Increasing the Performance of a Fiber-Reinforced Concrete for Protective Facilities

The use of fiber in cement materials is a promising and effective replacement for bar reinforcement. A wide range of fiber-reinforced concretes based on composite binders with increased impact strength characteristics have been developed. The synthesized composites included the composite binder made of Portland cement, silica, and carbonate additives. Basalt and steel were used as fibers. The nature of the influence of the composition and manufacturing technology of cement composites on the dynamic hardening coefficient has been established, while the growth of these indicators is achieved by creating a denser interfacial transition zone between the cement paste, aggregate, and fiber as a result of improving the homogeneity of the concrete mixture and controlling the consistency. Workability indicators (slump flow up to 730 mm; spreading time up to a diameter of 50 cm is up to 3 s) allow them to be classified as self-compacting concrete mixtures. An increase in the values of the impact strength coefficient by a factor of 5.5, the dynamic hardening coefficient by almost 70% as a result of interfacial interaction between fibers and binder matrix in the concrete composite, as well as absorption of impact energy by fiber, was revealed. The formula describing the effect of the loading rate on the coefficient of dynamic hardening of fiber-reinforced concrete has been refined. The fracture processes of the obtained materials have been established: after the initiation of primary cracks, the structure of the composite absorbs impact energy for a long time, while in the inelastic range (the onset of cracking and peak loads), a large number of secondary cracks appear.

Study on the causes of secondary cracks of the eave wall mural of Daxiong Hall at Fengguo Temple in Yixian, Liaoning, China

Built in Kaitai 9 years after the beginning of the Liao Dynasty (1020 A.D.), the Daxiong Hall of Fengguo Temple (Yixian County, Liaoning Province) is one of China’s largest existing ancient single-eave wooden architecture structures. In 2012, it was listed on the “Preliminary List of World Cultural Heritage in China.” Preserved Buddhist murals depicting the Yuan Dynasty cover approximately 470 m2 of the hall’s four walls. Since the in-situ reinforcement and protection of the mural, conducted in the 1980s in cooperation with the maintenance and restoration project of the main hall, seriously developed cracks—known as secondary cracks—have become a primary factor affecting the mural’s structural stability. In this study, we conducted a comprehensive investigation using a full-frame digital camera, an industrial endoscope, an infrared thermal imager, an online environmental monitoring system, and a three-dimensional laser scanner. Our results, and other relevant materials, allowed us to deepen our understanding of the existing structural features, the nature of the cracks, the deformation conditions, and the environmental characteristics of the mural. Moreover, we provide a further discussion on the macroscopic formation process of the secondary cracks.

Numerical Simulation of Fracturing in Coals Using Water and Supercritical Carbon Dioxide with Potential-Based Cohesive Zone Models

The Park-Paulino-Roesler (PPR) cohesive zone model (CZM) for coal was established for analyzing mixed-mode I/II fractures using semicircular specimens under punch-through shear (PTS) and three-point bending (SCB) tests. In these methods, the main parameters of the fracture were obtained through SCB tests and PTS tests. And according to the experimental results, the coal specimens show obvious characteristics of ductile fracture under mode I and II loading. Moreover, hydraulic and supercritical carbon dioxide (ScCO2) fracture tests were conducted, and accordingly, it was found that the crack initiation pressure of coal specimens for hydraulic fracturing is 17.76 MPa, about 1.59 times that driven by ScCO2. And the crack initiation time of coal with ScCO2 fracturing is 123.73 s, which is 1.58 times that for hydraulic fracturing. A macrocrack eventually formed in the coal specimen due to the hydraulic drive, which penetrated through the entire specimen. Yet, there was no crack penetrating the whole fracture specimen and several widely distributed secondary cracks in the fractured coal specimens by ScCO2. Furthermore, zero-thickness pore pressure cohesive elements were utilized to investigate multicrack propagation in coals undergoing hydraulic and ScCO2 fracturing. The constitutive relationships of the established PPR CZM were introduced into the cohesive elements. The obtained results are consistent with the hydraulic and ScCO2 fracturing experiment results for the coal specimens. This indicates that the established PPR CZMs can accurately represent the crack propagation behavior in coals for hydraulic and ScCO2 fracturing.

