Final Failure Research Articles

Mechanical behaviour of weakly filled granite and a unified evaluation method for macro-fine-micro multi-scale fracture characteristics

The mechanical properties of weakly filled rock are critical for engineering rock stability control. With the help of 3D DIC (Digital Image Correlation) and AE (Acoustic Emission) monitoring system, and the industrial CT (Computed Tomography), 3D laser scanning and SEM (Scanning Electron Microscope) technology, the mechanical behaviour and fracture characteristics of granite with different inclination angles of planar weak filling were analyzed comprehensively. Firstly, the influence law of the inclination angle of planar weak filling on the basic mechanical properties of the specimens was investigated, and it was confirmed that the specific angle had an obvious weakening effect on the specimen’s ability to resist deformation. Then, the spatial deformation field evolution and failure mode of the specimen were analysed, and it was found that when the inclination angle increased from 0° to 90°, the failure mode of the specimen will gradually change from “Y”-shaped shearing and splitting mixed failure to “chain” splitting failure, and then to overall splitting failure. Then, the spatial distribution of the micro-fracture event was analysed, and the results show that the spatio-temporal evolution and distribution characteristics of the micro-fracture events, the distribution characteristics of the spatio-temporal evolutionary local enlargement zone of the deformation field and the spatial distribution of the main fracture surface at the time of final failure are highly consistent and correlated. The spatio-temporal evolution and distribution characteristics of micro-fracture events provide important references for predicting the location, time and degree of spatial failure in weakly filled rock mass. Finally, based on the fractal theory, the fracture characteristics within the multiscale of the specimen were analysed, and the method of evaluating the unity of the multiscale fracture characteristics of the rock mass was summarised and given.

Experimental investigation of the behavior of UHPCFST under repeated axial tension

A ultra-high performance concrete filled steel tube (UHPCFST) is a composite structural component that extends the performance of both steel and concrete. It is a promising component to be used in a diagrid structure to further reduce self-weight. Compared with the research on compressive performance of UHPCFST, there is a lack of knowledge on the mechanical behavior of UHPCFST under axial tension. This paper fills this knowledge gap by carrying out experiments on UHPCFST subjected to monotonic and repeated tension. The test parameters are steel tube thickness and volume fraction of ultra-high performance concrete (UHPC). The failure modes, load-strain curves, tensile strength and tensile stiffness are studied in detail. Stiffness degradation is also studied. The test results show that: (1) under axial tension load, a UHPCFST typically experiences fracture failure of the outer steel tube, followed by section fracture of the UHPC, and notable deformation before a final ductile failure; (2) tensile strength increases with the increase of the thickness of the steel tube, while it is less obvious in a UHPCFST with a higher steel ratio; (3) the force-strain curve of a UHPCFST under monotonical axial tension is close to that of the UHPCFST under repeated axial tension, suggesting that the accumulated damage during unloading and reloading is limited. An exponential decay formula is proposed to predict stiffness degradation observed in the repeated axial tensile tests. It is found that the design codes from Europe, USA, and China underestimate tensile strength and stiffness of UHPCFST. Finally, a three-phase empirical model is proposed for the load-strain curve of UHPCFST under tension.

Analysis of mechanical response mechanism and energy evolution characteristics of saturated coal with a single pre-existing hole

It is essential to elucidate the mechanical mechanism of water injection and large-diameter drilling pressure relief measures on coal rock deformation and failure. This helps coal mines adopt more precise and effective dynamic disaster prevention and control measures. To obtain the mechanical response mechanism and energy evolution characteristics of saturated coal with a single pre-existing hole, uniaxial compression tests was carried out, and the deformation and failure process of coal samples were monitored by high-speed camera and acoustic emission monitoring systems. It is found that water saturation, single pre-existing hole measures, and their coupling effect resulted in a deterioration in the coal sample's strength by approximately 23.49%, 9.47%, and 47.95% respectively. The final failure mode of coal samples with different measures is shear failure. The law of fracture development and expansion and energy accumulation and release in different coal samples is roughly the same, the speed of energy accumulation and release in different stages is different. Water saturation and single pre-existing hole measures can affect the impact risk of coal samples, and water saturation can significantly reduce the impact tendency of coal samples, and reduce the impact energy index of complete coal samples by 56.59%. The impact tendency index of natural coal sample is slightly increased by single pre-existing hole measure, and the impact energy index of intact coal sample is reduced by 46.78% by the coupling of the two measures. These findings can provide technical support for coal mine power disaster prevention and control.

