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To deeply understand the rock failure characteristics under actual engineering condition, in which static geo-stress and dynamic disturbance usually act simultaneously, impact tests were conducted on sandstone subjected to axial static pre-stresses varying from 0 to 75 MPa by a modified split Hopkinson pressure bar. The fracturing process of specimens was recorded by a high speed camera. Dynamic parameters of sandstone, such as strain rate, dynamic strength and energy partition were acquired. Fracture mechanisms of pulverized specimens were identified by the method combining the displacement trend line and digital image correlation technique. Moreover, fragments of failed specimens were sieved to obtain the fragment size distribution. Test results revealed that, under the same incident energy, the dynamic compressive strength increases first, then decreases slowly and at last drops rapidly with the increase of pre-stress, and reaches the maximum under 24.4% of uniaxial compressive strength due to the closure of initial defects. Four final patterns were observed, namely intact, axial split, rock burst, and pulverization. The rock burst only occurs when the pre-stress lies in the elastic deformation stage or initial stable crack growth stage and the incident energy is intermediate. For pulverized specimens, the fracture mechanism is transformed into shear/tensile equivalent from tensile-dominated mixed mode as the pre-stress increases. Specimens with 75 MPa pre-stress release strain energy during failure process, contrary to specimens with lower pre-stresses absorbing energy from outside. The crushing degree of pulverized specimens exhibits a positive correlation with the pre-stress as a consequence of higher damage development in rock.

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.

A modified split Hopkinson pressure bar (SHPB) numerical testing system is established to study the characteristics of rocks under simultaneous static and dynamic loading conditions following verification of the capability of the SHPB numerical system through comparison with laboratory measurements (Liao et al. in Rock Mech Rock Eng, 2016 . doi: 10.1007/s00603-016-0954-8 ). Three different methods are employed in this numerical testing system to address the contact problem between a rock specimen and bars. The effects of stand-alone static axial pressure, stand-alone lateral confining pressures, and a combination of them are analyzed. It is determined that the rock total strength and the dynamic strength are greatly dependent on the static axial and confining pressures. Moreover, the friction along the interfaces between the rock specimen and bars cannot be ignored, particularly for high axial pressure conditions. Subsequently, the findings are applied to determine the dynamic behavior of rocks with in situ stresses. The effects of the magnitude of horizontal and vertical initial stresses at varied depths and their ratios are investigated. It is observed that the dynamic strength of deep rocks increases with increasing depth or the ratio of horizontal-to-vertical initial stresses ( K ). The dynamic behavior of deeper rocks is more sensitive to K , and the rock dynamic strength increases faster with depth in areas with higher K .

To investigate the mesodamage evolution of rock specimens containing a pre-existing opening under integrated static and dynamic loading, the nuclear magnetic resonance technique was utilised in this study. The changes in the transverse surface relaxation time (T2) distribution, peak area, and porosity of the specimens prior to and due to varying pre-stresses under an identical dynamic loading were systematically analysed. The experimental results show that as the pre-stress level increased the size and the amount of the pores, the peak area and the porosity increase, indicating that more damage is induced under a high pre-stress. Under identical loading conditions, the specimens having 30° and 60° square openings are more susceptible to damage than those having a 0° square opening. However, compared with the specimens containing a circular opening, the specimens with a square opening are more seriously damaged. The hoop stress concentration surrounding the opening induced by the pre-stress and subsequent dynamic loading was further demonstrated. The result indicates that the initiation of rock fracture occurs in the maximum hoop stress concentration area.

To investigate the disturbance‐induced shear instability mechanism of structural catastrophe in the deep rock mass, MTS 815 material testing machine was used to carry out quasi‐static loading tests and disturbance shear tests on symmetrical regular dentate joints of two materials at three undulation angles under specific initial static stress, disturbance frequency, and peak value. The test results indicate that: (i) the total ultimate instability displacement is only related to the intrinsic properties of the joints but not to the initial static stress and disturbance parameters; (ii) the cumulative irreversible displacement required for the disturbance instability conforms to the logistic inverse function relationship with the number of disturbances, displaying the variation trend of “rapid increase in the front, stable in the middle, and sudden increase in the rear”; (iii) the accumulation of plastic deformation energy is consistent with the evolution law of irreversible displacement of joints and the overall proportion of hysteretic energy is not large; (iv) the dissipated energy required for the instability of each group of joints is basically the same under various disturbance conditions, and this energy is mainly controlled by the initial shear stress and has no connection with the disturbance parameters. The stability of the total disturbance deformation and the disturbance energy law of the joints revealed in the tests provide data support for reasonably determining the disturbance instability criterion of joints. In this study, to investigate the effect and underlying mechanism of the failure process under disturbance, a series of quasi‐static loading tests and disturbance shear tests in the vicinity of the strength limit is conducted on symmetrical regular dentate joints. Combined with the “locked patches” theory of typical gradual sliding, the failure law and morphology of joints under quasi‐static loading are summarized. Further, the variation rule of irreversible displacement under disturbance and the fitting prediction is also examined. Besides, the energy evolution process of joints under disturbance is analyzed based on the energy damage and energy storage mechanism. Overall, the findings of this study can provide test support for reasonably determining the disturbance instability criterion of joints under high initial stress Highlights The total ultimate instability displacement is only related to the intrinsic properties of the joints. The cumulative irreversible displacement required for the disturbance instability is related to the initial static stress. The dissipated energy required for the instability of each group of joints is basically the same under various disturbance conditions.