A Modified Phase-Field Fracture Model for Simulation of Mixed Mode Brittle Fractures and Compressive Cracks in Porous Rock

In this work, we propose a modified phase-field model for simulating the evolution of mixed mode fractures and compressive driven fractures in porous artificial rocks. For the purpose of validation, the behaviour of artificial rock samples, with either a single or double saw cuts, under uniaxial plane strain compression has been numerically simulated. The simulated results are compared to experimental data, both qualitatively and quantitatively. It is shown that the proposed model is able to capture the commonly observed propagation pattern of wing cracks emergence followed by secondary cracks driven by compressive stresses. Additionally, the typical types of complex crack patterns observed in experimental tests are successfully reproduced, as well as the critical loads.

Study on Acoustic Emission Characteristics and Mechanical Behavior of Water-Saturated Coal

In terms of coal’s stability and failure, soaking time and water content play a significant role in geotechnical engineering practice. To determine the soaking time effect on the mechanical behavior of coal samples and the response of AE (acoustic emission) signal throughout loading, the samples with different soaking times (0–120 hours (h)) were prepared and tested under uniaxial compression. AE signals were continuously monitored during loading to examine the AE characteristic response via the AEwin Express-8.0 system. The results revealed that the mechanical characteristics of the coal samples decreased with an increase in soaking time. When coal samples were subjected to uniaxial compression, AE events occurred due to the formation of the cracks, which further propagated to cause coal fracture. AE counts and the accumulative counts fluctuated with time and corresponded very well to the load. Therefore, AE counts and the trend of the accumulative counts of AE qualitatively explained the rupture of the coal under stress. In addition, the variation in trends of AE counts, AE accumulative counts, and load with time at various phases of all samples were obtained. It is concluded that AE counts increase suddenly during a slow increase phase and peak at the active increase phase. During the attenuation phase, the AE counts first decrease significantly with stress drop, but also a slight increase was observed due to the initiation of secondary cracks. These research results are of great significance as a precursor in coal and rock failure.

Study on the Effect of Precrack on Specimen Failure Characteristics under Static and Dynamic Loads by Brazilian Split Test

To analyse the dynamic failure characteristics of the rock with a crack in rock engineering, the Brazilian split tests were conducted on the split Hopkinson pressure bar (SHPB) using precrack specimens under dynamic loads. In the study, five groups of different precrack angles are selected; they are 0°, 30°, 45°, 60°, and 90°, respectively. The results show that the static failure load of the specimen as a whole decreases to increase with the growth of the loading angle, and the DIF linear increases with the increase of the loading rate; the failure load of the specimen with an angle of 45° precrack is the most sensitive to the loading rate, followed by 0°, 60°, 30°, and 90°. The crack initiation time of specimen with 30°, 45°, and 60°precrack decreases with the loading rate, while it has no obvious change with the loading rate with 0° and 90°precrack. The failure mode of the specimen was controlled by the stress concentration at the crack tip; the main cracks all point from the crack tip to the loading end. When the precrack and the loading direction are at a certain angle, the failure process will produce secondary cracks; it would be particularly obvious under dynamic load splitting. Once the precrack and the loading direction are at a certain inclination angle, type-II secondary cracks will develop under dynamic load splitting.

Crack Propagation Law and Failure Characteristics of Coal-Rock Combined Body with the Different Inclination Angle of Prefabricated Fissure

Few studies have been conducted on the crack propagation law and failure characteristics of coal-rock combined body (CRCB) with prefabricated fissure. A sliding crack model was firstly presented to analyze the failure law of rock with a single fracture and the influence of the inclination angle of the fracture on the strength of the rock. The RFPA numerical models of the CRCB with different inclination angles of prefabricated fracture were then established to simulate the dynamic change process of crack propagation and shear stress of the CRCB with prefabricated fracture under uniaxial compression. The influence of the inclination angle of the fracture in the rock on the fracture expansion and failure characteristics of CRCB was further analyzed based on the acoustic emission data. The results showed that (1) when 2 β = arctan 1 / μ , σ cw takes the minimum value, and crack initiation is most likely to occur; (2) the strength of coal-rock assemblage shows different changing trends with the fracture inclination angle; (3) the secondary cracks of CRCB with prefabricated fracture of 0°, 15°, and 30° initiated and expanded near the tip of the main crack, and the secondary cracks of 45°, 60°, and 75° initiated and expanded from the tip of the main crack; (4) there are three failure modes of CRCB with prefabricated crack, the double-shear failure mode Λ , the tensile-shear composite failure mode along the fracture surface, and the tensile failure mode along the fracture surface; and (5) intact CRCB and CRCB with prefabricated crack when α = 75 ° and α = 90 ° have strong brittleness, and other CRCB with different prefabricated fracture inclination angles show a certain degree of postpeak plasticity. The results on the mechanical properties and damage characteristics of CRCB are of great significance for the safety and efficient mining of deep coal resources.

