Hole Sidewall Research Articles

Investigation on high-precision drilling of Cf/SiC composites with brazed diamond core-drill

Carbon fiber reinforced silicon carbide ceramic matrix (Cf/SiC) composites have promising applications as lightweight and hot end components due to their unique mechanical and high-temperature properties. However, high-precision drilling of Cf/SiC composites is still a difficult task, which is attributed to material's high hardness, anisotropy, and heterogeneity. In this work, a kind of core-drill with cemented carbide as shank and polycrystalline diamond (PCD) as grits, was prepared with vacuum brazing method. Experiments on drilling of 2D-Cf/SiC composite with the brazed core-drill were carried out. The effects of drilling parameters on the hole dimensional accuracy, surface roughness and defects were investigated. The results showed that with the increase of spindle speed and the decrease of feed rate, the dimensional accuracy was improved and surface roughness was reduced. Delamination, burr, and hole edge collapse were the main defects. The feed rate had a significant influence on the formation of defects. When the feed per revolution was equivalent to the diameter of a single carbon fiber, the defects were reduced effectively and the hole quality was improved substantially. In the investigated range of parameters, the optimal spindle speed and feed rate for high-precision drilling of Cf/SiC composites were 5000 r/min and 30 mm/min, respectively. Under this condition, the hole dimension accuracy was IT 10, and barely defects were observed on the machined surface. The surface roughness of hole sidewall reached a minimum value of 1.9 μm. This work provides a vital process for high-precision drilling on fiber reinforced ceramic matrix composites.

Plasma heating characterization of the large area inductively coupled plasma etchers with the plasma information for managing the mass production

Meter-scale of the large area inductively coupled plasma etchers with the capacitive power coupling are widely applied for the mass production of OLED (organic light emitting diode) display panels. Because of the large area-to-volume ratio of the etcher, the balance between the power loss and absorption is easily located in the capacitive coupling mode rather than the ideal inductively coupled mode. Therefore, the process results are sensitively governed by the power absorption and plasma heating properties of the reactors. We have introduced a new PI (plasma information) parameter, the ratio of the stochastic heating to Ohmic heating of the plasmas, which is monitorable by using the optical emission spectroscopy data of the processing etchers. With the help of this plasma heating characteristic index, we could optimize the process recipes with the detailed control of the etched hole sidewall passivation and related species generation rate in the plasmas; thus, chamber-to-chamber matching in the huge mass production fab with the higher efficiency was possible. It was demonstrated that the introduced PI index with plasma heating mechanism characterization could be applicable to the VM (virtual metrology) modeling as one of the good information supplying core variables. This PI index has shown a very high correlation with the plasma sheath and ion flux governing phenomena for a large number of mass-produced OLED display glasses. From these results, the introduced plasma heating mechanism-based PI index is expected to be utilized as a good reference index for their performance analysis or PI-VM modelings.

A MEMS Electrochemical Angular Accelerometer with Silicon-Based Four-Electrode Structure.

This paper presents a MEMS electrochemical angular accelerometer with a silicon-based four-electrode structure, which was made of thousands of interconnected microchannels for electrolyte flow, anodes uniformly coated on structure surfaces and cathodes located on the sidewalls of flow holes. From the perspective of device fabrication, in this study, the previously reported multi-piece assembly was simplified into single-piece integrative manufacturing, effectively addressing the problems of complex assembly and manual alignment. From the perspective of the sensitive structure, in this study, the silicon-based four-electrode structure featuring with complete insulation layers between anodes and cathodes can enable fast electrochemical reactions with improved sensitivities. Numerical simulations were conducted to optimize the geometrical parameters of the silicon-based four-electrode structure, where increases in fluid resistance and cathode area were found to expand working bandwidths and improve device sensitivity, respectively. Then, the silicon-based four-electrode structure was fabricated by conventional MEMS processes, mainly composed of wafer-level bonding and wafer-level etching. As to device characterization, the MEMS electrochemical angular accelerometer with the silicon-based four-electrode structure exhibited a maximum sensitivity of 1458 V/(rad/s2) at 0.01 Hz and a minimum noise level of -164 dB at 1 Hz. Compared with previously reported electrochemical angular accelerometers, the angular accelerometer developed in this study offered higher sensitivities and lower noise levels, indicating strong potential for applications in the field of rotational seismology.

