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Excavation and unloading operations in underground rock engineering can cause strong disturbances to the original in situ stress, potentially leading to disasters such as spalling and rockburst. To address the inadequacy of strength analysis research, this study employs the energy analysis method. By analysing the relationship between different confining pressure unloading paths, a series of constant deviatoric stress direct and cyclic unloading triaxial tests were conducted to obtain the energy evolution process. Based on this, the energy evolution mechanism of the unloading confining pressure test path was revealed, and the influence of this path on the brittleness of the specimen was discussed. The research results indicate that confining pressure unloading leads to active energy release, triggering energy imbalance release and energy transfer mechanisms, which generate tensile and shear fractures, respectively, leading to energy dissipation. Throughout the entire test process, energy dissipation mainly comes from the elastic energy stored in the specimen before the unloading point, while the external work is relatively small. The elastic energy ratio continuously decreases from the unloading point. In addition, different triaxial confining pressure unloading tests can be regarded as the coupling of two stress path mechanisms: when the stress deviation remains unchanged, the spherical stress decreases, and when the stress deviation changes, the spherical stress remains unchanged. Furthermore, tensile microcracks weaken the brittleness before the peak and enhance it after the peak. The research results can guide the construction schemes of rock underground engineering. Highlights: The unloading of confining pressure leads to the active release of energy, triggering energy imbalance release and energy transfer mechanisms, which generate tensile and shear fractures, respectively. During the unloading of confining pressure, the dissipation of energy primarily originates from the elastic energy stored in the specimen before the unloading point, with less contribution from external work. Different triaxial unloading tests can be considered as the coupling of three energy mechanisms, namely energy saturation, energy imbalance release, and energy transfer mechanism. The tensile microcracks generated during the unloading process of the specimen have weakened the brittleness before the peak and enhanced it after the peak. [ABSTRACT FROM AUTHOR]

Abstract Dam stability is one of the most important issues in hydraulic engineering. Microfractures and damage commonly occur during impoundment, which might lead to serious dam problems. In this study, based on the engineering background of the Sanhekou hydropower station, microseismic monitoring and numerical simulation were employed to systematically investigate the microfracture and damage characteristics of the dam body. First, the microseismic monitoring system was established to capture the microfractures inside the dam. The results indicated that the rise in water level elevation has a significant effect on the microfracture and damage characteristics of the dam body, especially during the early stage of impoundment. This can be reflected by the variation in the derived source parameters, i.e., the b value, daily energy release, daily apparent stress and daily apparent volume. In addition, the failure mode of the microfractures could be determined by using the E S /E P value of microseismic events and the moment tensor inversion method. The cracking orientation of the failure surfaces could also be determined by the moment tensor inversion method. Subsequently, numerical simulation was conducted where the initial damage of the dam was considered by integrating the microseismic monitoring data. The simulation results suggested that dam deformation under impoundment considering microseismic feedback agrees well with the real field measured results. The stress level of the dam toe was larger than that of the dam heel, and both the dam toe and dam heel were under compression before impoundment. However, with increasing water level elevation, the stress status of the dam heel area changes from compression to tension. The findings in this study will provide a better understanding of the damage and failure mechanism of dams during impoundment, which might be helpful for the design and support of dams in hydropower stations.

Based on the fractional order calculus theory and Weibull distribution, the fractional order nonlinear creep constitutive equation of green mudstone in the weak interlayer of slope under the combined action of initial damage and creep damage is established. In order to investigate the effect of damage degree on creep mechanical parameters and creep parameters, as well as the superiority of the improved creep model, creep tests were conducted on green mudstone samples with different initial damage degrees. The fractional order nonlinear creep damage coupling model outperforms the other two theoretical models in accurately describing the three stages of creep. The stress and damage variables in the constitutive equation will change the initial strain value of the creep curve. The fractional derivative and damage factor affect the shape of the creep curve, that is, change the time when the rock enters the third stage of creep. Therefore, to accurately predict the entire creep process of green mudstone, it is necessary to add fractional derivatives and damage factors to the creep model. This study is of great significance for analyzing the initial failure of slope creep under long-term gravity action and also provides a basis for predicting slope stability under coupled failure. [ABSTRACT FROM AUTHOR]

Given the corrosion protection failure of the cable-stayed bridge cable in the actual project, the impressed current cathodic protection (ICCP) technology was used to carry out the anti-corrosion tests from steel wire scale to cable scale, analyze the corrosion protection characteristics of steel wire ICCP, deduce the corrosion factor ICCP diffusion model, and reveal the space ICCP mechanism of the initial damaged cable. The analysis shows that ICCP can effectively inhibit the corrosive effect of steel wires and cables, and reduce the loss of mechanical properties of steel wires. Its protective efficiency increases with an increase in the absolute value of its protective potential and decreases with an increase in the dead load level. ICCP has a blocking effect on the diffusion of corrosion factors in the cable space, and the corrosion spatial distribution range, corrosion factor concentration, and diffusion coefficient of the initial damaged cable decrease significantly with the increase in the absolute value of the protection potential. The open circuit potential moves in a positive direction with the development of the corrosion process, and the blocking effect of corrosive factor diffusion is transformed from the joint participation by sacrificial anode cathodic protection and ICCP to the dominance of ICCP.

Damage can cause uncontrollable changes during the service of a structure, leading to a reduction in mechanical properties and ultimately to structural failure. This paper presents an experimental and numerical study of the crashworthiness of thin‐walled Al/CFRP hybrid tubes with pre‐existing transverse compression damage under transverse impact loading. Experiments shows that initial damage has a more limited effect on the energy absorption capacity of hybrid tubes under impact loading, but has a greater effect on their deformation pattern and the evolution of damage. After impact, cracks on the surface of the damaged tube propagate into multiple, discontinuous cracks with a total length similar to the length of the tube. Conversely, only short cracks are generated on the intact tube. Subsequent finite element simulations demonstrate the validity and accuracy of a coupled multi‐loads model and explore the effect of different structural parameters (winding angle and number of CFRP layers, and thickness of Al tube) on the crashworthiness of the hybrid tubes. Finally, a multi‐objective snake optimizer algorithm (MOSO) was used to obtain an optimal hybrid tube structure for multiple loading conditions in order to minimize the effect of initial damage on the crashworthiness of the hybrid tube. In comparison to the simulation outcomes for the original structure, the peak crushing force (PCF) decreased by 28.46%, whereas the specific energy absorption (SEA) increased by 44.59%. Highlight: The effect of pre‐existing damage on the crashworthiness and deformation pattern of hybrid tube structures is investigated by means of experimental and numerical simulations.A multi‐step finite element model was developed using the restart method to realize the crashworthiness analysis of the hybrid tube under multiple load coupling conditions.The prediction accuracy of four surrogate models for the crashworthiness of hybrid tubes is compared and analyzed.The multi‐objective snake optimizer algorithm (MOSO) is proposed to obtain the optimum hybrid tube structure for multiple loading conditions. [ABSTRACT FROM AUTHOR]