Directional hydraulic fracturing technology for prefabricated longitudinal guide seams in a coal mine tight sandstone roof

Triaxial hydraulic fracturing experiments were used to study the initiation pressure variation and acoustic emission characteristics of different guide seams sizes during roof hydraulic fracturing. Numerical simulations were used to explore the feasibility of multiple boreholes with prefabricated guide seams. An experiment of hydraulic fracturing on a pillar-free working face was also carried out in a coal mine. The results show that the specimens with guide seams reduced the initiation pressure, with the number of acoustic emission events and initiation pressure being inversely proportional to the size of the guide seams. Specimens without guide seams were deflected by stress and produced multi-level cracks, while the specimens with guide seams did not produce large secondary cracks and deflection. When the stress difference was small, three holes penetrated but not under large stress differences. The hydraulic fracturing technology of prefabricated longitudinal guide seams was tested in the Ningtiaota Coal Mine, and the auxiliary transportation roadway of S1201 working face was successfully retained for reuse in adjacent working faces.

Effect of solution aggressiveness on the crack - growth resistance and cracking mechanism of AA2024-T3

Aluminium alloy 2024-T3 was examined – using a range of microscopy techniques – at the early stages of corrosion attack to investigate the corrosion-induced cracking mechanism. Two different corrosive environments, exfoliation corrosion (EXCO) and 3.5 % wt. NaCl, were used for the exposure of tensile and pre-notched compact-tension C(T) specimens of AA2024-T3. Different embrittlement mechanisms are noticed for the two investigated corrosive environments. Significant intergranular corrosion (IGC) and grain boundary embrittlement is evident in the specimens exposed to EXCO solution, while this was not the case for the milder solution; comprising of 3.5 % wt. NaCl. With regards to the milder solution, corrosion attack is not restricted to the grain boundary, but evolves transgranularly to the neighbouring grains of the IGC attacked region and, consequently, the grain boundary strength in the direct vicinity is not notably affected. The extent of secondary cracks – after the exposure of C(T) specimens to EXCO solution and the subsequent crack-growth resistance evaluation – were found to correlate with the diameter of the plastically affected zone (≈ 3.78 ± 0.04 mm). Additionally, the depth of these cracks was found to correlate well with the thickness of the intergranular fracture surface, giving evidence that the secondary cracks form due to grain boundary embrittlement; probably attributed to hydrogen embrittlement phenomena.

Study on the Causes of Secondary Cracks of the Eave Wall Mural of Daxiong Hall at Fengguo Temple in Yixian, Liaoning, China

Built in Kaitai nine years of Liao Dynasty (1020 A.D.), the Daxiong Hall of Fengguo Temple in Yixian County, Liaoning Province, is one of the largest existing ancient single-eave wooden structure in China. There are nearly 470m 2 murals of Yuan Dynasty on the four walls of the hall. Since the in-situ reinforcement and protection of the mural were carried out in the 1980s in cooperation with the maintenance and restoration project of the main hall, the seriously developed cracks have become the primary factor affecting the structural stability of the mural. In order to find out the macroscopic causes of secondary cracks, endoscope, infrared thermal imager, three-dimensional laser scanner and other equipment were applied to study the relations between cracks’ attitude and structural defects of the eave wall, as well as the pathway that external environment act on the mural noumenon through comprehensive investigation, and the idea of restoring the stability of the mural by blocking the air channel and offsetting the environmental stress is put forward.