High-speed perforation of high-entropy alloy CrMnFeCoNi plates: Experiments and modeling

The perforation behavior of 2-mm thick CrMnFeCoNi high-entropy alloy plates is investigated via ballistic impact experiments using spherical stainless steel 304 projectiles with high-speed photography at impact velocities ranging from 500 ms−1 to 2213 ms−1. Postmortem target samples are characterized with macro photography, three-dimensional laser scanning, optical metallography, scanning electron microscopy, electron backscatter diffraction and microhardness tests. The absorbed energy and aperture area show a nearly linear increase with increasing the initial kinetic and absorbed energy, respectively. Dislocations, twins and kink bands are the main microstructural deformation modes. As the distance away from the bullet hole sidewall, both the deformation degree and hardness decrease. Besides, shear-dominated and tensile/shear-dominated fracture are found in the middle part and near the rear side of the bullet hole sidewall, respectively. Parameters of the Johnson–Cook(JC) constitutive model are obtained via the split Hopkinson pressure bar system. A finite element method (FEM) model comprised of the JC constitutive and JC progressive damage model is established and reproduces experimental observations.

Influence of lithology and bedding orientation on failure behavior of “D” shaped tunnel

To explore the influence of lithology on the failure behavior of layered tunnel, true triaxial compression experiments were undertaken on cubical phyllite and yellow sandstone samples containing a “D” shaped hole. The failure progress of the hole sidewalls was monitored and captured using a miniature camera. The results reveal that the initial vertical failure stress of the phyllite samples presents a “U” shaped change as the bedding angle increases. At the bedding angle of 45°, compared with the initial vertical failure stress of the yellow sandstone tunnel, that of the phyllite tunnel is lower, resulting in larger rock fragments and deeper V-shaped grooves. The failure pattern of the phyllite tunnel is primarily manifested as extensive shear sliding failure, and the failure is more severe. The failure of the yellow sandstone tunnel is primarily characterized by the sequential laminar fracturing along the maximum principal stress direction, predominantly manifesting as tensile failure. The primary factors influencing the failure of two types of layered rocks are the significant variations in the clay mineral content within the rocks. For the phyllite, it contains nearly one-third of montmorillonite (a clay mineral). This results in the formation of weak bedding planes within surrounding rocks, which induces shear slip failures along these bedding planes. In contrast, the yellow sandstone has a lower clay mineral content, leading to the absence of distinct weak bedding planes within the surrounding rock. In this case, bedding planes present ignorable effect on the surrounding rock.

Effects of water temperature on femtosecond laser layered-ring trepanning in superalloy with water-based assistance

The effects of the water temperature on the hole quality in water-assisted femtosecond laser layered-ring trepanning on superalloys were studied. The effects of the laser pulse repetition rate on the hole entrance/exit diameters, taper angle, and hole sidewall morphology and roughness at different water temperatures were compared and analyzed. The results show that at a water temperature of 70 ℃, the diameter of the hole entrance decreased, while that of the hole exit increased, compared with those at 20 ℃. Both the hole entrance and exit diameters decreased at 2 ℃. The hole taper angle was the largest when the water temperature was 20 ℃, followed by 2 ℃, and was the smallest at 70 ℃. When the laser pulse repetition rate was 300 kHz, the hole taper angle at 70 ℃ decreased by 43.6 % compared with that at 20 ℃. As the water temperature increased, the roughness of each part of the hole sidewall decreased. When the pulse repetition rate was 200 kHz, the quality of the hole sidewall improved under water-based assistance conditions, while the hole quality improved at a water temperature of 70 ℃, compared with drilling in air. As the water temperature increased, the water was more likely to evaporate, resulting in an increase in the amount of bubbles. As more bubbles moved upward, the water flow increased. The debris and materials generated by laser drilling were more effectively removed, thereby improving the drilling efficiency. The use of a higher water temperature was beneficial to obtain holes with smaller taper angles and a higher sidewall quality. The experimental results provide a reference for optimizing water-assisted femtosecond laser drilling. This methodology can be applied to the aerospace and semiconductor industries.

