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Considering that fluctuations in temperature can cause variations in both the internal structure as well as the mineral composition of rocks, their fracture characteristics must be impacted accordingly. With the exponential development of geotechnical engineering in cold regions, it is urgent to study the influence of the sub-zero temperature environment on the mechanical properties and dynamic properties of rocks. In order to investigate the influence of sub-zero temperature gradient on the dynamic fracture characteristics of rocks, red sandstone was used for the preparation of notched semi-circular bend specimens. First, a water-saturated machine and a sub-zero temperature incubator were utilized to pretreat the rock for 48 h, conducive for both satiation and freezing processes. Subsequently, the dynamic tests were carried out utilizing an improved split Hopkinson bar experimental system with a cryogenic sub-system. Concurrently, the striker velocity was modulated by setting distinctive air pressures, following which the rock was loaded at various loading rates. The test results demonstrate that the fracture toughness of the rock has an evident loading rate effect, and the fracture toughness proliferates exponentially with the increase in the loading rate. In the event that the loading rate is certain, the fracture toughness of the rock primarily increases gradually and then expeditiously over the course of advancement from room temperature to −20 ℃. Contradictorily, the rock fracture toughness diminishes abruptly with plummeting temperature. Analysis of the rock fracture process, accommodated by a high-speed camera, revealed that the fracture process of the rock at distinctive temperatures is fundamentally equivalent, and the crack propagation speed is negligibly influenced by the temperature. Furthermore, the rock fracture mode was analyzed by employing a scanning electron microscope (SEM) system. The SEM images of the rock depicted that the fracture of red sandstone at sub-zero temperature is predominantly intergranular fracture and cement tearing, accompanied by a trace of transgranular fracture. Meanwhile, the experimentation also revealed that the number of micro-cracks in the rock significantly multiplied when the temperature declined to −25 ℃, illustrating that sub-zero temperature has a deteriorating effect on the rock. Conclusively, the influence mechanism of temperature on the internal structure of the rock is discussed, and it is assumed that the change in the internal structure of the rock is the collaborative effect of thermal expansion-cold contraction and ice-water phase transition. The interpretation of this study has substantial reference significance for the further consequential analysis of frigidity on the fracture properties of the rock.

The conventional fracture in shale hydraulic fracturing belongs to the type-I fracture, and the size of the fracture process zone (FPZ) is an important index to measure the fracability of rock mass. This index is also one of the feasible entry points to study the complexity of the fracture network. In order to visually observe the type-I FPZ at the tip of shale fractures, and to study the relationship between the mechanical properties, the shape and size of the FPZ, and the bedding structure, Notched Semi-Circular Bend (NSCB) tests were conducted with three typical fracture direction-bedding orientations (splitter, arrester, divider). The digital image correlation (DIC) method was used to realize the intuitive observation of the real fracture process and the FPZ near the fracture tip. The test found that the FPZ of shale is narrow and long as a whole and is "flame-like". The height-to-length ratio of the FPZ at the fracture tip determines whether bending and deflection happen between the new fracture and the prefabricated cracks when the fracture occurs. Most of the specimens often appear in the FPZ with a beaded high shear strain zone before the fracture, which is caused by the oblique communication of micro-cracks in the FPZ before the fracture. The appearance of a beaded zone of high shear strain indicates that macroscopic fracture is imminent. The research results can be used for the design of disaster early warning and prevention programs. [ABSTRACT FROM AUTHOR]

• The crack propagation process and its strain evolution were captured using the split Hopkinson pressure bar (SHPB) device combined with high-speed digital image processing. • The relationship between dynamic mode Ⅰ fracture toughness and crack propagation velocity of bedding shale with bedding and loading rate was analyzed qualitatively and quantitatively. • Based on the Traction-Separation Law, the method of global cohesive element was used to simulate the dynamic crack propagation and calculate the fracture toughness of bedding shale. The fracture behavior and various of mechanical mechanisms of shale are greatly influenced by the bedding surface. To study the effects of bedding angles on the crack propagation patterns and dynamic fracture properties, the shale notched semi-circular bend (NSCB) specimens were studied by the modified split Hopkinson pressure bar (SHPB) device, three-dimensional digital image correlation (3D-DIC) system with the high-speed (HS) photography and ABAQUS finite element software. The research results showed that at the same impact velocity, the 30°, 45° and 60° beddings played an obvious deflexion effect on the shale crack propagation. However, the 0° and 90° bedding did not deflect the crack propagation. The dynamic fracture toughness of shale NSCB specimens increased with the increase of impact loading rate but the increase magnitude decreased with the higher impact rate. The kinetic energy during specimen fracture was calculated by the extracting displacement data from the 3D-DIC, and it was found that the kinetic energy accounted for less than 6% of the absorbed energy during the dynamic compression of the shale. The crack propagation characteristics of ABAQUS simulation was approximately consistent with the experimental results. It is conducive to the study of shale gas fracturing and the formation of complex seam networks in shale and it is of great research significance for the efficient extraction of shale gas. [ABSTRACT FROM AUTHOR]