To understand the time-dependent fracture behaviour of hard rock in the excavation damage zone, a series of stress paths are designed to obtain the same initial damage extent induced by excavation. The influence of initial damage under different stress states on the creep process of Beishan granite is investigated experimentally. A unified indicator, i.e., the time-dependent fracture potential index (ξTFPI), is proposed to conveniently describe the effect of applied stress on the creep process. The results show that the time-dependent strain increases with increasing initial damage extent and is significantly affected by the applied stress at a given initial damage extent. Under the same initial damage extent, with the increase in ξTFPI, the larger the primary creep strain rate and secondary creep strain rate are, and this phenomenon is more obvious in the lateral creep results. The secondary creep strain rate increases with increasing ξTFPI, and the increasing trend of the axial secondary creep strain rate with ξTFPI is greater with a higher initial damage. Finally, a quantitative relationship of the secondary creep strain rate with the initial damage and ξTFPI is established and validated against the experimental results. [ABSTRACT FROM AUTHOR]

As large-span structures, reticulated shells are widely used in large-scale public building and act as emergency shelters in the event of sudden disasters. However, spatial reticulated shells are dynamic-sensitive structures; the effect of the initial structural damage on dynamic stability should be considered. In this study, a new nonlinear dynamic model of cylindrical reticulated shells with initial damage is proposed to investigate the effect of initial damage accurately. Firstly, the damage constitutive relations of the building steels are built based on the irreversible thermodynamic theory; furthermore, its fundamental equations are obtained using simulated shell methods. Then, the nonlinear vibration differential equations with damage are obtained and studied with support. Meanwhile, the nonlinear natural vibration frequency with initial damage is derivatized. After that, a bifurcation problem with initial damage is studied by using Flouquet Index, and the dynamic stability state at the equilibrium point is analyzed in depth. It is found that the local dynamic stability of the system is determined via its initial condition, geometric parameters, and initial damage. Moreover, the initial damage dominates over other influence factors due to its strong randomness and uncertainty for the same structure. The damage accumulation results in the transition of the equilibrium point. In addition, the nonlinear natural vibration frequency decreases to zero with the accumulation of the damage reaching 0.618; the local stability of cylindrical reticulated shells fails and they even lose whole stability. This study provides a theoretical foundation for the future investigation of whole stability with initial damage. [ABSTRACT FROM AUTHOR]

In light of the limited research on latent damages during the construction of large-span suspension arches, this study introduced a method to simulate structural damage utilizing random porosity. Initially, based on data from real-world engineering projects, the most susceptible areas within the arch structure were pinpointed. Subsequently, multiple test arch simulation models were constructed. Employing Python, commands for random porosity were implemented within ABAQUS and distinct mesh modules were devised to depict structures under varying degrees of damage. The current investigation delved into the structural responses of these susceptible areas under different damage rates, shedding light on damage progression patterns. Notably, our findings demonstrated that concealed damages on the top plate of the arch foot profoundly influenced structural integrity, whereas damages at the arch hance were comparatively minimal and predominantly manifest at the arch base. The pronounced localized damage at both the arch base and hance initiated and intensified at sectional corners, necessitating enhanced anti-crack measures in these regions. Moreover, depending on the stresses of the arch structure, diverse reinforcement strategies could be employed, optimizing the balance between load-bearing efficiency and cost considerations. [ABSTRACT FROM AUTHOR]

An estimate of the effect of initial damage, such as delamination in the area of a structural hole, on the static and fatigue strength of polymer composite material (PCM) based on computational mechanics methods is presented. Calculation for durability of structural elements made of PCM is conducted using Simcenter 3D—Samcef package and Specialist Durability module. A typical carbon fiber-reinforced plastic with the available physical and mechanical characteristics obtained from the tests was chosen as the study material. Fatigue characteristics of the typical carbon fiber-reinforced plastic were approximated for subsequent calculation on durability. In the durability calculation, the observed parameter is the degradation of the material stiffness under repeated loading of the investigated area. The convergence with the experimental results of the fatigue strength modeling for a defect-free sample, which is a strip with a hole, is estimated. The fatigue strength of a sample with a delamination-type defect is also compared with the fatigue strength of a damage-free sample. [ABSTRACT FROM AUTHOR]

In the mining of nearly horizontal open-pit coal mines, transportation flat plates are usually arranged on the end-slope. Reciprocating transport operations of large heavy-duty trucks cause significant cyclic loading and unloading effects on the overburden rock, leading to initial damage to the coal rock body. At the same time, the strong impact load generated by large-scale blasting during open-pit mining further deteriorates the mechanical properties of the coal rock body, thereby inducing large area instability of the end-slope. Therefore, it is crucial to conduct experimental research on the dynamic mechanical properties of coal rock under cyclic loading and unloading damage conditions for the stability of the end-slopes and safe mining in open-pit coal mines. The dynamic mechanical properties of coal samples under the initial damage condition of cyclic loading and unloading and their damage and fracture mechanism were systematically analyzed by using the split Hopkinson pressure bar test system and scanning electron microscope test system. The results show that: ① With the increase of the loading strain rate, the dynamic compressive strength and elastic modulus of the coal sample increase rapidly, and the strengthening effect of the strain rate on the mechanical properties of the sample is significant. The dynamic compressive strength and elastic modulus decrease linearly and rapidly as the initial damage increases under the condition of fixed strain rate, indicating that the initial damage weakens the mechanical properties of the coal sample; ② There exist small-scale primary cracks and pore defects in the natural state of the coal samples, and the occurrence of damage leads to the emergence of new cracks and pores inside the coal sample. With the increase of the damage intensity, the defect scale and number gradually increase and expand rapidly, which is the fundamental reason for the weakening of its mechanical properties; ③ The failure mode of the coal sample changes from tension failure to shear failure with the increase of the initial damage and strain rate, and at the same time, its degree of damage, dissipation energy during the failure process, and energy dissipation capacity show a rapid increasing trend; ④The dynamic failure of the damaged coal sample is mainly brittle failure, but under the condition of high initial damage, significant sliding separation and toughness fracture morphology can be observed locally, which is also the main reason for the high energy dissipation capacity of the coal sample under the condition of high initial damage.