Microfabricated Atomic Vapor Cells with Multi-Optical Channels Based on an Innovative Inner-Sidewall Molding Process

Existing microfabricated atomic vapor cells have only one optical channel, which is insufficient for supporting the multiple orthogonal beams required by atomic devices. In this study, we present a novel wafer-level manufacturing process for fabricating multi-optical-channel atomic vapor cells and an innovative method for batch processing the inner sidewalls of millimeter glass holes to meet optical channel requirements. Surface characterization and transmittance tests demonstrate that the processed inner sidewalls satisfy the criteria for an optical channel. In addition, the construction of an integrated processing platform enables multilayer non-isothermal anode bonding, the filling of inert gases, and the recovery and recycling of noble gases. Measurements of the absorption spectra and free-induction decay signals of xenon-129 (129Xe) and xenon-131 (131Xe) under different pump-probe schemes demonstrate the suitability of our vapor cell for use in atomic devices including atomic gyroscopes, dual-beam atomic magnetometers, and other optical/atomic devices. The proposed micromolding technology has broad application prospects in the field of optical-device processing.

Effects of fiber orientations on fracture mechanisms in rotary ultrasonic drilling of carbon fiber reinforced plastic laminates

The fracture mechanisms of the carbon fibers seriously depend on fiber orientations which significantly influence the surface quality of carbon fiber reinforced plastics (CFRPs). Rotary ultrasonic drilling (RUD), possessing some excellent characteristics including the superior sidewall quality and high cylindricity error, offers a possibility for the efficient drilling of the CFRP components. In contrast, the effects of fiber orientations on fiber fracture mechanisms are still not fully recognized in RUD of CFRP composites. Incorporated with kinematic characteristics of the abrasive, a novel theoretical model was developed to quantitatively describe the effective orientation of the fiber. Afterwards, the drilling experiments with and without ultrasonic were conducted under the same parameters. The surface topographies of the hole sidewalls at various orientations were comparatively characterized to explore the fiber fracture mechanisms. Subsequently, the influences of these mechanisms and the drilling parameters the surface qualities were also investigated. Moreover, the bending failure mechanisms of the carbon fibers were explored through theoretical analysis. As a result, another fiber fracture mechanism, bending failure, was identified with the smooth fracture and micro-fragmentation topographies at upwind and leeward parts of the fiber cross-section, respectively. Ultrasonic superimposition by means of the immense inertia force of the abrasive promoted the micro-fracture of the carbon fiber. The weaker sustainment from the back-up materials brought about the serious bending failure of fiber oriented at 135°, while the axial load component aggravated the tensile fracture of the carbon fiber with the effective orientation of 45°. The critical moment of the fiber bending failure was also established, by incorporating the beam theory and linear elastic fracture mechanics. The fundamental investigation provided a favorable foundation for further optimizing the parameters to improve the drilling quality and efficiency in formal RUD of CFRP composites.

Failure analysis and improvement of a 42CrMo crankshaft for a heavy-duty truck

The crankshaft of a heavy-duty truck engine fractured and failed. The failure occurred in the left crank web of the seventh main journal. To investigate the cause of the crankshaft fracture failure, macroscopic fracture and microscopic morphological analysis, chemical composition and mechanical properties testing, metallurgical examination and numerical simulation methods were used to analyse the failure of the fractured crankshafts and to propose improvement measures. The results show that the crack source was located at the junction of the threaded bottom hole column surface and taper surface, and the failure mechanism was high cycle fatigue fracture failure. The main reason for the failure was the stress concentration caused by the lack of an obvious transition fillet at the junction of the threaded bottom hole column surface and taper surface, with the maximum stress value of 486.9 MPa. In addition, the metallographic structure transformation of the surface layer of the bottom hole sidewall (maximum depth of about 22 μm) caused by the high machining temperature also had an important influence on the fracture. The proposed improvement scheme of increasing the transition fillet and using an internal cold machining tool was proposed, and the feasibility of the proposed scheme was verified by a fatigue bench test.

