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Determination on underlying mechanisms of crack initiation is of vital importance to understand the failure processes of geomaterials in practical engineering. In this study, uniaxial compression experiments of granitic samples containing a single pre-existing flaw were conducted and the failure processes were recorded by using the high-speed camera. To quantitatively determine the crack initiation mechanism, a novel method was first proposed based on digital image correlation (DIC) analysis and then its validity was confirmed. By utilizing this method, three types of cracks with different initiation mechanisms were identified and the effect of flaw inclination angle on crack initiation mechanisms was discussed from the viewpoint of theoretical analysis. With the increase of inclination angles, wing cracks change from mixed mode I/II cracks to mode I cracks, while anti-wing cracks have no evident changes and are dominated by mode II cracks. Under compressive pressure, the upper and bottom surfaces of pre-existing flaw deform to each other and the distributions of full-field tangential stress around flaw are different, which might induce the variation of crack initiation mechanisms with regard to the inclination angle.

In recent years, many studies have shown that it is meaningful to place rocks under stress paths corresponding to various loading and unloading conditions. However, the deformation evolution of rock under cyclic loading with consideration of the mechanical behavior and characteristics has rarely been studied under triaxial cyclic unloading and loading conditions. In practical engineering, particularly in underground or mining engineering, the stress increase in the rock mass in areas affected by mining is mainly caused by crack initiation and development when the rock is subjected to the effects of cyclic unloading and loading. In this study, variations in the stress–strain curves, irreversible strain, elastic modulus, and Poisson’s ratio are discussed and explained. The test results demonstrate that in comparison with a lower initial confining stress, increasing the initial confining pressure restrains the radial deformation of sandstone samples, and the degree of compaction of the sandstone samples rapidly increases in the failure stage. This results in the loss of the failure buffering process of the sandstone sample. Changes in the degree of compaction of the rock samples lead to obvious differences in the irreversible strain of the rock under different initial confining pressures and different limit unloading and loading cyclic confining stresses. The scanning electron microscopy and analysis results demonstrate that the macroscopic mechanical and microscopic physical properties of sandstone show different characteristics under different initial confining stresses.

Several studies have been conducted on the fatigue behavior of copper and 7-3, and 6-4 brasses. However, there have been fewer studies on the fatigue behavior and fatigue crack growth (FCG) properties of free-cutting brass, primarily because emphasis has been placed on the development of lead-free free-cutting brass. In this study, fatigue experiments were performed in the atmosphere at room temperature using three types of free-cutting, two types of bismuth (Bi)-based (with different grain sizes), and lead (Pb)-based brasses. It was found that lead-free Bi-based free-cutting brass had approximately the same fatigue performance as that of Pb-based free-cutting brass. It was also clarified that the addition of Bi or Pb initiated fatigue cracks, and that the crack growth period occupied most of the fatigue life. Differences in the FCG behavior of the three free-cutting brasses were observed in the low ΔK range. The modified linear fracture mechanics parameter M was used to quantitatively analyze the fatigue life and FCG behavior (short surface cracks). A comparison between the calculated and experimental results showed that M was useful.

Experiments on man-made flawed rock-like materials are applied extensively to study the mechanical behaviour of rock masses as well as crack initiation modes and crack coalescence types. A large number of experiments on specimens containing two or three pre-existing flaws were previously conducted. In the present work, experiments on rock-like materials (formed from a mixture of sand, plaster, limestone and water at mass ratio of 126:9:9:16) containing multiple flaws subjected to uniaxial compression were conducted to further research the effects of the layout of pre-existing flaws on mechanical properties, crack initiation modes and crack coalescence types. Compared with previous experiments in which only three types of cracks were found, the present experiments on specimens containing multiple flaws under uniaxial compression revealed five types of cracks, including wing cracks, quasi-coplanar secondary cracks, oblique secondary cracks, out-of-plane tensile cracks and out-of-plane shear cracks. Ten types of crack coalescence occurred through linkage among wing cracks, quasi-coplanar secondary cracks, oblique secondary cracks, out-of-plane shear cracks and out-of-plane tensile cracks. Moreover, the effects of the non-overlapping length and flaw angle on the complete stress–strain curves, the stress of crack initiation, the peak strength, the peak strain and the elastic modulus were also investigated in detail.