The conventional fracture in shale hydraulic fracturing belongs to the type-I fracture, and the size of the fracture process zone (FPZ) is an important index to measure the fracability of rock mass. This index is also one of the feasible entry points to study the complexity of the fracture network. In order to visually observe the type-I FPZ at the tip of shale fractures, and to study the relationship between the mechanical properties, the shape and size of the FPZ, and the bedding structure, Notched Semi-Circular Bend (NSCB) tests were conducted with three typical fracture direction-bedding orientations (splitter, arrester, divider). The digital image correlation (DIC) method was used to realize the intuitive observation of the real fracture process and the FPZ near the fracture tip. The test found that the FPZ of shale is narrow and long as a whole and is “flame-like”. The height-to-length ratio of the FPZ at the fracture tip determines whether bending and deflection happen between the new fracture and the prefabricated cracks when the fracture occurs. Most of the specimens often appear in the FPZ with a beaded high shear strain zone before the fracture, which is caused by the oblique communication of micro-cracks in the FPZ before the fracture. The appearance of a beaded zone of high shear strain indicates that macroscopic fracture is imminent. The research results can be used for the design of disaster early warning and prevention programs.

Dynamic fracture failure of rocks subjected to static hydrostatic pressure is commonly encountered in deep underground rock engineering. The static fracture behavior of rocks under hydrostatic stress has been well studied in the literature. However, it is desirable to investigate the dynamic fracture failure of rocks under various hydrostatic pressures. In this study, a triaxial split Hopkinson pressure bar (SHPB) system is used to measure the dynamic fracture toughness of rocks under five hydrostatic pressures. The results show that dynamic fracture toughness under a certain hydrostatic pressure enhances with the increase of the loading rate, and the dynamic fracture toughness at the similar loading rate increases with the hydrostatic pressure due to the closure of microcracks in rocks. An empirical formula is proposed to describe the influences of the loading rate and the hydrostatic pressure on the rock dynamic fracture toughness. [ABSTRACT FROM AUTHOR]

• The dynamic mechanical properties and fracture energy of the NSCB samples is presented. • All fracture modes of concrete exhibit the mode Ⅰ failure in NSCB tests. • The dynamic fracture energy shows a significant dependence on loading rate, increasing with the loading rate. • The repaired specimens consume more energy than the notched specimens and have a lower kinetic energy of flying fragments. Concrete constructions commonly contain cracks or defects during forming and service, repair operations are necessary in many situations. However, limited investigations conducted on the dynamic fracture toughness (DFT) of repaired concrete structures. This paper investigates the cracking behavior of concrete structures through notched dynamic semi-circular bend (NSCB) tests performed using a split Hopkinson pressure bar (SHPB) apparatus with two measurement techniques, namely digital image correlation (DIC) and crack propagation gauge (CPG), to capture the flying half angular velocity and crack propagation speed of NSCB concrete specimens under various loads. The results demonstrate a linear increase in both DFT and crack propagation speed of concrete with the loading rate. Additionally, a threshold value exists beyond which the crack speed stabilizes, and the DFT is not evidently influenced by the notch depths. The cracks propagate along the aggregates and remain mostly intact under a relatively low loading rate. With an increased loading rate, the damage pattern of some otherwise deflected aggregates changes resulting in crack penetration and significantly reducing the fracture surface roughness. Repaired specimens exhibit greater energy dissipation at the same loading rate, thereby producing flying fragments with lower kinetic energy, which can prevent secondary damage to internal personnel and structures. [ABSTRACT FROM AUTHOR]