Flexural behaviour of built-up I-shaped bamboo scrimber beams: experimental tests and design method

ABSTRACT This paper presents the flexural behaviour of built-up I-shaped bamboo scrimber beams under four-point bending. Ten bamboo scrimber beams were tested, employing different types of connections between the web and flanges. The load-deflection relationship, failure mode and load-slip relationship of beams were obtained through experimental tests. Besides, the measured strains of bamboo scrimber in various regions were used to analyse the flexural mechanisms of I-shaped beams. Test results showed that the beam with a glued connection exhibited debonding at the web-flange interface, hindering the development of flexural capacity of the beam. By using self-tapping screws or bolted steel angles to strengthen the connection, the slip between the web and flanges was considerably reduced, resulting in the increased flexural capacity of beams. The rupture of the bottom flange near the concentrated point load led to the final failure of strengthened beams. The design method for mechanical fastened glulam beams in Eurocode 5 was adopted to assess the flexural capacity of bamboo scrimber beams. Comparisons between experimental results and calculated results per Eurocode 5 indicate that the method is capable of predicting the flexural resistances of I-shaped bamboo scrimber beams with reasonably good accuracy.

Mechanism of principal stress rotation and deformation failure behavior induced by excavation in roadways

The failure modes of rock after roadway excavation are diverse and complex. A comprehensive investigation of the internal stress field and the rotation behavior of the stress axis in roadways is essential for elucidating the mechanism of roadway failure. This study aimed to examine the spatial relationship between roadways and stress fields. The law of stress axis rotation under three-dimensional (3D) stress has been extensively studied. A stress model of roadways in the spatial stress field was established, and the far-field stress state at different spatial positions of the roadways was analyzed. A mechanical model of roadways under a 3D stress state was established using far-field stress solutions as boundary conditions. The distribution of principal stresses σ1, σ2 and σ3 around the roadways and the variation of the stress principal axis were solved. It was found that the stability boundary of the stress principal axis exhibits hysteresis when compared with that of the principal stress magnitudes. A numerical analysis model for spatial roadways was established to validate the distribution of principal stress and the mechanism of principal axis rotation. Research has demonstrated that the stress axis undergoes varying degrees of spatial rotation in different orientations and radial depths. Based on the distribution of principal stress and the rotation law of the stress principal axis, the entire evolution mechanism of the two stress adjustments to form the final failure form after roadway excavation has been revealed. The on-site detection results also corroborate the findings presented in this paper. The results provide a basis for the analysis of the failure mechanism under a 3D stress state.

Comparative analysis of experimental and simulation results of composite circular tube

Abstract In this work, the internal pressure of the composite pipe is analysed and studied when transporting liquid or gas. The quasi-static hydraulic test is designed to simulate the internal pressure of the composite pipe. By using the ABAQUS finite element method, a numerical model of the composite pipe under hydraulic load is established. The strain curves and final failure modes of the test and simulation are compared to verify the feasibility of the finite element method model. Finally, the damage mechanism of the composite tube is analysed according to the numerical model, and it is found that the carbon fibre composite material has an important effect on the bearing capacity of the composite tube.

Research on the upper limit of constant strain rates and dynamic fracture behavior for hybrid woven composite

Based on the analysis of stress wave propagation at both ends of the specimen, a formula for the upper limit of the constant strain rate is derived. The upper limit of constant strain rate is applicable to all the ratios of wave impedance, providing a broader applicability. The in-plane dynamic compressive behavior was obtained for 2D plain weave carbon/aramid hybrid composites at strain rates ranging from 195 to 790 s−1. The results validate the prediction of the upper limit of the constant strain rate. The strength exhibited sensitive to strain rate. A nonlinear constitutive model described the dynamic constitutive relationship effectively under different strain rates. The distribution of delamination failure significantly influenced the final failure shape of the specimens. Microscopic observations indicated that carbon fiber yarns and aramid fiber yarns exhibited different fracture modes.