Understanding the shear mechanics mechanism of bolted joints is of great significance for predicting and preventing geological disasters. Most current studies seldom consider the rheological effects of bolted joints. In this paper, a comprehensive rheological constitutive model is proposed, accounting for initial damage and damage evolution across different rheological stages and bolt characteristics. The model incorporates an elastoplastic Hooke body for instantaneous deformation, parametric nonlinear Kelvin and viscous models for attenuation and steady creep stages, and a visco-plastic model based on time-dependent shear strength for accelerated creep stage. Additionally, a bolt-rock cooperative deformation model is introduced, considering the evolution of the bolt's elastic modulus. The resulting elasto-viscoplastic constitutive model effectively describes the shear rheological behavior of bolted joints, with its validity and superiority demonstrated through comparisons with shear creep tests and the Maxwell model. This research aims to provide valuable theoretical guidance for the construction and reinforcement of rock mass engineering projects. [ABSTRACT FROM AUTHOR]

To study the seismic strengthening of damaged reinforced concrete (RC) frames using CFRP sheets, this study designed and tested the scaled 2-bay and 2-storey RC frame specimens. After applying a low cyclic horizontal load to simulate the initial damage to the specimen in an earthquake disaster, CFRP was used to strengthen the joints of the damaged RC frame. Pseudo-static tests of strengthened specimens and counterpart specimens were then carried out. Seismic performance, including stiffness, load capacity, ductility and energy dissipation were further analyzed. The failure mode of strengthened RC frame structures showed excellent ductility. The results demonstrated that the strengthening method involving wrapping CFRP can significantly improve the maximum horizontal bearing capacity, initial stiffness and energy dissipation capacity of the non-ductile reinforced concrete frame structure. The average displacement ductility coefficient of strengthened specimen can be enhanced to 3.41 compared with that of counterpart specimen (3.00). The pushover analysis based on the OpenSees model determined that the prototype frame with CFRP strengthening can maintain structural integrity and safety, with its maximum interstorey displacement angle below the limit of seismic specification (i.e., 1/50 in a severe earthquake). This study can contribute to the development of practical and efficient methods for restoring and improving the performance of damaged RC frames in seismic-prone regions. [ABSTRACT FROM AUTHOR]

This work studies the creep characteristics of concrete sustaining initial damage by considering both the initial damage to concrete and the damage caused by crack growth in the creep process. The damage state of concrete is characterized by the characteristics of the amplitude of ultrasonic waves. Creep tests on concrete prisms with different degrees of initial damage were carried out at loads of 40%, 50%, and 60% compressive strength. The results show that the initial damage to concrete significantly impacts its creep behavior. The modified creep curve of concrete, including the initial damage variable, is consistent with the creep curve of undamaged concrete at different stress levels, proving the applicability of the damage variable. [ABSTRACT FROM AUTHOR]

Abstract The design of the contact surface has a big impact on how well water jets penetrate coal rock. The influence of water jets on contact surfaces with various curvatures is studied using numerical modeling to elucidate the flow field and coal seam damage characteristics. The water jet impingement flow field region can be separated into three areas: velocity concentration, divergent flow, and wall recirculation. As the jet water pressure rises, the flow field's distribution features become more apparent. Water jet impact morphology and flow field properties on contact surfaces of various curvatures are comparable. The peak velocity ratio of the wall and axis falls first, then increase as the curvature radius ratio (n) of the contact surface increases. Near n = 0.8, the peak velocity ratio of the wall and axis reaches a minimum, indicating that the kinetic energy loss in the process of water jet impact to the contact surface is the greatest. When water jets of various pressures impact the contact surface, the peak velocity drops by two‐thirds before and after impact, and nearly 89% of the kinetic energy is lost in the impact process. The range of the initial damage and failure area in the center of the contact surface and the initial damage and failure area on both sides of the wall caused by water jet impact decreases gradually as n increases. When n = 2, the initial damage and failure region on both sides of the wall has virtually vanished; when n = 0.2. The contact surface's center failure depth induced by water jet impact is the deepest. The initial injury is unaffected by changes in jet water pressure.

This research aims to evaluate the influences of the freeze–thaw (F-T) effect on the energy dissipation mechanism and damage evolution characteristics of muddy siltstones containing initial damage. At first, four initial damage levels were achieved by applying different impact loads to the intact rock, and the damage stresses for levels I, II, III, and IV initial damage were 9.80 Mpa, 17.00 Mpa, 23.34 Mpa, and 32.54 Mpa, respectively. Then dynamic compression tests were conducted on the muddy siltstones containing initial damage after 0, 5, 10, 15, 20, 25, 30, and 40 F-T cycles in the temperature range from −20 to 20 °C. The damage variable of the muddy siltstones was determined by studying energy distribution during fracture of the rock. The damage evolution characteristics of the muddy siltstone containing initial damage under the F-T effect were explored combined with the fractal theory. Test results show that (1) the dynamic compressive strength of the muddy siltstones decreases exponentially with the increasing number of F-T cycles; the dynamic compressive strength of muddy siltstone with different initial damage decreased by 54.9%, 48.4%, 39.4%, 42.5%, and 44.5% after 40 freeze-thaws, respectively, compared with that of intact. (2) The absorbed energy, reflected energy, and transmitted energy of the muddy siltstones subject to different levels of initial damage exhibit step-like changes under the effect of F-T cycles and the rate of decrease in absorbed energy decreases in the late stage of F-T cycles. (3) Both the damage variable and the fractal dimension of the muddy siltstones show an increasing trend with an increase in the number of F-T cycles, and it is more difficult for damage to become superimposed as the damage accumulates to that range causing fatigue (the damage variables ranged from 0.73 to 0.97) while the fractal dimension of the fracture surfaces in the rock still increases. (4) With the gradual increase in the damage variable, the energy absorption density of the rock is negatively correlated with the fractal dimension of the rock fragments. [ABSTRACT FROM AUTHOR]

As large-span structures, reticulated shells are widely used in large-scale public building and act as emergency shelters in the event of sudden disasters. However, spatial reticulated shells are dynamic-sensitive structures; the effect of the initial structural damage on dynamic stability should be considered. In this study, a new nonlinear dynamic model of cylindrical reticulated shells with initial damage is proposed to investigate the effect of initial damage accurately. Firstly, the damage constitutive relations of the building steels are built based on the irreversible thermodynamic theory; furthermore, its fundamental equations are obtained using simulated shell methods. Then, the nonlinear vibration differential equations with damage are obtained and studied with support. Meanwhile, the nonlinear natural vibration frequency with initial damage is derivatized. After that, a bifurcation problem with initial damage is studied by using Flouquet Index, and the dynamic stability state at the equilibrium point is analyzed in depth. It is found that the local dynamic stability of the system is determined via its initial condition, geometric parameters, and initial damage. Moreover, the initial damage dominates over other influence factors due to its strong randomness and uncertainty for the same structure. The damage accumulation results in the transition of the equilibrium point. In addition, the nonlinear natural vibration frequency decreases to zero with the accumulation of the damage reaching 0.618; the local stability of cylindrical reticulated shells fails and they even lose whole stability. This study provides a theoretical foundation for the future investigation of whole stability with initial damage.