The work aims to improve the quality of workpieces obtained by breaking rolled stock products by researching the separation process under dynamic and static-dynamic loading on a press-hammer. Combined static-dynamic loading during cold breaking by bending allows to reduce of high-frequency vibrations of the “tool-specimen-support” system, to eliminate the violation of the contact of the specimen with the supports, and to reduce the peak values of the forces from the side of a punch and supports. The static force at the moment of impact provides a certain initial level of tensile stresses in the area of the stress concentrator. This has a positive effect on the quality of the obtained workpieces. The magnitude of the static force required to exclude the separation of the specimen from the supports depends on the stiffness of the contact between the head and the intermediate punch and increases with increasing this stiffness. The obtained experimental results confirm the adequacy of the mathematical model of specimen separation by the breaking method under dynamic and static-dynamic loading. It has been experimentally established that the value of the preliminary static force must be at least 40% of the force at which the specimen is broken down. Obtained results can be used to improve the technology of separating rolled stock into dimensional workpieces by the method of cold breaking by bending.

Under the action of dynamic loadings such as earthquakes and volcanic activities, the mechanical properties of gas-hydrate-bearing sediments will deteriorate, leading to a decrease in the stability of hydrate reservoirs and even inducing geological disasters such as submarine landslides. In order to study the effect of dynamic loading on the mechanical properties of hydrate sediments, triaxial compression tests of numerical specimens were carried out by using particle flow code (PFC2D), and the macro-meso mechanical behaviors of specimens were investigated. The results show that the loading frequency has a small effect on the stiffness of the hydrate sediment, while it has a large effect on the peak strength. The peak strength increases and then decreases with the increase in loading frequency. Under the same loading frequency, the peak strength of the hydrate sediment increases with the increase in loading amplitude, and the stiffness of the specimen decreases with the increase in loading amplitude. The maximum shear expansion of the specimen changes with the movement of the phase change point and the rearrangement of the particles. The maximum shear expansion of the specimen changes with the movement of the phase change point and the change of the bearing capacity of the particles after the rearrangement, and the more forward the phase change point is, the stronger the bearing capacity of the specimen in the plastic stage. The shear dilatancy angle and the shear dilatancy amount both increase linearly with the increase in loading amplitude. The influence of loading frequency and amplitude on the contact force chain, displacement, crack expansion, and the number of cementation damage inside the sediment is mainly related to the average axial stress to which the specimen is subjected, and the number of cracks and cementation damage of the sediment specimen increases with the increase in the average axial stress to which the sediment specimen is subjected. As the rate of cementation damage increases, the distribution of shear zones becomes more obvious.

The stress distribution around constructions in deep rock can be modified by dynamic disturbances, such as earthquakes. For this, we proposed a new loading method applied to granite sample under coupled static and dynamic cyclic loading (CSDCL) condition. The variations in P-wave velocity, gas permeability, and mechanical properties of granite before and after CSDCL are revealed. It shows that with increasing axial static stress, the dynamic cyclic loading amplitude and cycle number, P-wave velocity, uniaxial compressive strength (UCS) and elastic modulus decrease, whilst the permeability and Poisson’s ratio increase. It seems that variations in those parameters have a close relation with the exciting frequency. In this case, CSDCL is applied for crack development along the axial direction in the rock samples, and subsequently results in degradation of mechanical properties, delay of P-wave propagation, and increase of the transport paths. The results show that the permeability and Poisson’s ratio are likely to be more sensitive to the CSDCL, in particular under the axial static stress. The empirical relationships of the P-wave velocity with permeability, UCS, and elastic modulus are established for a pragmatic monitoring purpose. From design point of view, we have established the relationships of the disturbance factors to damage variable. Then the correlations of the damage variable to permeability, UCS, and elastic modulus are analyzed. The testing results in this context could facilitate our understanding of rock stability upon excavation subjected to dynamic disturbances.

The present study assessed the Bailey Bridge’s condition and investigated its adaptation as a permanent structure, targeted the Acrow Bailey Bridge in Japan. Field diagnostic loading experiments were performed under various loading conditions, such as dynamic and static loading tests. The onsite data were obtained using a transducer, friction strain gauge, target measurements for the image processing approach, and accelerometer. From the field measurements, the deflection and stresses of the bridge were found to operate within the linear elastic region. The bridge was then accurately modeled based on the in situ geometric configuration of the bridge, and Finite Element Analysis was performed. The model’s accuracy was validated with the onsite data under the linear elastic domain. The model was deployed to check for resistance of critical members. A nonlinear analysis based on the linear and nonlinear buckling method was performed to determine the subject bridge’s Serviceability Limit State and Ultimate Limit State. The results showed that the first out-of-plane eigenvalue buckling analysis could monolithically assess bridge members. Further, the study established digital twin models resolve for historical data through in situ modeling measurements. Therefore, the findings obtained in this study highlight the bridge’s Structural Health Condition, bearing capacity, and propose a framework for adaptation as a permanent structure.

The soft sandstone is taken as samples, and the RLW-2000M triaxial rheological testing machine is used as the main equipment to test the axial stress-strain, the axial rheological deformation in this paper. The axial strain is selected as damage variables, the damage degree expression is derived, and the effect of each factor on damage degree is studied. Research results show that axial deformation increases with the passage of time, and the hysteresis curve change from sparse to dense under the action of the dynamic loading. The axial deformation increase with the increase of the dynamic loading amplitude, and it increases with the decrease of frequency. The damage degree of specimen tends to be stable after mutation and increases with the increase of the dynamic loading amplitude. The research results will provide the scientific basis for us to disclose the rule of rheology-damage evolution of sandstone under the action of dynamic loading.