Stress Analysis and Spalling Failure Simulation on Surrounding Rock of Deep Arch Tunnel

To study the stress distribution characteristics of surrounding rock and the spalling mechanism of deep hard rock tunnels with different arch heights, the complex variable function and angle-preserving transformation method in elasticity theory were applied to the analytic solution of tangential stress distribution of arch tunnels during stress adjustment. In addition, true triaxial tests were conducted on granite cube specimens (100 mm × 100 mm × 100 mm) containing holes with three arch heights (including the 25 mm semi-circular arch, 16.7 mm three-centered arch, 12.5 mm three-centered arch) to simulate the spalling process under different initial ground stresses. The stress distribution solution and experimental results show that the initial failure stress of arch holes is 0.39–0.48 times the uniaxial compressive strength (UCS) of the rock. The initial failure location occurs at the arch foot, where tangential stress maximizes. When the lateral pressure coefficient is in the range of 0.38–0.50, the tangential stress is 3.2–3.5 times the UCS. The rock debris of the hole wall are in thin flake shapes. Symmetrical V-shaped or curved failure zones occurred on hole sidewalls. The stress distribution resolution of the surrounding rock of tunnels with different arch heights shows that with the increasing burial depth, the bearing performance of the semi-circular arch tunnel is optimal. In addition, the maximum tangential stress increases as the height of the arch decreases or the lateral stress increases, making it easier for the initial failure to occur at the foot of the arch.

Process Optimization and Performance Evaluation of TSV Arrays for High Voltage Application.

In order to obtain high-quality through-silicon via (TSV) arrays for high voltage applications, we optimized the fabrication processes of the Si holes, evaluated the dielectric layers, carried out hole filling by Cu plating, and detected the final structure and electric properties of the TSVs. The Si through-hole array was fabricated in an 8-inch Si substrate as follows: First, a blind Si hole array was formed by the Si deep reactive etching (DRIE) technique using the Bosch process, but with the largest width of the top scallops reduced to 540 nm and the largest notch elimidiameternated by backside grinding, which also opens the bottom ends of the Si blind holes and forms 500-μm-deep Si through holes. Then, the sidewalls of the Si holes were further smoothed by a combination of thermal oxidation and wet etching of the thermal oxide. The insulating capability of the dielectric layers was evaluated prior to metal filling by using a test kit. The metal filling of the through holes was carried out by bottom-up Cu electroplating and followed by annealing at 300 °C for 1 h to release the electroplating stress and to prevent possible large metal thermal expansion in subsequent high-temperature processes. The TSV arrays with different hole diameters and spacing were detected: no visible defects or structure peeling was found by scanning electron microscopy (SEM) observations, and no detectable interdiffusion between Cu and the dielectric layers was detected by energy dispersive X-ray (EDX) analyses. Electric tests indicated that the leakage currents between two adjacent TSVs were as low as 6.80 × 10-10 A when a DC voltage was ramped up from 0 to 350 V, and 2.86 × 10-9 A after a DC voltage was kept at 100 V for 200 s.

Experimental Characterization of Laser Trepanned Microholes in Superalloy GH4220 with Water-Based Assistance.