In light of the limited research on latent damages during the construction of large-span suspension arches, this study introduced a method to simulate structural damage utilizing random porosity. Initially, based on data from real-world engineering projects, the most susceptible areas within the arch structure were pinpointed. Subsequently, multiple test arch simulation models were constructed. Employing Python, commands for random porosity were implemented within ABAQUS and distinct mesh modules were devised to depict structures under varying degrees of damage. The current investigation delved into the structural responses of these susceptible areas under different damage rates, shedding light on damage progression patterns. Notably, our findings demonstrated that concealed damages on the top plate of the arch foot profoundly influenced structural integrity, whereas damages at the arch hance were comparatively minimal and predominantly manifest at the arch base. The pronounced localized damage at both the arch base and hance initiated and intensified at sectional corners, necessitating enhanced anti-crack measures in these regions. Moreover, depending on the stresses of the arch structure, diverse reinforcement strategies could be employed, optimizing the balance between load-bearing efficiency and cost considerations.

An estimate of the effect of initial damage, such as delamination in the area of a structural hole, on the static and fatigue strength of polymer composite material (PCM) based on computational mechanics methods is presented. Calculation for durability of structural elements made of PCM is conducted using Simcenter 3D—Samcef package and Specialist Durability module. A typical carbon fiber-reinforced plastic with the available physical and mechanical characteristics obtained from the tests was chosen as the study material. Fatigue characteristics of the typical carbon fiber-reinforced plastic were approximated for subsequent calculation on durability. In the durability calculation, the observed parameter is the degradation of the material stiffness under repeated loading of the investigated area. The convergence with the experimental results of the fatigue strength modeling for a defect-free sample, which is a strip with a hole, is estimated. The fatigue strength of a sample with a delamination-type defect is also compared with the fatigue strength of a damage-free sample.

Damaged rock masses on the slopes of open pit coal mines are prone to geological disasters such as landslides under low temperatures and dynamic loads such as blasting impacts. Based on the Low Temperature Split Hopkinson Pressure Bar (LT-SHPB) system, dynamic compressive tests were done on sandstone specimens, which were damaged by uniaxial loading and unloading test. Dynamic stress–strain curves and dynamic mechanical properties of frozen sandstone with initial damage were analyzed as well as the energy dissipation characteristics. The results indicate that both compressive state and plastic deformation state of the dynamic stress-strain curves increase with the increase of the damage value. Dynamic peak stress and dynamic elastic modulus exhibit an evident damage weakening effect while the dynamic peak strain, in contrast, exhibits a damage enhancement effect. In addition, all three dynamic mechanical properties of the damaged frozen sandstone exhibit an impact effect. The dissipation energy ratio and reflection energy ratio of frozen sandstone increase with the increase of initial damage value while the transmission energy ratio decreases. With the increase of initial damage value and strain rate, the energy utilization rate during the sandstone failure process increases, resulting in more small fragments and powders.

Microscopic voids/defects always occur in brittle piezoelectric fiber during the preparation. In addition, inconsistent material properties between the reinforced fibers and the matrix led to some microcracks located at the interface in matrix materials. In order to obtain equivalent properties of piezoelectric composites with respect to initial voids/defects, a parametric microscopic finite-volume model was constructed in this paper. To this end, the mechanical-electric coupling constitutive relation was implanted into the parametric FVDAM model to describe their piezoelectric characteristics. To verify the presented coupled mechanical-electric model, the numerical results of local stress and electric field distributions acquired by finite element method were also indicated for a comparison. On this basis, equivalent modulus and equivalent electrical properties with full consideration of local damages were also investigated by the presented microscale model. [ABSTRACT FROM AUTHOR]

High dispersion of rock mass strength causes significant difficulties in strength prediction. This study aims to investigate experimentally the strength prediction model for brittle damaged rock with multiscale initial damage based on P-wave velocity using regression analysis. Intact dolomitic limestone was collected from a deep metal mine in Southern China. Rock specimens with different initial damage degrees were prepared through the application of uniaxial compressive stress. Both intact rock and damaged rock specimens were tested for P-wave velocity and uniaxial compressive strength (UCS). The test results indicate that the method of prefabricating initial damage to the rock mass through uniaxial compressive stress is feasible. The UCS values of the damaged rock specimens were correlated with the square of the P-wave velocity (linearly positive) and the initial damage (linearly negative). The parameters of the new strength prediction model have a physical significance, and its results are within the upper and lower limits of the 95% confidence interval of the UCS. The strength prediction model considering multiscale initial damage based on P-wave velocity could reasonably predict the strengths of brittle rock masses. [ABSTRACT FROM AUTHOR]

To study the seismic strengthening of damaged reinforced concrete (RC) frames using CFRP sheets, this study designed and tested the scaled 2-bay and 2-storey RC frame specimens. After applying a low cyclic horizontal load to simulate the initial damage to the specimen in an earthquake disaster, CFRP was used to strengthen the joints of the damaged RC frame. Pseudo-static tests of strengthened specimens and counterpart specimens were then carried out. Seismic performance, including stiffness, load capacity, ductility and energy dissipation were further analyzed. The failure mode of strengthened RC frame structures showed excellent ductility. The results demonstrated that the strengthening method involving wrapping CFRP can significantly improve the maximum horizontal bearing capacity, initial stiffness and energy dissipation capacity of the non-ductile reinforced concrete frame structure. The average displacement ductility coefficient of strengthened specimen can be enhanced to 3.41 compared with that of counterpart specimen (3.00). The pushover analysis based on the OpenSees model determined that the prototype frame with CFRP strengthening can maintain structural integrity and safety, with its maximum interstorey displacement angle below the limit of seismic specification (i.e., 1/50 in a severe earthquake). This study can contribute to the development of practical and efficient methods for restoring and improving the performance of damaged RC frames in seismic-prone regions.

The design of the contact surface has a big impact on how well water jets penetrate coal rock. The influence of water jets on contact surfaces with various curvatures is studied using numerical modeling to elucidate the flow field and coal seam damage characteristics. The water jet impingement flow field region can be separated into three areas: velocity concentration, divergent flow, and wall recirculation. As the jet water pressure rises, the flow field's distribution features become more apparent. Water jet impact morphology and flow field properties on contact surfaces of various curvatures are comparable. The peak velocity ratio of the wall and axis falls first, then increase as the curvature radius ratio (n) of the contact surface increases. Near n = 0.8, the peak velocity ratio of the wall and axis reaches a minimum, indicating that the kinetic energy loss in the process of water jet impact to the contact surface is the greatest. When water jets of various pressures impact the contact surface, the peak velocity drops by two‐thirds before and after impact, and nearly 89% of the kinetic energy is lost in the impact process. The range of the initial damage and failure area in the center of the contact surface and the initial damage and failure area on both sides of the wall caused by water jet impact decreases gradually as n increases. When n = 2, the initial damage and failure region on both sides of the wall has virtually vanished; when n = 0.2. The contact surface's center failure depth induced by water jet impact is the deepest. The initial injury is unaffected by changes in jet water pressure. [ABSTRACT FROM AUTHOR]

Existing rock material damage models always ignore the initial damage characteristics of rock materials, and the actual rock materials have initial damage characteristics. To consider the rock's initial damage characteristics, a series of compression tests for yellow sandstone was carried out. First, the acoustic emission characteristics and damage model of yellow sandstone, considering the loading rates, were analyzed. Second, an initial damage model, which can better describe the initial damage characteristics of yellow sandstone materials, is presented. The research results show that the strength and elastic modulus of yellow sandstone depends on the loading rate, and increases as the loading rate increases. [ABSTRACT FROM AUTHOR]