Highlights • The dynamic fracture toughness anisotropy of black shale is studied via SHPB system. • Fracture toughness of transversely isotropic black shale is calculated by FEM with the J-integral method. • Variations of fracture toughness versus loading rate and loading angle are obtained. • Two envelope lines are proposed to enclose the variation of fracture toughness with loading rate. Abstract This paper investigates the dynamic fracture toughness anisotropy of black shale (BS). Notched semi-circular bend (NSCB) BS specimens are prepared with different loading angles (the angle between the stratification plane and the loading direction). NSCB specimens are dynamically loaded using a modified split Hopkinson pressure bar (SHPB) system. Formulae for calculating mode-I fracture toughness of transversely isotropic material are obtained by finite element method (FEM) in combination with the J-integral method. Dynamic fracture toughness of the NSCB sample is calculated using stress waves recorded in SHPB tests and the obtained formulae. The results show that the fracture toughness of specimens for all loading angles exhibits obvious loading rate dependence, with the fracture toughness increasing with the loading rate. Similar to the static case, the specimens with higher loading angle have larger fracture toughness values as compared with ones with lower loading angles under similar loading rates. We proposed two envelope lines to enclose the fracture toughness of specimens under different loading rate. The difference between two envelope lines decreases with the increase of the loading rate. The anisotropy of black shale diminishes under extreme high loading rate. Anisotropy index, defined as the ratio of the maximum to the minimum of fracture toughness, is equal to 1.75 under static loading and 1.52 under smaller dynamic loading. These conclusions may have significant implications to the design of hydraulic fracturing in shales, e.g., fracture fluids pumping could be considered as static loading and perforation can be considered as dynamic loading. The anisotropy of shales should be considered in both these cases. [ABSTRACT FROM AUTHOR]

This study investigated the effect of water content on quasi-static fracture behavior of sandstone. Notched semi-circular bending (NSCB) tests were conducted on a total number of 20 sandstone specimens with different water contents (0, 1.0, 2.0 and 3.5%) to determine their fracture toughness. During the NSCB tests, the cracking process and acoustic emission (AE) signals were recorded continuously with the aid of a charged couple discharge (CCD) camera and an AE system, and the crack propagation velocity was also measured accurately via a crack propagation gauge (CPG). Test results demonstrated that both the fracture toughness and crack propagation velocity observably decreased with the increase of water content, the variation trend of which could be described by exponential equations. The cumulative AE counts of wet specimens in the NSCB tests were much less than those of dry ones, which indicated that the sandstone specimens underwent more ductile failure and released less elastic energy due to water-softening effects. [ABSTRACT FROM AUTHOR]

• The mixing of PVA and MSF can retain the strain hardening ability of MKPC after high temperature • The dynamic fracture toughness were calculated by finite element based on the parameters. • The dynamic fracture toughness reaches the minimum at 500 °C, but increases at 800 °C and 1100 °C. • The dynamic fracture toughness of PVA/MSF-MKPC increases with the increase of loading rate. • Crack arrest toughness is greater than initial toughness than extension toughness. In order to improve the high-temperature resistance of engineering cementitious composite (ECC), magnesium potassium phosphate cement (MKPC) was used to replace portland cement, and micro steel fiber (MSF) was used to replace polyvinyl alcohol (PVA) fiber in equal volume. And the fiber-reinforced potassium magnesium phosphate cement matrix composites (MSF/PVA-MKPC) were prepared. The microstructure of the MKPC matrix and the condition of the fibers after subjecting to high temperature were observed by scanning electron microscope (SEM), and the split Hopkinson pressure bar (SHPB) was used to conduct the impact test on notched semi-circular bend (NSCB) specimens after thermal treatment. The dynamic propagation behavior of mode I crack of MSF/PVA-MKPC after heat treatment was studied by crack propagation gauge (CPG) and experimental–numerical method. The results showed that: (1) MSF/PVA-MKPC remained intact after high temperature exposure and did not completely lose the strain hardening and strain softening ability; (2) Dynamic fracture toughness is negatively correlated with crack propagation speed; (3) Dynamic fracture toughness increases with the increase of loading rate and first decreases and then increases with the increase of maximum exposure temperature; (4) Crack arrest exists in the process of crack propagation, and the crack arrest toughness is greater than the initial toughness and the average propagation toughness. [ABSTRACT FROM AUTHOR]

A dynamic load superposed on a static pre-load is a key problem in deep underground rock engineering projects. Based on a modified split Hopkinson pressure bar test system, the notched semi-circular bend (NSCB) method is selected to investigate the fracture initiation toughness of rocks subjected to pre-load. In this study, a two-dimensional ANSYS finite element simulation model is developed to calculate the dimensionless stress intensity factor. Three groups of NSCB specimen are tested under a pre-load of 0, 37 and 74 % of the maximum static load and with the loading rate ranging from 0 to 60 GPa m s. The results show that under a given pre-load, the fracture initiation toughness of rock increases with the loading rate, resembling the typical rate dependence of materials. Furthermore, the dynamic rock fracture toughness decreases with the static pre-load at a given loading rate. The total fracture toughness, defined as the sum of the dynamic fracture toughness and initial stress intensity factor calculated from the pre-load, increases with the pre-load at a given loading rate. An empirical equation is used to represent the effect of loading rate and pre-load force, and the results show that this equation can depict the trend of the experimental data. [ABSTRACT FROM AUTHOR]

