Search

Defects such as voids, pores, and joints will transform into big scale cracks in the rock of tunnel surrounding under dynamic load like blasting and earthquake. In this paper, three kinds of symmetrical cracks were chosen as an example, and experiments and numerical simulations were conducted to study the effect of symmetric cracks on a blast-induced crack. The relationship of main crack propagation characteristic and distribution of symmetrical cracks was investigated. Some circular specimens using two kinds of material, PMMA and sandstone, including a center hole charged with a detonator and pre-existing cracks were used in the experiments. The test system consisted of an oscilloscope and an ultradynamic strain amplifier and crack propagation gauges (CPGs) were employed in monitoring propagation velocity. AUTODYN code was applied in numerical simulation to investigate the propagation behavior of main crack between symmetrical cracks. Linear equation of state and a modified major principal stress failure criterion was utilized to describe the status of rock material. Based on experimental and numerical results, it can be concluded that (1) the pre-existing symmetrical cracks have arrest effect on main crack propagation, (2) compressive stress in y-direction plays very important roles in crack arrest, and (3) the spacing of parallel cracks has a great influence on crack propagation length and velocity.

Radial cracks may exist around tunnel edge, and these cracks may propagate and weaken tunnel stability under nearby blasting operations. In order to study the blast-induced fracture behavior of radial cracks emanating from a tunnel spandrel, a tunnel model containing a spandrel crack (TMCSC) with different inclination angles was proposed in this paper. Crack propagation gauges (CPGs) and strain gauges were used in the experiments to measure crack initiation moment and propagation time. Finite difference models were established by using AUTODYN code to simulate crack propagation behavior and propagation path. ABAQUS code was used to calculate dynamic stress intensity factors (SIFs). The results show that (1) crack inclination angles affect crack initiation angles and crack propagation lengths significantly; (2) critical SIFs of both mode I and mode II decrease gradually with the increase of the crack propagation speed; (3) the dynamic energy release rates vary during crack propagation; and (4) there are “crack arrest points” on the crack propagation paths in which the crack propagation speed is very small.

Dynamic fracture behavior under impacting loads has been well studied, but for that under blasting loads, less attention has been paid. In order to investigate mode I crack propagation behavior of brittle materials under blasting, a new configuration specimen, i.e. single internal crack circular disc (SICCD) specimen was proposed in this paper, and it was applied in the blasting experiments. Crack propagation gauges (CPGs) were stuck along crack propagation paths to measure crack initiation and propagation time and crack propagation speeds. Green sandstone and PMMA were selected to make the SICCD specimens. Finite difference models were established by using AUTODYN code according to the SICCD specimen dimension and the loading curve measured near the borehole. Generally, the simulation results of crack propagation paths agree with the test results. Finite element code ABAQUS was applied in the calculation of dynamic stress intensity factors (SIFs), and the curves of dynamic SIFs versus time were obtained. By using these curves and the breaking time of the CPG wires, the mode I critical dynamic SIFs in initiation and in propagation were obtained. The results show that the measuring method of the critical dynamic SIFs of brittle materials under blasting presented in this paper is feasible and applicable. During crack propagations, the crack speed is not a constant, and the critical dynamic SIFs in propagation decreases with the increase of crack propagation speeds.

High in-situ stress can limit the generation of rock fractures induced by blasting, which usually shows different states of rock fragmentation with those under low-stress conditions. In this paper, the stress distribution around the blasthole under coupled in-situ stress and blasting load was theoretically analyzed. Then, the single-blasthole blasting process, which is calibrated by field blasting tests, was numerically investigated using the Riedel-Hiermaier-Thoma (RHT) model, and the effects of in-situ stress magnitudes and lateral pressure coefficients on the crushed zone and the crack propagation were investigated. After that, influences of lateral pressure coefficients, buried depths, and blasthole layouts on the behavior of double-blasthole blasting were studied. It is concluded that in-situ stresses can increase the compressive stress and reduce the tensile stress caused by blasting load. The area of the crushed zone decreases with increasing in-situ stresses. The crushed zone is elliptical in shape in anisotropic pressure conditions. The gap between the long axis and the short axis of the crushed zone widens as the difference between the stress in the horizontal and vertical directions increases. Cracks preferentially propagate in the higher stress direction. At a buried depth of 1000 m, connecting cracks can be formed at lateral pressure coefficients ranging from 0.25 to 3.0 when blastholes are drilled along the horizontal direction. The rise in buried depths and the angle between the centerline of adjacent blastholes and the higher stress direction can limit the formation of connecting cracks. The research results can provide guidance for analyzing the behavior of rock blasting in deep underground.

Drilling and blasting method is a common method of rock fragmentation in engineering. Meanwhile, the dynamic behaviours of rocks under blasting loads are different from those under impact loads. In this paper, a large-size arc-edge rectangle with edge notches (LAREN) specimen was posed to investigate the dynamic fracture parameters of brittle rock materials under blasting load. Three types of radial pre-cracks with different lengths were used, combining with the crack propagation gauge velocity test system, and the crack initiation time and propagation time were obtained, so as to obtain the crack propagation speed which is to be modified by the fractal theory. Also, the specimen rationality was analyzed by AUTODYN code in depth. At last, the dynamic stress intensity factors were estimated by using nonlinear finite element software ABAQUS and experimental results. It is concluded that the LAREN specimens can effectively lower the influence of the reflected tensile stress wave (by 54.18%) on the dynamic crack initiation and propagation, and have good applicability in the measurement of dynamic fracture parameters under blasting. With the development of pre-crack length, the crack time becomes longer, the average propagation speed becomes smaller, and the fluctuation of speed becomes smaller and tends to be stable. The increase of the pre-crack length also leads to the gradual decrease in the test value of the crack initiation toughness, while the test value of the propagation toughness is not constant.