As the mining depth increases, in underground tunnels, caverns or pillar goaf, a rock burst is one of the important accidents that threaten mine safety. Drilling pressure relief becomes one of the main means of preventing rock bursts, which affect the mechanical properties and stability of the coal and rock in underground excavations. However, the surrounding rock of the roadway or the coal body is usually broken, and the pressure relief of the large-diameter borehole will affect its support. A segmented hole-reaming technology is proposed and applied in a coal mine in China. A pressure-relief mechanic model of segmented hole reaming was built. The coal sample had an elastic modulus of 0.35 GPa, the UCS and UTS were 17.4 MPa and 1.41 MPa, the Poisson ratio was 0.27, the cohesive force was 2.81 MPa, and the friction was 23.7°. The pressure relief range of the boreholes with different diameters, horizontal in situ stress coefficients, cohesive forces, and friction angles were analyzed. When the drill hole was increased from 120 mm to 200 mm, the pressure relief range was increased by 57.1%. The stress distribution of the staged reaming and pressure-relief drilling was also obtained. In the vertical direction, the vertical stress of the borehole first decreased and then was restored to the original rock stress area, and in the horizontal direction, it first increased and then was restored to the original rock stress area. A CMS1-6200 segmented drilling rig was used to construct the pressure relief hole. The weight of the drilled pulverized coal was monitored at different depths. The results showed that the amount of pulverized coal in all of the drilled holes was less than 3 kg/m, indicating that the effect of reducing the pressure relief is obvious. The study is of great significance to research the pressure relief range, mechanical characteristics and stress distribution of segmented hole reaming; it also provides insight into the rock burst prevention and the design of drilling in the mine site. [ABSTRACT FROM AUTHOR]

The service life of piezoelectric devices is significantly affected by the electric fatigue load. The randomness of loads and nonuniformity of material properties caused by random distribution of internal defects in piezoelectric body will lead to the dispersion of the failure life of piezoelectric devices. Predicting failure probability of piezoelectric materials under electric fatigue load is essential for the reliability and life prediction of piezoelectric structures and devices. In this paper, the initial damage of piezoelectric materials is modeled as a random variable. Based on the methods of probability theory and statistics, the failure probability assessment model of piezoelectric materials under electric fatigue load is proposed. The probability density function of initial damage is obtained by observation and statistics of initial defect area. The failure probability assessment model of piezoelectric materials under electric fatigue load can be obtained by probability theory and integral method. Meanwhile, the failure probability evolution of piezoelectric materials under various electric fatigue loads is considered. The results show that the failure life and dispersion of piezoelectric materials is significantly affected by electric fatigue load, and the variation of failure probability under various electric loads is similar. Based on the proposed failure probability assessment models, the randomness of the electric load and randomly distributed initial defects of materials are considered in the damage evolution and performance prediction of piezoelectric structures, which is an important supplement to the reliability prediction and application of piezoelectric devices. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Engineering Geology / Gongcheng Dizhi Xuebao is the property of Journal of Engineering Geology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

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In deep coal and rock mass engineering, extensive initial damage problems caused by excavation and unloading are critical to the long-term deformation and stability of underground projects. An innovative initial damage coal sample test method was proposed, and coal samples with different initial damage were prepared by the MTS pressure tester's circular displacement cyclic loading and unloading control. The creep characteristics of coal samples with different initial damages were tested by the staged loading creep test method. With the increase of initial damage, the coal samples’ creep time became shorter, and the rate of accelerated creep damage increased sharply. The relationship between fractional viscosity coefficient and stress was introduced to describe the non-linear relationship of creep strain increase at each stage. The creep model in the acceleration phase was improved by establishing the relationship between the damage variable and the initial damage. Based on the fractional calculus theory, a viscoelastic-plastic model considering the initial damage of coal samples was established, and the fractional order model was verified. The coefficient of correlation between the experimental data and the creep model was higher than 0.95, indicating that the fractional creep model can better describe the creep characteristics of coal samples with initial damage.

In order to study the instability development process of the slope reinforced by anti-slide piles under earthquake conditions, the dynamic response characteristics of the slope are usually taken as the main characteristics, and the model test and numerical simulation are the main research methods. In this paper, a shaking table model test is designed and completed to investigate the influence of anti-slide piles with different initial damage on the failure mode of high and steep slope under earthquake conditions. The changes in velocity, strain and natural frequency during slope vibration are tested in combination with cloud maps when sinusoidal waves of different accelerations with a peak value of 5 Hz are applied. Thus, the differences of slope failure development process and dynamic response characteristics are obtained. The experimental results show that the anti-slide pile with different initial damage has obvious influence on the slope instability process. Under the condition of good anti-slide pile quality, the failure development of the slope behind the pile is limited to soil sliding on top of the slope, slope sliding and overburden sliding; the front slope foot of pile mainly forms shear belt and local sliding. With the decrease in the initial mass of the anti-slide pile, the slope failure develops into topsoil sliding, slope sliding and deep integral sliding; analogously, the failure of the slope in front of the pile develops into a whole slip along the slip belt. The natural frequency cloud map can directly reflect the damage location of the slope, and the frequency change rate is positively correlated with the cumulative shear strain. It shows that the macro-failure characteristics of the model slope change well when the natural frequency is used as the sensitive index to measure the influence of vibration on the model slope. The threshold value of the natural frequency change rate can distinguish different development stages of the slope; 1% is the threshold value of stage II, and 1.5% is the threshold value of stage III. [ABSTRACT FROM AUTHOR]

• A meso -scale numerical method for concrete beam with BFRP bars was built, in which concrete heterogeneity and concrete/bars interaction were considered. • The effect of the initial shear static loading and equivalent strain rates on the torsional performance of BFRP bars −RC beams were analyzed. • The torsional size effect of BFRP bars-RC beams with initial damage under different strain rates were explored. To reveal the influence of the initial static shear on the torsional performance of Basalt Fiber Reinforced Polymer Bars-Reinforced Concrete (BFRP bars-RC) beams, in this study, 52 BFRP bars-RC beam models with geometric similarity using a three-dimensional mesoscale numerical simulation method were established. The effects of initial shear static loadings (F 0 = 0, 0.25 V u , 0.5 V u , and 0.75 V u) on the failure modes, torsional strength, ductility, and size effect of BFRP bars-RC beams under various equivalent strain rates were quantitatively analyzed. The results show that, regardless of whether the beams have initial damage, their load-bearing capacity and deformation ability increase as loading strain rates increase. For beams with initial damage, the increased degrees in load-bearing capacity relative to their respective initial static shear decreases as the initial static shear level increases but then increases as the subsequent loading strain rate increases. Increasing the beams' initial damage degree makes the beams' ductility worse. In addition, the more severe the beams' initial damage, the more significant the strength size effect. However, increasing the strain rate can weaken the beams' strength size effect. [ABSTRACT FROM AUTHOR]