An experiment using water-assisted millisecond laser trepanning on superalloy GH4220 was carried out, and the effects of pulse energy on the hole entrance morphology, diameter, roundness, cross-section morphology, taper angle, sidewall roughness, and recast layer in air and with water-based assistance were compared and analyzed. The results show that, compared with the air condition, the water-based assistance improved the material removal rate and hole quality, increased the diameter of the hole entrance and exit, increased the hole roundness, decreased the hole taper angle, decreased the hole sidewall roughness, and reduced the recast layer thickness. In addition, under the combined action of water and steam inside the hole, the sidewall surface morphology quality was improved. Compared with the air condition, the spatter around the hole entrance was reduced, but the oxidation phenomenon formed by the thermal effect surrounding the hole entrance with water-based assistance was more obvious. The research provided technical support for the industrial application of millisecond laser drilling.

A comparative study on high pulse energy femtosecond laser drilling of high-aspect-ratio holes under different pressure conditions

Laser drilling of high-aspect-ratio holes is gradually on the agenda in the field of aerospace manufacturing. We present a comparative study of high pulse energy femtosecond laser micro-hole machining of thermal barrier coated superalloys under different pressure conditions. The ablation results showed that severe contaminative ablation patterns occurred near the geometric focus position, which is related to the remarkable refraction of the plasma near the geometric focus. Adopting negative defocus positions can restrain the plasma defocusing effect and eliminate these ablation defects. As compared to the case in the atmospheric pressure condition, the drilled holes had good retention in diameters, and the aperture size as small as 30 μm can be achieved in the low-pressure environment, which is attributed to a good suppression of the plasma defocusing effect. The drilled samples have convergent and divergent shapes struck within the hole in the low-pressure condition while the hole profiles show good collimation along the thickness direction in the atmospheric pressure environment. This can be attributed to the fact that the pulse energy can be well dispersed to the hole sidewall because of the plasma defocusing effect in the atmospheric pressure environment, and the sidewall materials can be effectively removed. The maximum aspect ratio of the hole can reach 15.7 based on the drilling results. The presented results provide some guidance for the machining of micro-holes with high aspect ratios on other occasions.

Evaporation effect of infiltration hole and its comparison with mulching

Reducing ineffective evaporation of soil water is an important way to relieve drought in semi-arid rainfed agricultural areas. An infiltration hole is a supplemental measure for slope rainwater harvesting measure and is commonly used in vegetation restoration and orchard management of slopes. However, the evaporation effects of infiltration holes are unclear. In this study, we determined the evaporation effects of infiltration holes under a high initial soil water content and compared the differences in inhibition of soil evaporation among infiltration holes, mulching, and their combination after a rainstorm event. The results showed that the infiltration holes at different depths increased soil daily evaporation in the first 26 days under the high initial soil water content, after which it decreased. Compared with the control, the average daily evaporation while covering the top surface and the hole (sidewall and bottom) decreased by 74.3% and 4.3%, respectively. After the rainstorm event, cumulative evaporation (97 days) significantly decreased by 42.0% and 34.0% under the infiltration hole compared to gravel mulching and branch mulching, respectively. Overall, the infiltration hole increased soil evaporation at the early stage. Soil water mainly evaporated through the top surface of the soil column rather than through the hole itself. In the long term after rainstorms, infiltration holes have better effects in inhibiting evaporation than mulching. Our findings determine the evaporation effects of infiltration holes in the process of harvesting rainwater to replenish soil water in semi-arid rainfed agricultural areas.

Surface morphology and microstructure of the hole sidewall during drilling of Polyether-Ether-Ketone(PEEK) via single-pulse laser