During tunnel construction and service, the rock surrounding tunnel is often subjected to multiple factors that influence its behaviour, such as dynamic disturbances (explosions, mechanical excavations, etc.) and existing cracks. These factors can readily induce safety issues, such as rock bursts and collapses. To investigate the effect of the loading direction on the failure modes of fractured tunnels, this study numerically investigated the destructive behaviour of tunnel models under the coupling effect of dynamic disturbance loads and external cracks using the finite-difference method (FDM). Additionally, a physical tunnel model with prefabricated cracks was created using green sandstone. A drop weight impact testing device (DWITD) was employed as the dynamic disturbance loading apparatus, while the relative azimuth angle between the tunnel and the cracks was varied. The crack initiation, arrest time, and extension rate were obtained using a crack fracture tester (CFT). The research results indicated that the preexisting cracks propagated continuously and eventually connected with the tunnel on the incident side under impact loads. The failure area of the tunnel was primarily controlled by the loading direction, exhibiting different modes of failure, often occurring at the bottom and arch of the tunnel. New cracks on the transmitted side of the tunnel appeared at different locations for different impact angles. The presence of cracks around the tunnel had a significant impact on the dynamic stress concentration factor (DSCF) of the rock surrounding the tunnel. The findings of this research can provide valuable guidance for tunnel stability analysis and the optimization of support schemes.HighlightsCrack parameter test was applied in crack propagation speed calculation.A large specimen with tunnel was used to calculate rock dynamic fracture toughness.The displacement trend line diagram was used to identify the fracture pattern of the crack under impact.The fracture toughness of rock is calculated by experimental numerical method. Stress wave and fracture mechanics theories were used to explain the interaction mechanism between cracks and tunnel.

To experimentally investigate the stability of underground excavations under high in situ stress conditions, several rock samples with a mini-tunnel were prepared and subjected to monotonic axial and coupled static–dynamic loading until failure. Mini-tunnels were generated by drilling circular or cubic cavities in the centre of granite rock blocks. Strain gauges were used to monitor the deformation of the mini-tunnels at different locations, and a high-speed camera system was used to capture the cracking and failure process. We found that the dynamic crack initiation stress, failure mode and dynamic crack velocity of the specimen all depend on the pre-stress level when the sample is under otherwise similar dynamic disturbance conditions. The crack initiation stress threshold first increased slightly and then decreased dramatically with the increase in the pre-stress value. The specimens were mainly fractured by tensile cracks parallel to the compression line under lower pre-stress, while they were severely damaged with additional shear cracks under higher pre-stress. Furthermore, the propagation velocity of the primary crack was significantly larger than that of the subsequent cracks. The effect of applying different amounts of static pre-stresses on the velocity of the primary tensile crack was similar to that observed for the crack initiation stress threshold; however, it did not affect the velocity of the secondary and subsequent tensile cracks.

In the Al alloy A2024-T3 extruded material, a rod-like structure is generated parallel to the extrusion direction. In this study, the effects of rod-like structures on fatigue crack initiation and growth behavior were comprehensively investigated. Two types of specimens were used in a fatigue experiment, in which the direction of the load stress amplitude was parallel (specimen P) and perpendicular (specimen V) to the rod-like structure. Based on the experimental and analytical results, the following findings were obtained regarding the fatigue life, location of crack initiation, and fatigue crack growth behavior. Because the fatigue life of specimen P was longer than that of specimen V, it is inferred that the rod-like structure significantly affects the fatigue life. In specimen P, fatigue cracks were generated from the grain boundaries of the Al matrix. By contrast, in specimen V, cracks were generated from the Cu–Mg-based intermetallic compound in the Al matrix. In specimen P, fatigue cracks were more likely to propagate across the rod-like structure, which decreased the fatigue crack growth rate. In specimen V, fatigue cracks did not propagate across the rod-like structure; instead, they propagated through the Al matrix. Therefore, the fatigue crack growth resistance of specimen V was lower than that of specimen P. The relationship between the fatigue crack growth rate and the modified linear elastic fracture mechanics parameter could be used to predict the S–N curve (stress amplitude vs. fatigue life) and fatigue crack growth behavior. The predicted results agreed well with the experimental results.