Abstract The previous researches on the degradation process of concrete under sulfate attack mainly focus on non-damaged concrete. It may lead to an excessive evaluation of the durability of the structure, which is detrimental to the safety of the structure. In this paper, three different damage degrees of concrete specimens with non-damaged (D 0) and initial damage of 10% (D 1) and 20% (D 2) were prefabricated and subjected to sulfate attack and wetting–drying cycles. With the increase of sulfate attack cycles (0, 30, 60, 90, 120, 150 cycles), the changes in mass loss, relative dynamic modulus of elasticity, and the stress–strain curve were studied. The results show that the mass of the D 0 specimen had been increasing continuously before 150 sulfate attack cycles. The mass of D 1 and D 2 had been increasing before 60 cycles, and decreasing after 60 cycles. At 150 cycles, the mass loss of D 0, D 1, D 2 were − 1.054%, 0.29% and 3.20%, respectively. The relative dynamic modulus of elasticity (RDME) of D 0 specimen increases continuously before 90 sulfate attack cycles. After 90 cycles, the RDME gradually decreases. However, for D 1 and D 2 specimens, the RDME began to decrease after 30 cycles. The damage degree has an obvious influence on the compressive strength and elastic modulus. For the D 0 specimen, the compressive strength and elastic modulus increased continuously before 90 cycles and decreased after 90 cycles. The compressive strength and elastic modulus of D 1 and D 2 specimens began to decrease after 30 cycles. The stress–strain curves of concrete with different initial damage degrees were established, and the fitting results were good. Finally, based on the analysis of experimental data, the degradation mechanism of concrete with initial damage under the sulfate wetting–drying cycle was discussed.

The concepts of initial damage, loaded damage, and total damage were introduced for cemented paste backfills (CPBs) with stratified structures. Based on these, the damage evolution model, damage constitutive model, and strength criterion of CPB with stratified structures were formulated based on damage theory and the total differential rule. Finally, compression experiments and numerical simulations of CPBs were conducted with three-dimensional particle flow code with the layer numbers of 0, 1, 2, and 3, a solid content of 75% and a cement–sand ratio of 1:4. The theoretical curves of the damage constitutive model and strength criterion of the layered CPB closely matched the curves generated based on experimental tests and numerical simulations. This finding verified the correctness of the model. The initial increase of the damage fits closely to a quadratic polynomial as a function of the layer number, the total damage rate first increased and then decreased, while the maximum of the total damage rate increased.

This research aims to evaluate the influences of the freeze–thaw (F-T) effect on the energy dissipation mechanism and damage evolution characteristics of muddy siltstones containing initial damage. At first, four initial damage levels were achieved by applying different impact loads to the intact rock, and the damage stresses for levels I, II, III, and IV initial damage were 9.80 Mpa, 17.00 Mpa, 23.34 Mpa, and 32.54 Mpa, respectively. Then dynamic compression tests were conducted on the muddy siltstones containing initial damage after 0, 5, 10, 15, 20, 25, 30, and 40 F-T cycles in the temperature range from −20 to 20 °C. The damage variable of the muddy siltstones was determined by studying energy distribution during fracture of the rock. The damage evolution characteristics of the muddy siltstone containing initial damage under the F-T effect were explored combined with the fractal theory. Test results show that (1) the dynamic compressive strength of the muddy siltstones decreases exponentially with the increasing number of F-T cycles; the dynamic compressive strength of muddy siltstone with different initial damage decreased by 54.9%, 48.4%, 39.4%, 42.5%, and 44.5% after 40 freeze-thaws, respectively, compared with that of intact. (2) The absorbed energy, reflected energy, and transmitted energy of the muddy siltstones subject to different levels of initial damage exhibit step-like changes under the effect of F-T cycles and the rate of decrease in absorbed energy decreases in the late stage of F-T cycles. (3) Both the damage variable and the fractal dimension of the muddy siltstones show an increasing trend with an increase in the number of F-T cycles, and it is more difficult for damage to become superimposed as the damage accumulates to that range causing fatigue (the damage variables ranged from 0.73 to 0.97) while the fractal dimension of the fracture surfaces in the rock still increases. (4) With the gradual increase in the damage variable, the energy absorption density of the rock is negatively correlated with the fractal dimension of the rock fragments.

In the current work a new equation for initial damage assessment of limestone based on plane strain theory is proposed. Detailed investigations of the static and dynamic characteristics of limestone with different initial damage degree, using longitudinal wave speed, and static-dynamic compression tests are performed. This study investigated the static and dynamic characteristics of limestone with different initial damage degree, using longitudinal wave speed, and static-dynamic compression tests. Experimental results show that the degree of initial damage decreases with increasing longitudinal wave speed, which reaches the minimum when the longitudinal wave speed is approximately 6000 m/s, and the smaller the longitudinal wave velocity, the greater the degree of initial damage. The static and dynamic compressive strengths of limestone increase with the longitudinal wave velocity and strain rate, but the elastic modulus and Poisson's ratio do not change significantly. Finally, based on the experimental results, the definitions of damage threshold value and strain softening are proposed, which further verify the influence of strain rate and initial damage on rock compression characteristics. The present study sheds light on the importance of initial damage for the mechanical state of rock in underground engineering. [ABSTRACT FROM AUTHOR]

In deep coal and rock mass engineering, extensive initial damage problems caused by excavation and unloading are critical to the long-term deformation and stability of underground projects. An innovative initial damage coal sample test method was proposed, and coal samples with different initial damage were prepared by the MTS pressure tester's circular displacement cyclic loading and unloading control. The creep characteristics of coal samples with different initial damages were tested by the staged loading creep test method. With the increase of initial damage, the coal samples' creep time became shorter, and the rate of accelerated creep damage increased sharply. The relationship between fractional viscosity coefficient and stress was introduced to describe the non-linear relationship of creep strain increase at each stage. The creep model in the acceleration phase was improved by establishing the relationship between the damage variable and the initial damage. Based on the fractional calculus theory, a viscoelastic-plastic model considering the initial damage of coal samples was established, and the fractional order model was verified. The coefficient of correlation between the experimental data and the creep model was higher than 0.95, indicating that the fractional creep model can better describe the creep characteristics of coal samples with initial damage. [ABSTRACT FROM AUTHOR]

Existing rock material damage models always ignore the initial damage characteristics of rock materials, and the actual rock materials have initial damage characteristics. To consider the rock’s initial damage characteristics, a series of compression tests for yellow sandstone was carried out. First, the acoustic emission characteristics and damage model of yellow sandstone, considering the loading rates, were analyzed. Second, an initial damage model, which can better describe the initial damage characteristics of yellow sandstone materials, is presented. The research results show that the strength and elastic modulus of yellow sandstone depends on the loading rate, and increases as the loading rate increases.