Polyether-ether-ketone (PEEK), as one kind of thermoplastic polymers, exhibits a great deal of remarkable properties and plays an important role in aerospace, electronics, automotive and medicine. An investigation of single-pulse laser drilling on Polyether-ether-ketone (PEEK) was reported to show the influence of irradiation energy on the hole sidewall. In-situ evolution of drilling process was observed by the high-speed camera. The surface morphology and microstructural characteristics of the hole sidewall were evaluated by different characterization techniques. The hole sidewall was divided into three parts: expanded zone at the entrance, linear zone in the middle, and U-shaped zone at the bottom. It was found that the quality of inner sidewall could be obviously improved under high single-pulse energy (>1 J), in terms of the surface morphology with small heat-affected zone (HAZ) thickness and surface roughness, as well as the least adhesion. Besides, chemical and structural characteristics of the hole sidewall changed a lot under high energy. The C content of three parts increased as the energy increased. The crystallinity of the hole sidewall decreased from 35 % to 25 % after laser-energy irradiation, but there was little change under different single-pulse energy. New diffraction peaks appeared in X-ray diffraction images and the intensity of the relative peaks in Raman spectra decreased when the energy was over 1 J. It was proven that some chemical bonds (phenyl ring (1597 cm−1) and C–O–C (1148 cm−1)) would break under high energy, contributing to mass removal of the keyhole. Moreover, the HAZ thickness of the hole sidewall was calculated based on a viscous flow model, which showed a good consistency with experimental results. These results reveal material-removal mechanism accompanying the appearance of direct decomposition of the material and breaking of chemical bonds under high energy. It can provide references for understanding mechanisms of HAZ formation and material-removal and improving surface quality for laser drilling of polymers.

Femtosecond laser drilling in superalloy with water-based magnetic assistance

Experiment of femtosecond laser drilling in superalloy with water-based magnetic assistance was carried out. The effects of different environments (in air, magnetic assistance, water assistance and water-based magnetic assistance) on the hole morphology, hole diameter, taper angle and sidewall roughness after femtosecond laser drilling were compared and analyzed. The effects of magnetic flux density on the hole diameter, taper angle and sidewall quality with water-based magnetic assistance were discussed. The mechanism of femtosecond laser drilling in superalloy with water-based magnetic assistance was revealed. The results show that transverse magnetic assistance and water assistance could both improve the hole quality and drilling efficiency, and the effect of water assistance was greater than that of magnetic assistance. The drilling efficiency was the highest with water-based magnetic assistance, which was mainly manifested in the increase of the hole diameter, and the increase of the hole cross-section size; on the other hand, the hole quality was the best, which were mainly manifested in the increase of the perpendicularity of the hole cross-section, the decrease of the taper angle, the reduction of irregular micro scale defects such as grooves on the hole sidewall, and the decrease of the roughness of the hole sidewall. Compared with that in air, the hole taper angle was reduced by 24.63 %, and the hole sidewall roughness was reduced by 38.38 % with the water-based magnetic assistance. With the water-based magnetic assistance, the higher the magnetic flux density, the higher the hole quality. This research provided theoretical basis and technical support for femtosecond laser drilling.

Competitive mechanism of laser energy and pulses on holes ablation by femtosecond laser percussion drilling on AlN ceramics

In laser microstructure machining, results are derived through the action of multiple laser processing parameters (LPPs); therefore, multiple studies on the effect of LPPs on machining results are available. Further, few studies observed how competitive mechanism of each LPP affects machining results. As the most important LPPs, single-pulse energy and the number of pulses were varied to assess their effect on hole dimension and morphology. Additionally, competitive mechanism of LPPs on hole ablation was further explored. The study was based on femtosecond laser percussion drilling on aluminum nitride (AlN) ceramics surface. It was found that laser energy has a dominant role in initial ablation, determining both ablation and ablation mechanism. Additionally, it also determined hole inlet surface morphology and ablation mechanism remained the same regardless of increase in the number of pulses. Laser pulses assumed a dominant role after intersection point of LPPs acceleration rate and hole dimension acceleration rate. Hence, final hole dimensions and its sidewall morphology were defined. It was found that deep holes can be processed both in gentle ablation and strong ablation with lots of pulses. However, there are opposite mechanisms for inducing hole sidewall morphology; for example, in strong ablation, solidification effect is enhanced with increase of hole depth, while it is decreased in gentle ablation.