• The porosity of rock samples increases with the number of freeze–thaw cycles, while static and dynamic fracture toughness decreases, indicating that freeze–thaw cycles degrade the microstructure and adversely affect the mechanical properties. • The fractal dimensions increased with the freeze–thaw cycles increased. The freeze–thaw effect increases the roughness of fracture surface, which corresponds to the decrease of fracture toughness. • The dynamic fracture toughness increases with the increase of loading rate. This indicates that the freeze–thaw rock under high dynamic load has greater fracture resistance than that under slow dynamic load. In this study, the dynamic fracture toughness of freeze-thawed granite was studied. Different freeze–thaw (F-T) cycles were used to treat notched semicircular bending (NSCB) granite samples. The microstructure changes of samples treated with different numbers of F-T cycles were measured by a nuclear magnetic resonance (NMR) testing system. The dynamic fracture toughness was obtained by dynamic testing with split Hopkinson pressure bar (SHPB) equipment. In addition, the roughness of the rock fracture surface was studied using fractal geometry, and the F-T damage was defined using fractal dimensions. On this basis, a dynamic fracture toughness prediction model is proposed. The influence of the F-T cycle and loading rate on fracture toughness are discussed. [ABSTRACT FROM AUTHOR]

The basic properties of reef limestone were measured to explore the dynamic fracture characteristics of reef limestone. The test results show that the physical and mechanical properties of blocky reef limestone are linearly related to the density. Hence, accurate mechanical parameters can be ascribed to standard mechanical specimens. Next, the fracture toughness of the reef limestone was measured using notched semi-circular bend (NSCB) specimens. Meanwhile, fracture energy and characteristics were studied with the help of a high-speed camera and scanning electron microscope. The findings show that the fracture toughness and energy of reef limestone are linearly related to the density with a rate effect. Moreover, the dynamic compressive strength is also linearly correrlated with the dynamic fracture toughness of reef limestone under the same loading rate; the fracture toughness of saturated reef limestone is larger than that of dry reef limestone. Besides, reef limestone has a rough fracture surface for its various crystal shapes and arrangements, with cracks propagating along the interface of varied crystals. • For blocky reef limestones, its physical and mechanical parameters have a linear relationship with density. • The fracture surface of reef limestone is rough due to its complicated crystal shape and arrangement. • The fracture toughness of reef limestone increases with the increase of density and has obvious rate effect. [ABSTRACT FROM AUTHOR]

Loading rate is the main controlling factor in dynamic failure of rocks. In this paper, digital image correlation (DIC) combined with ultra-high speed photography is utilized to study the loading rate effect of a granitic rock – Laurentian granite using notched semi-circular bend (NSCB) method. The dynamic stress intensity factors and crack tip positions are determined from the displacement fields obtained using DIC. Fracture time, fracture toughness and crack growth velocity all exhibit loading rate dependence. The dependence of fracture propagation toughness on crack growth velocity is also obtained, which is in good agreement with that reported in the literature. [ABSTRACT FROM AUTHOR]

Coupled static-dynamic tests were carried out on notched semi-circular bend (NSCB) granitic specimens using a modified split Hopkinson pressure bar (SHPB) device, to investigate the effects of axial pre-force and loading rate on dynamic fracture behavior of rocks. Digital image correlation (DIC) method was adopted to obtain the deformation field and fracturing process. The results indicate that the pre-force (up to 50% static peak force) enhances the fracture parameters overall, including fracture initiation toughness, fracture energy, crack propagation velocity (CPV) and fracture propagation toughness, but the effect of loading rate is more significant than that of the pre-force. Digital images show that the increase in pre-force would cause the dominant crack to symmetrically bifurcate since the pre-force strengthened a fan-shaped area for the specimen near the incident bar. The loading rate larger than 124 GPa·m1/2/s could lead to the secondly fracture of the specimen and thereby more fragments. Compared with previous studies, the present investigation deepens the understanding of fracture characteristics of rocks under coupled static and dynamic loads by further reducing the pre-force interval (0.25 kN). In addition, a new method based on the DIC technique is proposed to calculate crack opening distance (COD) and CPV, which can improve the efficiency and accuracy compared to the strain gauge method. Moreover, fracture propagation toughness is also emphasized apart from the initiation toughness. The findings can provide some enlightenments for rock fracture dynamics and crack propagation of highly stressed rocks. [ABSTRACT FROM AUTHOR]