The nuclear magnetic resonance (NMR) technique was used to study freeze–thaw damage degradation of sandstone with initial damage. The initial damage was fabricated by preloading, and the pore structure was detected using the NMR technique. Porosity, T2 spectrum distribution, and T2 spectral area were obtained. The magnetic resonance imaging (MRI) technique was used to obtain two-dimensional pore distribution images. Combined with uniaxial compressive strength tests, the effects of initial damage and freeze–thaw cycles on the mechanical properties of rock were studied and the failure mode of rock was analyzed. The results show that in the first 20 cycles, the numbers of micropores (d ≤ 1 μm) and mesopores (1 μm < d ≤ 10 μm) both increased. As the number of cycles further increased, the porosity of the sample was mainly affected by micropores. When the initial damage level is 50% or lower, as the number of freeze–thaw cycles increases, the ability of sandstone to accommodate permanent strain before failure gradually weakens and then increases again after 40 cycles. When the initial damage level reaches 70% or higher, the ability to accommodate permanent strain continues to weaken. The strain produced by sandstone before elastic deformation is roughly positively correlated with the number of mesopores and macropores in the sandstone. Under loading, the sandstone presents four failure modes: splitting failure, single shear failure, cone failure, and multi-crack failure. [ABSTRACT FROM AUTHOR]

To study the damage characteristics of rock mass under multi-level creep load, damage variable D was defined based on the spatio-temporal evolution characteristics of deformation modulus E, and the Kachanov damage theory is used to describe the damage evolution, then the damage evolution equation of the rock mass under multi-level creep load is obtained. Combining the damage evolution equation with the Lemaitre strain equivalence principle, the creep damage constitutive model of rock mass under multi-level creep load considering initial damage is obtained. By comparing the results of uniaxial and triaxial tests with the calculated values of the model, the rationality, reliability, application range of the model proposed in this paper is verified. According to the results of parameter inversion, obtain the relationship between damage, stress and time. Results show that time and stress are the important factors influencing the damage of rock mass under multi-level creep loading, the damage increases with time and stress level. However, the influence of time and stress on damage has a significant stress response characteristics: under low stress, the instantaneous damage Dis caused by the instantaneous stress loading is the main reason for the damage. With the increase of the load level, the main cause of the damage gradually changes from the instantaneous loading of the stress to the creep accumulation of the damage, and the greater the initial damage, the higher the time-dependent damage DiT proportion in the global damage. [ABSTRACT FROM AUTHOR]

Brittle fracture of stirrups, due to effects such as diseased concrete expansion, is frequently reported and poses a potential threat to the load-bearing performance of structures. The brittle fracture was speculated to be directly related to the initial damage formed to bent steel bar during the bending process. However, few researches have been carried out on the formation of initial damage and its effect on mechanical performance. In this study, bending processes were conducted based on commonly used crescent-shaped steel bars; the effects of bending parameters and rib morphologies on the degree of initial damage were investigated in combination with data on bamboo-shaped steel bars, which often undergo brittle fracture. At the same time, tensile tests were carried out using V-shaped bent steel bars. The results show that as the bar diameter increases and the bending diameter decreases, the degree of initial damage in the bent steel bar increases. For example, for a crescent-shaped steel bar with a diameter φ of 16 mm, the initial damage when bending with a diameter 2.0 φ was 430 µm, which exceeded the extent of the bamboo-shaped steel bar. At the same time, the ultimate tensile strength of bent steel bars was reduced by 20% compared to straight steel bars, and can be reduced by approximately 90% in severe cases; the deformation capacity of bent steel bars after yielding was also reduced by a maximum 90%. SEM analyses showed that the fracture surfaces of bent steel bars exhibit typical brittle fracture morphologies with the cleavage and quasi-cleavage fracture, indicating influences of initial damage and its propagation. Finally, based on the study results, some recommendations are made regarding the improvement of the transverse rib morphology, construction measures and the design of mechanical performance for bent steel bars, to prevent, for example, the premature failure of stirrups and the concrete components in which they are located. • Initial cracks with the max. length as 2.7% of the bar diameter were formed due to bending process. • Greater bar and smaller bending diameter induce more severe initial damage. • Bent steel bars have a 20% and 90% reductions in fracture strength and deformation. • Recommendations were made to prevent the premature failure of stirrups. [ABSTRACT FROM AUTHOR]

Roller compacted concrete (RCC) often remains in a damage state as a result of improper vibrating compaction control, which makes it more susceptible to the action of freeze thaw (FT) cycles in cold regions and thus results in the degradation of mechanical behaviors. In this paper, the influences of initial damage and FT cycles on the physical and mechanical behaviors of RCC were systematically examined through physical tests, including FT cycle tests, pore structure tests and uniaxial compressive tests. An in situ vibrating compaction procedure was employed in the experiment to accurately replicate the field compaction process for RCC under laboratory conditions, and the dynamic elasticity modulus (DEM) was adopted as the non-destructive testing (NDT) index to quantify the initial damage caused by improper RCC construction control. The results indicated that the higher values of initial damage and FT cycles exhibited a greater influence on the frost resistance and pore structure distribution. Remarkable attenuation of compressive strength and elastic modulus and an increase in peak strain were observed to occur in RCC when the initial damage degree exceeded 0.1 and the number of FT cycles exceeded 75. Finally, the uniaxial compressive stress strain model was established through segmented linear regression analysis based on the Weibull-Lognormal model. The proposed model was validated to be effective and reasonable in predicting the mechanical properties of RCC under the combined effects of initial damage and FT cycles. • In-situ vibrating compaction method was used to simulate field-compaction process. • Initial damage from improper construction control was quantified by NDT methods. • Effects of initial damage and FT cycles on RCC properties were investigated. • Uniaxial compressive constitutive model for DRCC under FT cycles was established. [ABSTRACT FROM AUTHOR]

In order to study the instability development process of the slope reinforced by anti-slide piles under earthquake conditions, the dynamic response characteristics of the slope are usually taken as the main characteristics, and the model test and numerical simulation are the main research methods. In this paper, a shaking table model test is designed and completed to investigate the influence of anti-slide piles with different initial damage on the failure mode of high and steep slope under earthquake conditions. The changes in velocity, strain and natural frequency during slope vibration are tested in combination with cloud maps when sinusoidal waves of different accelerations with a peak value of 5 Hz are applied. Thus, the differences of slope failure development process and dynamic response characteristics are obtained. The experimental results show that the anti-slide pile with different initial damage has obvious influence on the slope instability process. Under the condition of good anti-slide pile quality, the failure development of the slope behind the pile is limited to soil sliding on top of the slope, slope sliding and overburden sliding; the front slope foot of pile mainly forms shear belt and local sliding. With the decrease in the initial mass of the anti-slide pile, the slope failure develops into topsoil sliding, slope sliding and deep integral sliding; analogously, the failure of the slope in front of the pile develops into a whole slip along the slip belt. The natural frequency cloud map can directly reflect the damage location of the slope, and the frequency change rate is positively correlated with the cumulative shear strain. It shows that the macro-failure characteristics of the model slope change well when the natural frequency is used as the sensitive index to measure the influence of vibration on the model slope. The threshold value of the natural frequency change rate can distinguish different development stages of the slope; 1% is the threshold value of stage II, and 1.5% is the threshold value of stage III.