Under dry-wet cycles, rock mechanical parameters may be degraded with material strength reduced and structure stability weakened. The impact test was conducted with large single cleavage semicircle compression (LSCSC) specimens to investigate the effect of dry-wet cycles on rock crucial parameters including dynamic parameters, fracture toughness, elastic modulus and tensile strength. Crack extending gauges (CEG) were applied to investigate fracture time and crack velocity. The fractal theory was introduced to correct the extended length and the speed of the crack. A RSM-SY5 (T) non-metal ultrasonic detector was employed to measure P-wave and S-wave under different dry-wet cycles. Additionally, the chemical composition and microscopic morphology features of sandstone in different dry-wet cycle numbers were determined by using X-ray diffraction (XRD) and a scanning electron microscope (SEM-TM3000). The dynamic stress intensity factors (DSIFs) were computed by ABAQUS software. The fracture toughness under different dry-wet cycle numbers was confirmed by using numerical-experimental method. The investigation demonstrates that the mode I crack propagation was influenced by the number of dry-wet cycles with the crucial parameters, fracture toughness, material parameters, cracking time and average crack propagation speed affected. The fracture toughness decrease and disperse with the increase of dry-wet cycles. With the increase of dry-wet cycle number, the crack propagation speed increases, and the crack initiation time decreases.

The interface transition zone (ITZ) between aggregate and mortar of concrete has a weak resistance to static or dynamic loading and failure usually starts from this zone. Therefore, it is essential to study the failure properties and tensile strength of ITZ of concrete under static and dynamic loading. Accordingly in this paper, bi-material specimens of rock-mortar Brazilian Disc were designed. Two types of rock granite and sandstone with smooth and rough surface were selected to prepare the bi-material Brazilian disc (BBD) specimens. Meanwhile, to compare the difference of failure properties with single material, the Brazilian disc (BD) specimen of single granite, sandstone and mortar were also prepared. The static and impact tests were conducted using the BBD and BD specimens, and a split Hopkinson pressure bar (SHPB) device was applied in the impact tests. Because of asymmetry stress distribution in the bi-material Brazilian disk, the tensile strength was determined by an experimental-numerical method. In addition, to simulate the failure process in ITZ, numerical models of the BD and BBD specimens embedded with cohesive elements using Python language were established. The results show that under static loading, the tensile strength of bi-material sandstone-mortar is less than that of both the single sandstone and mortar, while the tensile strength of granite-mortar is slightly lower than that of the mortar but much less than that of the granite. The tensile strength of bi-material with rough rock surface is generally higher than that with smooth rock surface. The dynamic tensile strength is more than 20 times the corresponding static tensile strength, and the dynamic tensile strength increases with loading rates.

The study on dynamic fracture toughness (DFT) is of considerable practical interest because it can be used to predict cracked rock structure stability under blasting. The measurement method of dynamic fracture toughness under blasting loads is less addressed until now. In order to study the measurement method of DFT and crack propagation behavior under blasting, a new configuration specimen, i.e. single internal cracked circular disc (SICCD) specimen was proposed in this paper. Four types of brittle materials, black sandstone, red sandstone, green sandstone and PMMA were selected to prepare the SICCD specimens. Crack propagation gauges (CPGs) were glued along crack propagation paths to measure crack initiation and propagation time and crack propagation speeds. An oscilloscope and an ultra-dynamic strain amplifier were employed in the blasting tests. X-ray diffraction (XRD) and scanning electron microscope (SEM) were used to analyze the ingredient and microstructure of the brittle materials respectively. Finite element code ABAQUS was applied in the calculation of dynamic stress intensity factors (DSIFs), and the curves of the DSIF versus time were obtained. By using these curves and the breaking time of CPG filaments, the critical DSIFs of mode I crack at initiation and propagation were obtained. The results show that the measurement method of the critical DSIF of brittle materials under blasting is feasible and applicable. During crack propagations, the crack propagation speed and dynamic energy release rate are not constant. Comparing the test results of these four brittle materials, the initiation toughness and crack propagation speed increase with the elastic modulus of the brittle material under same blasting conditions.

In order to study fracture toughness of mode I crack under blast load, a new specimen of a rectangle plate with a crack and edge notches (RPCEN) was proposed in this study. The PMMA was selected to prepare the RPCEN specimens. The testing system consists of a dynamic strain amplifier, an oscilloscope, a constant source, strain gauges and crack propagation gauges. The strain gauges and crack propagation gauges (CPGs) were used to measure the blast load and crack propagation speed, respectively. The scanning electron microscope was applied in studying the property of fracture surface. The explicit dynamic software AUTODYN was applied in simulating crack dynamic propagation behavior. The JWL equation of state (EOS) was applied to the explosive detonation products, and a linear EOS was used to describe the relationship between pressure and density of PMMA. A modified principle stress failure criterion was employed to assess the material state. The experimental-numerical method was used to determine the dynamic initiation and propagation toughness of the PMMA. The dynamic stress intensity factors (DSIFs) were calculated by the finite element code ABAQUS. The experimental and numerical results show that: (1) The RPCEN specimen proposed in this paper has the function to minimize the reflected tensile stress waves; (2) During crack propagation the fracture surface roughness and the nucleation rate increase with crack propagation speed; (3) Crack propagation speed is not a constant, and propagation toughness is related to the crack propagation speed.