Shear failure analysis of UHPC-NC composites interface with U-shaped studs

Given the extensive utilization of composite structures of ultra-high performance concrete (UHPC) and normal concrete (NC) in engineering applications, there has been a growing emphasis on studying the shear failure and mechanical properties analysis of the UHPC-NC interface. U-shaped studs assume a crucial role in reinforcing the UHPC-NC interface. A three-dimensional finite element model was constructed with different parameters to investigate the shear failure and mechanical properties of UHPC-NC incorporating U-shaped studs. The failure mode and the influences of diameter, internal span, stud number and concrete compressive strength on the shear characteristics of the UHPC-NC interface were analyzed. The numerical analysis indicates the final failure mode of the UHPC-NC interface with U-shaped stubs is the fracture of the U-shaped stud. The diameter and internal span of studs influence the interface shear capacity, while interface shear capacity is not sensitive to the compressive strength of UHPC and NC. Moreover, a calculation formula for the shear capacity of the UHPC-NC interface with U-shaped studs was proposed, considering the influence of stud diameter and internal span. An exponential function is recommended for predicting the interface shear slip and the stiffness corresponding to 0.55 times the shear capacity is adopted as the elastic shear stiffness. The interface shear slip model and the expression for elastic shear stiffness were established, providing a reference for approximate calculation without experimental data.

Spatial reconstruction, microstructure-based modeling of compressive deformation behavior, and prediction of mechanical properties in lightweight Al-based entropy alloys

Lightweight Al-based entropy alloys are newly emerged materials with promising potential for structural applications; however, their modeling and strengthening prediction require further exploration. In this study, the spatial distributions of various phases in three Al-based entropy alloys were investigated. A closely interconnected intermetallic compound (IC) network comprising Al2Cu and Al45Cr7 phases was identified. Finite element (FE) modeling was conducted based on the reconstructed three-dimensional microstructure to simulate compressive deformation behavior. The IC network served as the main stress bearer during deformation. Thin sections in the Al2Cu network were the weak sites where stress concentration and damage first occurred. However, the breakage of this limited region contributes to relatively coordinated deformation and extended plasticity. The breakage of large particles accounts for the final alloy failure. The results of the FE model were compared with the experimentally measured stress–strain behavior and mechanical properties, showing very good agreement. Given the non-uniform distribution of strain and stress during deformation, a strengthening model, merging the Voigt and Reuss models, was also developed to predict the mechanical properties of Al-based entropy alloys with the aim of facilitating Al-based entropy alloy development.

Seismic and radon signatures: A multiparametric approach to monitor surface dynamics of a hazardous 2021 rock–ice avalanche, Chamoli Himalaya

AbstractThe observation of precursory signals of the 2021 Chamoli rock–ice avalanche provides an opportunity to investigate the multidisciplinary analysis approach of rock failure. On 7 February 2021, a huge rock–ice mass detached from the Raunthi peak at Chamoli district in Uttarakhand, India. The tragic catastrophe resulted in more than 200 deaths and significant economic losses. Here, we analyse radon concentration and seismic signals to characterise the potential precursory anomalies prior to the detachment. Continuous peaks of radon anomalies were observed from the afternoon of 5 to 7 February and decreased suddenly after the event, while a cumulative number of seismic tremors and amplitude variations are more intensified ~2.30 h before the main event, indicating a static to dynamic phase change within the weak zone. This study not only characterises abnormal signals but also models the rock failure mechanisms. The analysis unveils three time‐dependent nucleation phases, physical mechanisms of signal generation and a complete scenario of physical factors that affected the degree of criticality of slope failure. The results of this study suggest gradual progression of rock cracks/joints, subsequent material creep and slip advancement acceleration preceded the final failure. Furthermore, the study highlights the importance of an early warning system to mitigate the impact of events like the 2021 Chamoli rock–ice avalanche.

Numerical and experimental studies on bending behavior of circular aluminium-concrete-steel double skin tubular cross-sections and bending capacity design approach

Considering the excellent corrosion resistance of aluminium alloy, a new-type aluminium-concrete-steel double skin tubular (ACSDST) member is proposed by replacing the outer steel tube of concrete-filled double skin steel tubular members with an aluminium alloy tube to improve structures' corrosion resistance and durability. Until now, no literature has been available on ACSDST cross-section in bending. This paper presents experimental and numerical investigations on the flexural behaviour of circular ACSDST cross-sections in bending. Broad parameters are examined in this research, including varying diameter-to-thickness ratios of the outer tube (20–120), hollow ratio (0.2–0.75), concrete compressive strength (30–70 MPa) and steel yield strength (235–355 MPa). The experimental results show that the final failure is controlled by the tensile rupture of aluminium alloy tubes, but it generally happens after the maximum moment, and the composite section behaves in good curvature ductility. The parametric analysis based on the developed finite element (FE) model discloses the different influences of diameter-to-thickness ratios of the outer tube, hollow ratio, and concrete and steel strengths on the bending moment capacity. Among them, the diameter-to-thickness ratio of the outer tube and hollow ratio significantly affect the bending moment capacity. The design equations for bending moment capacity are induced based on the plastic stress distribution method. Those equations are safe and accurate for the maximum moment (a widely used definition of bending moment capacity) within a certain range of parameters. However, a reduction factor 0.9 is proposed for the equations to predict the moment at a longitudinal flexural tensile strain of 1% (another widely used definition of bending moment capacity). Moreover, the cross-section slenderness limit for the compact ACSDST section is recommended based on the analysis of the experimental and FE results.