The effect of thermal treatment on the dynamic fracture toughness of Laurentian granite (LG) was investigated in this work. Notched semi-circular bend (NSCB) LG specimens are heat treated at temperatures up to 850 °C. The micro-cracks in the rock samples induced by thermal treatment are examined by scanning electron microscope (SEM). The microscopic observations are consistent with the subsequent P-wave velocity measurements, which shows that the P-wave velocity decreases with the treatment temperature monotonically when the temperature is higher than 250 °C. Dynamic fracture toughness measurements are then carried out on these samples with the dynamic load exerted by a modified split Hopkinson pressure bar (SHPB) system. The relationship between fracture toughness and treatment temperature is investigated. Experimental results show that fracture toughness increases with the loading rate but decreases with the treatment temperature. However, when the heating temperature is below 250 °C and above 450 °C, the dependence of dynamic fracture toughness on the temperature is different from other temperatures, which can be explained by the physical processes at the microscopic level of the rock due to heating. At treatment temperatures below 250 °C, the thermal expansion of grains leads to an increase in the toughness of the rock. At treatment temperatures above 450 °C, the sources of weakness such as grain boundaries and phase transition of silicon are depleted, and as a result the decrease in fracture toughness is not as significant as other treatment temperature ranges. [ABSTRACT FROM AUTHOR]

Beishan area in Gansu province of China has been listed as a major candidate for underground constructions of high-level radioactive waste repository. Therefore, to explore the dynamic responses of Beishan granite is important to the related excavation engineering. This paper investigates the rate dependent effect on the dynamic fracture behaviors of the Beishan granite. The notched semi-circular bend (NSCB) and cracked chevron notched semi-circular bend (CCNSCB) specimens are prepared with speckles uniformly distributed on surface for digital image correlation (DIC) image processing. An ultra-high-speed camera is employed for the modified split Hopkinson pressure bar (SHPB) system to capture the complete cracking process of the rock specimen. The dimensionless stress intensity factors (SIFs) of NSCB and CCNSCB specimens are calculated numerically by the J-integral method in ABAQUS simulation software. To systematically reveal the dynamic fracture mechanisms of Beishan granite specimens, the key dynamic fracture parameters including the dynamic fracture toughness, the evolution of dynamic stress intensity factor (DSIF) during fracturing and the dynamic fracture energy are obtained for further investigations and comparisons. The results show that the dynamic fracture toughness of NSCB and CCNSCB specimens exhibits a distinct rate dependence-the fracture toughness increases with the loading rate rises. Furthermore, the toughness value of the CCNSCB specimen are mostly higher than that of the NSCB specimen due to a more severe stress concentration level near the notch tip. The evolutions of DSIFs of Beishan NSCB and CCNSCB specimens are quite similar to each other, and the corresponding variation curves are all subjected to three typical developing phases until specimens are totally fractured. The dynamic fracture energy of NSCB and CCNSCB specimens is relatively consistent and shows a positive relationship with the loading rate. However, the fracture energy absorption rate (defined as the ratio between the fracture energy and the energy generated in the incident bar) gradually decreases with the loading rate rising, indicating a lower loading rate may be more suitable to improve rock fragmentation efficiency. • The mode I DSIFs are extracted from DIC analysis and the cracking process of Beishan NSCB and CCNSCB specimens is obtained. • The dimensionless SIFs of NSCB and CCNSCB specimens are determined using J-integral method in ABAQUS simulation software. • Key dynamic fracture parameters of Beishan granite specimens are obtained to study rate effects on dynamic mechanisms. [ABSTRACT FROM AUTHOR]

• As the number of wetting-drying cycles increases, the density and P-wave velocity of sandstone gradually decrease and the porosity increases. • The density and interconnectivity of the internal microcracks increase with wetting-drying cycles. • Under the same loading rate, the fracture toughness and energy dissipation of rock decreases as the increasing cyclic number. • The primary mechanisms for the rock deterioration after wetting-drying cycles are the dissolution of calcite and the intergranular cracking induced by clay swelling. Rocks are usually subjected to cyclic wetting-drying due to the periodical changes in moist condition. To investigate the effect of wetting-drying cycles on the fracture behavior of sandstone, notched semi-circular bending tests were performed on sandstone samples after every ten cycles (a total of 50 cycles) under both quasi-static and dynamic loading conditions. Tests results indicate that both of fracture toughness and energy dissipation of sandstone significantly decrease with the increase of cycle number. Moreover, the microstructural characteristics reveal that after cyclic wetting-drying treatments, the calcite cementation between grains gradually dissolves and the density and width of inter-granular fracture increase, which could be responsible for the deterioration of rock mechanical properties. [ABSTRACT FROM AUTHOR]