ the value measured at the delamination onset, leading to conservative results since the increase resistance of the delamination is neglected. In this paper, the fracture toughness and the R-curves of carbon/epoxy IM7/8552 are experimentally evaluated in coupons with delamination positioned at 0°/0° and 45°/−45° interfaces using Double Cantilever Beam (DCB) and Mixed-Mode Bending (MMB) tests. A simplified numerical approach based on the Virtual Crack Closure Technique (VCCT) is developed to simulate variable fracture toughness with the delamination length within a Finite Element code using a predefined field variable. The results of the numerical analyses compared with the experimental data in terms of load-displacement curves demonstrate the effectiveness of the proposed technique in simulating the increase resistance in delamination positioned between plies at 45°/−45° interface.