Experimental study on I/II/III mixed mode fracture characteristics of a combined rock mass under creep loading

I/II/III mixed mode fractures of intersecting joint fissures often occur in natural rock masses, and jointed rock masses are prone to rockbursts in deep underground engineering when subjected to long-term crustal stresses. However, most studies of the mechanical mechanisms of these intersected joints have been conducted by simplifying two-dimensional joint model tests. Furthermore, the fracture mechanisms of two-dimensional intersected joints under tension and compression are completely different from those of three-dimensional joints. This paper presents a novel prefabricated specimen with combinations of intersecting joints capable of detecting the failure behaviours of rock I/II/III mixed mode fractures under creep loading. Uniaxial compression and multistage creep tests are performed on prefabricated sandstone specimens with intersecting joints of 0°/0°, 0°/30°, 0°/60°, and 0°/90°. The experimental results show that with the increase in the number of prefabricated intersecting joints, the uniaxial compressive strength and elastic modulus values of the sandstone specimens gradually decrease. In addition, the sandstone specimens experience relatively few AE events and minor axial strain variations in the first creep stage and the second creep stage of the multistage creep test. The axial strain increases sharply due to the sharp increase in the number of AE events in the third creep stage. The 0°/60° sandstone specimen undergoes accelerated creep failure, resulting in mixed X-shaped tensile‒shear rupture. The RA value is high based on the quantification of the creeping cracks using the acoustic emission parameters of the rise angle (RA) and average frequency (AF). The AF values of the 0°/0°, 0°/30°, and 0°/90° sandstone specimens are high. The experimental results show that a larger joint intersection angle leads to greater mutual restraints and greater effects of prefabricated crack propagation in the rock specimens, thus increasing the final failure strength. Finally, based on the acoustic emission count, a characteristic variable D suitable for characterizing the creep damage evolution of a joint rock mass is established. The findings of this paper can facilitate an effective understanding of the creep effect of I/II/III mixed mode fracture and its micromechanism. The research results will have a certain reference value for the detection and risk mitigation of instantaneous and time-delayed rockbursts.

Strengthening the bonding interfaces of carbon reinforced aluminum laminate by slot structure

AbstractA reliable strategy to enhance the bonding ability in carbon reinforced aluminum laminates (CARALL) is proposed, which is based on the mechanical interlocking effect generated by the surface slot structure of aluminum alloy enhances the structural interface between fibers/resins and aluminum alloy. The characteristic of slot structure is determined, and the strengthening mechanism of bonding interfaces is revealed. The CARALL samples with slot structure are fabricated via an hot pressing process. The micro texture morphologies, mechanical, interlayer bonding properties under different conditions are studied. The results indicate that forming a slot structure on the aluminum surface is beneficial for improving the interfacial bonding performance. Compared with the non‐slotted structure, the interlayer type II fracture toughness of the slotted structure is increased by 112.99%. The slotted structure can reduce the defects at the interface between the layers and improve the interface bonding force. In this work, the interlaminar shear strength (ILSS) of the slotted structure sample was 27.08% higher than that of the non‐slotted structure sample.Highlights A slotted structure was proposed to solve the problem of insufficient bonding strength. A slotted structure consistent with the direction of adjacent fibers is designed. The interlayer type II fracture toughness of the slotted structure is increased by 112.99%. The final crack propagation of the slotted structure sample is reduced by 40%. The final failure area of the slotted structure sample is reduced by 53.85%.