Uniaxial Compressive test (UCS) results are essential in evaluation the values of Poisson’s ratio. However, according to the suggestion of the International Society for Rock Mechanics, Poisson’s ratio can be determined using three alternative methods: the secant, average and tangent. Applying these methods causes discrepancies in the results; according to our experiences, the differences can be threefold or more. This paper aims to study the process of changes of Poisson’s ratio for intact rock during loading from micro-crack initiation to failure stage. The objective of this theoretical investigation is to establish a straightforward mathematical formulation between σ/σ c and intact rock’s Poisson’s ratio value. To outline these changes forty two granite rocks were investigated from Bátaapáti radioactive waste repository (Hungary) and the calculation was performed by using the new formula from the beginning of loading till failure stage at UCS test. In the laboratory test program, Poisson’s ratio derived from standard tests varies with momentary stress; it steadily increases as stress rises until reaching the stress level causing unstable crack propagation. Additionally, the Poisson’s rate follows a linear increase with stress, up to the point of unstable crack propagation stress. The research demonstrated that the proposed equations provide competent values for the root mean squared error value (ranging from 0 to 0.04), the mean absolute percentage error (ranging from 0.6% to 18%) and the mean absolute error (ranging from 0 to 0.04). Contrary to previous ideas, our results suggest that the Poisson’s ratio is not a constant for rigid rocks.

The Drucker-Prager (D-P) model is a representative elastoplastic model for geomaterials because hydrostatic pressure is considered. This paper proposes an ordinary state-based peridynamic (OSB-PD) model with shear deformation based on D-P model and the associated flow rule to study the plastic and damage behaviors of geomaterials. By considering the second invariant of the stress deviator J2 and the first invariant function of stress tensor I1 as the function of peridynamic energy density, the D-P yield function in nonlocal form can be used in peridynamics. In addition, the equivalent stress and equivalent plastic strain in this PD model with shear deformation are determined. Several examples are used to verify the validity of the proposed PD model. The PD results are compared with those obtained by the finite element method (FEM). It implies that the proposed method is effective and accurate.

Drill and blast method is widely applied in deep rock engineering, and the in-situ stress poses a great challenge to blasting excavation. The failure mechanism of rock under coupled dynamic and static loads and the effect of in-situ stress on blasting effects are major concerns when dealing with deep rock blasting excavation. In this study, lab-scale crater blasting experiments on sandstone specimens under various equal biaxial compressive stresses were conducted to investigate the effects of in-situ stress on blasting effects and the mechanism of in-situ stress affecting rock blasting. The initiation and propagation of crack network, morphological characteristics of blasting crater, and distribution characteristics of blasting fragments under biaxial in-situ stress were studied. Besides, the quantitative relationships between biaxial in-situ stress and blasting crater parameters (diameter, area, and volume) were analyzed. The experimental results show that the biaxial static stress inhibits the formation of radial cracks and promotes the formation of circumferential cracks, resulting in the time delay of initial crack formation and the change of initial crack type from radial crack to circumferential crack. With increasing biaxial static stress, the diameter, area and volume of blasting crater, and the size and quantity of blasting fragments gradually increase. Meanwhile, blasting craters are all circular under the various biaxial static stresses. Biaxial static stress has significant influences on the evolution of flaky failure zone, while the effect on the block failure zone and transition failure zone is relatively small. Finally, the mechanisms of the effect of biaxial in-situ stress on the initiation and propagation of blast-generated cracks, blasting crater morphology, and blasting fragments’ distribution were analyzed theoretically.HighlightsCrater blasting experiments on hard stone under various biaxial in-situ stresses were conducted.The effects of static stress on cracks, blasting crater, and blasting fragments are investigated.Quantitative relationships between in-situ stress and blasting crater parameters are examined.Mechanism analysis of biaxial in-situ stress affecting blasting.