Investigating the effect of different tool electrodes in electric discharge drilling of Waspaloy on process responses

Despite the wide application of electric discharge machining in the fabrication of holes in difficult-to-cut materials, the process's performance and accuracy are governed by the choice of an appropriate tool material and process parameters. Moreover, the process becomes less productive due to unstable machining and inadequate debris flushing while drilling high aspect ration holes, resulting in a low material removal rate (MRR) and high average surface roughness (Ra). Further, the holes' accuracy deteriorates due to a high tool wear rate (TWR). To overcome these issues, a rotary tool electrode setup is proposed for the drilling of Waspaloy. The influence of three different tool materials, viz. copper, brass, and graphite, along with various process parameters, was studied on the process performance. The electrical discharge drilling (EDD) operation was performed by varying the input process parameters, viz. current (I), pulse-on-time (TON), pulse-off-time (TOFF), tool electrode speed (TES), and voltage (V), using copper, brass, and graphite tool electrodes. The influence of machining conditions was investigated on response variables, namely, Ra, TWR, and MRR. Taguchi-based L16 orthogonal array was used to design the experiments, and results were analysed using Analysis of Variance (ANOVA). An improvement in the performance of rotary EDD was observed due to proper flushing, efficient debris evacuation, and steady machining condition. The current was found to be the most significant electrical parameter. The graphite tool electrode had shown the highest MRR of 32.056 mm3/min at 25 A current which was 13.09% and 131.32% higher than that of the copper and brass tool electrodes, respectively. However, the graphite tool resulted in a poor surface quality, and a high energy transfer coefficient, indicating the highest Ra value of 8.385 μm. The brass tool electrode, owing to a lower thermal conductivity and melting point, caused a higher TWR than the copper and graphite tools. Finally, the microscopic images for drilled holes with different electrodes at various energy settings were captured using field emission scanning electron microscopy (FESEM) and compared for surface quality. In addition, the surface elemental analysis was performed using Energy Dispersive X-Ray Analysis (EDX) to investigate material migration from the tool electrodes to hole's side walls.

Failure process and characteristics of three-dimensional high-stress circular tunnel under saturated water content

True-triaxial compression tests were carried out on cubic granite samples with a circular through hole using a true-triaxial testing system to investigate the influence of saturated water content (SWC) on the failure process and characteristics of a circular tunnel of surrounding rocks. The spalling failure under SWC can be divided into four periods: calm period, buckling deformation period, period of rock fragment gradual buckling and exfoliation, and period of formation of symmetrical V-shaped notches. When the horizontal axial and vertical stresses were constant, the spalling failure severity was reduced with the increase in lateral stress. Under natural water content, a strong rockburst with dynamic failure characteristics occurred on the circular hole sidewall. Under SWC, the failure severity was reduced and the circular hole sidewall experienced spalling failure, exhibiting progressive static failure characteristics. Therefore, water can reduce the failure severity of surrounding rocks in deep underground engineering, which has a certain guiding significance for the prevention and control of rockbursts.

Experimental investigation on rockburst process and characteristics of a circular opening in layered rock under three-dimensional stress conditions

To investigate the influence of the bedding angle on the failure process and characteristics of circular hole sidewalls, true-triaxial lab tests were conducted on cubic phyllite specimens with different bedding angles. When the tunnel axis direction was parallel to the bedding plane, the initial failure vertical stress of the hole sidewall first decreased and then increased with an increase in the bedding angle β. A V-shaped notch was formed in the surrounding rock, and the characteristics of the V-shaped notch boundaries were such that one side was approximately parallel to the bedding plane, and the other side crossed the bedding plane. Three failure modes of surrounding rock under different bedding planes were observed: large-scale slip failure along the bedding plane (β = 0°, 15°, and 30°), large-scale shear failure across the bedding plane (β = 45° and 60°), and local rock fragment exfoliation failure (β = 75° and 90°). The large-scale slip (or shear) failure of the surrounding rock generated high acoustic emission (AE) counts, whereas the local failure of the surrounding rock generated relatively low AE counts. By comparison, the initial failure vertical stress was much lower than that when the bedding plane was perpendicular to the direction of the hole axis. Therefore, the tunnel layout method perpendicular to the bedding plane is beneficial for improving the stability of the surrounding rock in underground engineering and reduces the risk of rockburst occurrence.