Investigation of three-dimensional model reconstruction and fractal characteristics of crack propagation in jointed sandstone

The presence of random joints, cracks, and other defects significantly affects the meso-damage mechanism and macro-mechanical behavior of the rock. This study employed micro-CT scanning, digital image processing (DIP), and the rock failure process analysis system (RFPA3D) to reconstruct a genuine mesostructure, creating a three-dimensional (3D) numerical model of jointed sandstone. Under uniaxial stress, this model facilitated the meso-damage evolution process of prefabricated cracks in sandstone with varying dip angles. Additionally, the influence of jointed sandstone heterogeneity and prefabricated cracks with various dip angles on its failure mode and meso-damage mechanical properties were investigated. Utilizing the MATLAB platform, a 3D box dimension algorithm was developed to analyze the fractal characteristics of the mesodamage evolution in the sample. This algorithm enabled the quantitative characterization of the meso-damage evolution of sandstone. This study categorized three types of sandstone final failure modes: composite shear failure, shear failure along the joint surface, and tensile failure. Additionally, linear variations in the elastic modulus and compressive strength of the jointed sandstone were observed with increasing prefabricated fracture inclination, highlighting significant anisotropy. The presence of joints was found to induce and control the failure mode of sandstone. The meso-damage evolution process of sandstone was described in terms of the fractal dimension, indicating that more severe damage corresponded to a larger fractal dimension. This approach offers a novel statistical method for studying the progression of rock damage.

Comparative study of eco-friendly wire mesh configurations to enhance sustainability in reinforced concrete structures

Recent and past studies mainly focus on reducing the dead weight of structure; therefore, they considered lightweight aggregate concrete (LWAC) which reduces the dead weight but also affects the strength parameters. Therefore, the current study aims to use varied steel wire meshes to investigate the effects of LWAC on mechanical properties. Three types of steel wire mesh are used such as hexagonal (chicken), welded square, and expanded metal mesh, in various layers and orientations in LWAC. Numerous mechanical characteristics were examined, including energy absorption (EA), compressive strength (CS), and flexural strength (FS). A total of ninety prisms and thirty-three cubes were made. For the FS test, forty-five 100 × 100 × 500 mm prism samples were poured, thirty-three 150 × 150 × 150 mm cube samples were made, and forty-five 400 × 300 × 75 mm EA specimens were costed for fourteen days of curing. The experimental findings demonstrate that the FS was enhanced by adding additional forces that spread the forces over the section. One layer of chicken, welded, and expanded metal mesh enhances the FS by 52.96%, 23.76%, and 22.2%, respectively. In comparison to the remaining layers, the FS in a single-layer hexagonal wire mesh has the maximum strength, 29.49 MPa. The hexagonal wire mesh with a single layer had the greatest CS, measuring 36.56 MPa. When all three types of meshes are combined, the CS does not vary in this way and is estimated to be 29.79 MPa. In the combination of three layers, the chicken and expanded wire mesh had the most energy recorded prior to final failure, which was 1425.6 and 1108.7 J, whereas it was found the highest 752.3 J for welded square wire mesh. The energy absorption for the first layer with hexagonal wire mesh increased by 82.81% prior to the crack and by 88.34% prior to the ultimate failure. Overall, it was determined and suggested that hexagonal wire mesh works better than expanded and welded wire meshes.

Application of AIE luminogen-loaded core–shell fibers in self-warning and self-healing polymer coatings with enhanced corrosion resistance

Aggregation-induced emission (AIE) materials have been used for non-destructive visual detection of cracks and damages to prevent the final structural failure and increase the service life of coatings. In this study, a self-warning and self-healing core–shell fiber is synthesized for developing AIE composite coatings. In these core–shell fibers, a mixture of hmethylene diisocyanate (HDI) and tetraphenylethylene (TPE) is used as core constituent, and polyacrylonitrile (PAN) is used for preparing the shell, which are continuously wrapped with a high loading rate through coaxial electrospinning technology. The parameters of electrospinning process are optimized to achieve core–shell fibers with smooth surface and average diameter of 100 ± 50 nm. The composite coating reveals effective fluorescence in case of cracks in a corrosive environment. The maximum self-healing efficiency of the fiber coating is 94.8 % after soaking for 120 h. The introduced composite coating paves the way for improving the safety and reliability of critical engineering components.

Effects of heat treatment on tensile properties of 3D-printed short fiber-reinforced composites

3D-printed carbon fibre-reinforced composites (CFRC) using fused deposition modelling (FDM) offer the potential for building complex geometries and low waste. However, these composites have weaker interlaminar bonding and higher void content than traditional composites. This paper explores the effect of temperature on improving the mechanical properties of 3D-printed short carbon fibre- reinforced polyamide (PACF). Two types of printed structures were tested: one with a 0° build orientation (parallel to extruder movement) and the other with a 90° build orientation (perpendicular to extruder movement). Both samples were heat-treated at 150°C. Force vs. displacement data was obtained from the MTS testing machine. After that, the tested samples were viewed under an optical microscope. Images obtained from the optical microscope are then analyzed to see how the samples failed by checking the microstructure. The average ultimate strength, ultimate strain, and elastic modulus were used for the analysis. The sample follows a trend where the strength and elastic modulus increase after heat treatment. The result also showed that the 0° build orientation samples have higher mechanical performance than the 90° build orientation sample. Also, from the ultimate strain values, it was evident that samples printed in the 0° absorbed more energy, exhibiting 83% higher resistance before final failure. Lastly, an optical microscope was used to investigate the failure mechanisms of the samples.