Before a coal mining operation, it is necessary to carry out coal-bed methane (CBM) pre-extraction to prevent coal and gas explosions, causing accidents. However, gas extraction and coal mining will lead to coal damage, which results in a change in the gas migration law. Especially, under deep mining conditions, the gas migration mechanism is more complicated owing to a high ground temperature. Coal permeability is the most important constituent that determines gas flow properties. Therefore, the coal permeability evolutionary law related to damage-induced conditions under different temperatures should be further researched. In this paper, a series of triaxial seepage experiments during the whole stress-induced process, and in the changing of the effective stress process were carried out. The results have shown that during the whole stress–strain process, with an increase in axial strain, coal permeability gradually decreases to a minimum value at first, then increases sharply, and finally keeps nearly constant. A higher temperature resulted in a lower elastic modulus, peak strain, and peak strength, but it caused higher thermal damage. When the coal fractures, coal permeability increases with the increase in temperature. During a change in the effective stress process, higher temperatures resulted in higher permeability. Under higher effective stress, the impact of temperature on permeability was not significant. Based on the above results, a novel damage-based permeability model was developed to describe the permeability evolutionary law caused by damage-induced conditions under compaction, and in a fracturing situation. In the proposed model, an exponential function has been used for the combination between permeability and damage variable. The damage variable is composed of thermal damage and mechanical damage. In addition, the damage variable has been modified by introducing a modified function of the initial damage. Finally, the proposed model has been applied to fit two sets of experimental data available. The fitting results showed that the proposed permeability model could well reflect the permeability behaviors of damage-induced coal at different temperatures. [ABSTRACT FROM AUTHOR]

The time-dependent behaviour of a rock mass is crucial to the long-term stability of underground engineering projects, such as large tunnels, caverns and roadways. This paper focused on the experimental and theoretical studies of marble creep behaviour considering its initial damage due to excavation damage. Based on the onsite investigation of the creep performance of Jinping marble, detailed creep tests were carried out on damaged marble under different confining stresses, which indicated that damaged marble exhibited more notable creep properties than the intact marble. A new time-dependent creep constitutive model was proposed to characterize the time-dependent mechanical behaviour of damaged marble in terms of fractional derivatives and damage variables by replacing the Newtonian dashpot with the Abel dashpot. Furthermore, this one-dimensional creep equation was extended to a three-dimensional numerical model for analyzing the long-term deformation of hydraulic caverns. The simulation results indicated that the proposed model provided a precise description for the full creep process of marble containing initial damage. [ABSTRACT FROM AUTHOR]

The previous researches on the degradation process of concrete under sulfate attack mainly focus on non-damaged concrete. It may lead to an excessive evaluation of the durability of the structure, which is detrimental to the safety of the structure. In this paper, three different damage degrees of concrete specimens with non-damaged (D0) and initial damage of 10% (D1) and 20% (D2) were prefabricated and subjected to sulfate attack and wetting–drying cycles. With the increase of sulfate attack cycles (0, 30, 60, 90, 120, 150 cycles), the changes in mass loss, relative dynamic modulus of elasticity, and the stress–strain curve were studied. The results show that the mass of the D0 specimen had been increasing continuously before 150 sulfate attack cycles. The mass of D1 and D2 had been increasing before 60 cycles, and decreasing after 60 cycles. At 150 cycles, the mass loss of D0, D1, D2 were − 1.054%, 0.29% and 3.20%, respectively. The relative dynamic modulus of elasticity (RDME) of D0 specimen increases continuously before 90 sulfate attack cycles. After 90 cycles, the RDME gradually decreases. However, for D1 and D2 specimens, the RDME began to decrease after 30 cycles. The damage degree has an obvious influence on the compressive strength and elastic modulus. For the D0 specimen, the compressive strength and elastic modulus increased continuously before 90 cycles and decreased after 90 cycles. The compressive strength and elastic modulus of D1 and D2 specimens began to decrease after 30 cycles. The stress–strain curves of concrete with different initial damage degrees were established, and the fitting results were good. Finally, based on the analysis of experimental data, the degradation mechanism of concrete with initial damage under the sulfate wetting–drying cycle was discussed. [ABSTRACT FROM AUTHOR]

A remotely triggered rockburst is a typical engineering geology disaster that occurs during the excavation of hard rock at depth. Damage in the rock mass naturally exists and will also be induced by the construction of underground excavation. These damages, in turn, influence the occurrence and intensity of a rockburst. However, this problem, which draws significant public attention, remains unsolved. In the present study, the effect of the initial damage on a triggered rockburst of granite is investigated. Rectangular prismatic rock specimens with different initial damage D (0, 0.2, 0.3, 0.4, 0.6, and 0.7) are prepared by using heat-treatment (25, 200, 300, 400, 500, and 600, respectively). The initial damage is determined by the ratio of the P wave velocity of the specimen heat-treated at a specific temperature to that without heat-treatment. Then, remotely triggered rockburst simulations are conducted on the specimens with coupled static-dynamic loads exerted using an improved true-triaxial testing machine. The failure mode, deformation characteristics, fracture features, and kinetic energy of the ejected fragment of the tested specimen are systematically investigated. The experimental results indicate that for a given initial damage, a threshold of static stress exists, and beyond this threshold, a dynamic disturbance can trigger a rockburst. Additionally, it is found that a dynamic disturbance can much more easily induce a triggered rockburst as the level of initial damage increases. However, the kinetic energy of ejected fragments increases to a peak and then decreases as the initial damage increases. The maximum kinetic energy appears at an initial damage D = 0.3. Therefore, another interesting finding is a specific initial damage can induce the most violent triggered rockburst. [ABSTRACT FROM AUTHOR]

The experimental results show that the creep properties of the rocks are affected by the initial damage, and the damage evolution also has a significant impact on the time-dependent properties of the rocks during the creep. However, the effects of the initial damage and the damage evolution are seldom considered in the current study of the rocks' creep models. In this paper, a new nonlinear creep damage model is proposed based on the multistage creep test results of the sandstones with different damage degrees. The new nonlinear creep damage model is improved based on the Nishihara model. The influences of the initial damage and the damage evolution on the components in the Nishihara model are considered. The creep damage model can not only describe the changes in three creep stages, namely, the primary creep, the secondary creep, and the tertiary creep, but also reflect the influence of the initial damage and the damage evolution on creep failure. The nonlinear least squares method is used to determine the parameters in the nonlinear creep damage model. The consistency between the experimental data and the predicted results indicates the applicability of the nonlinear damage model to accurately predict the creep deformation of the rocks with initial damage. [ABSTRACT FROM AUTHOR]