Towards automated characterisation of fatigue damage in composites using thermoelastic stress analysis

Composite materials demonstrate complicated fatigue behaviour due to their microstructure and the varied types of defects that can occur during loading. This necessitates experimentation to determine their performance under loading. In this study an algorithm is introduced for identifying and categorising different defects forming during fatigue tests. Thermoelastic stress analysis was used to obtain high spatial and temporal resolution stress information from the surface of notched composite specimens. Specimens with three different geometries were loaded in tension–tension fatigue to failure. An algorithm was used to identify when and where matrix cracking and delaminations formed within the specimens as well as quantify how this changed over time. By improving how damage events are identified and characterised, the algorithm reduces the amount of time needed to process experimental fatigue data and helps to provide greater understanding of fatigue processes in new materials from early small-scale cracking all the way to final failure.

Investigation on fatigue behavior of GFRP hybrid bonded/bolted joint under shear loading based on nondestructive monitoring test and numerical modeling

The GFRP (Glass Fiber Reinforced Polymer) is prospective in bridge and building engineering for its advantageous material property. The connection configuration of hybrid bonded/bolted joint of GFRP is thought superior in mechanical performance compared with the traditional bonded and bolted joints. However, the fatigue issue of this type of joint remains unclear which impedes its engineering applications. The presented work experimentally and numerically investigated the fatigue behavior of the GFRP hybrid bonded/bolted single-lap joints under shear loading. High-cycle fatigue tests on single-bolt, four-bolt, and nine-bolt specimens were conducted to study the failure process, rigidity degradation, failure mode, and fatigue life of the hybrid joints, where the nondestructive monitoring techniques of AE (Acoustic Emission) and 3D-DIC (Three-Dimensional Digital Image Correlation) were adopted, respectively to detect the material damages and overall deformation of specimens. Finite element modelling was further carried out to reveal the fatigue failure mechanism of hybrid joints, in which the property degradation of GFRP plates and adhesive layer were considered. Results showed that the failure process of GFRP hybrid single-lap joints under fatigue shear loading can be divided into four stages: (1) steady and (2) rapid development of adhesive damage, (3) steady and (4) rapid development of GFRP damage, of which the (2) and (4) stages account for less than 3% of the total fatigue life of joint, respectively. Stiffness degradation of 14–35% were read for the tested joints before final failure, and the joints with more bolts performed a superior degradation resistance compared to the less ones. Besides debonding and bolt inclination, the single-bolt joints failed with bearing failure of GFRP material, while the failure mode of multi-bolt joints were dominated by shear failure in GFRP plate with Y-shaped zone. The simulation results well supported the observations in experimental test. Based on the experimental results, the S-N curve of the GFRP hybrid single-lap joints is proposed, which can provide support for the engineering design of this type of joints.

Experimental study on the dynamic mechanical and progressive fracture behavior of multi-jointed rock mass under repetitive impact loading

The failure of rock masses is often controlled by structural planes, and the study on the dynamic mechanical behavior of multi-jointed rock masses under repetitive dynamic loading is still few. A series of repetitive impact tests on multi-jointed gabbro (MJG) are conducted with the split Hopkinson pressure bar (SHPB) testing apparatus. The effects of repetitive impact and joint inclination angles (α) on the dynamic mechanical behavior of MJG from the perspectives of impact number, stress equilibrium, dynamic mechanical properties, energy evolution, and macro and micro failure modes are deeply investigated. The results show that the joint inclination angle affects the impact resistance of the MJG. Under repetitive impact loadings, the mechanical behavior of the MJG is consistent, with tensile wing cracks initiating from the joint tips and propagating along the loading direction until specimen failure. During this process, cracks propagate along the weakest paths, thereby consuming the energy generated by the impact and preventing the activation of other minor defects. The macroscopic failure of the MJG undergoes four stages: the appearance of white patch, crack coalescence, crack penetration, and fracture, with the final failure mode being splitting. The microscopic formation stage of white patch is divided into microcrack activation and microcrack widening.