Shear Failure Research Articles

Failure Modes and Fault Morphology

Abstract Fault failure modes determine the geometric characteristics of faults and fault zones during their formation and early development. These geometric properties, in turn, govern a wide range of fault processes and behaviors, such as reactivation potential, fault propagation, and growth, and the hydraulic properties of faults and fault zones. Here, we use field data and close-range digital photogrammetry to characterize, in detail, the surface morphology of three normal faults with cm-scale displacements in mechanically layered carbonates of the Cretaceous Glen Rose Formation at Canyon Lake Gorge, Comal County, Texas. Analyses demonstrate complex fault surface geometries, a broad spectrum of slip tendency (Ts) and dilation tendency (Td), and variable failure behavior. We show that (i) fault patches coated with coarse calcite cement tend to have moderate to high dips, low to high Ts, and high to very high Td; (ii) slickensided fault patches exhibit low to moderate dips, moderate to very high Ts, and moderate to high Td; and (iii) slickolite patches exhibit low dips, moderate Ts, and low to moderate Td. Calcite-coated patches are interpreted to record hybrid extension-shear failure, whereas slickensided and slickolite patches record shear and compactional shear failure, respectively. Substantial variability in both Ts and Td across the exposed fault surfaces reflects complex fault morphology that is not easily measured using traditional field techniques but is captured by our photogrammetry data. We document complex fault geometries, with kinematic (displacement) compatibility indicating the various failure modes were active coevally during fault slip. This finding is in direct contrast with the often-assumed concept of faults forming by shear failure on surfaces oriented 30° to σ1. Distinct failure behaviors are consistent with patchworks of volume neutral, volume gain, and volume loss zones along the fault surfaces, indicating that the characterized faults likely represent dual conduit-seal systems for fluid flow.

Size Effect on Shear Behaviors of 3D-Printed Soft Rock-like Materials Under Different Shear Velocities

The shear failure of rock masses is one of the primary causes of underground engineering instability. The shear mechanical behavior of rocks at different sizes is of great significance for studying the shear failure pattern of engineering rock masses. However, due to the presence of various joints and defects in natural rocks, the obtained rock specimens exhibit significant discreteness, making it difficult to customize specimen sizes for size effect studies. In recent years, 3D printing (3DP) technology has gained widespread application in rock mechanics tests due to its high printing precision and ability to form specimens in a single step with minimal discreteness. Among these, specimens prepared using sand-powder 3DP exhibit elastoplastic mechanical characteristics similar to those of natural rocks. Therefore, this study utilized sand-powder 3DP to prepare rock-like specimens of four different sizes and conducted compression shear tests under three different shear velocities. The shear strength, shear strain, and wear of the shear surfaces were analyzed as functions of specimen size and shear velocities. The results indicate that under the same shear velocity, the shear strength of the specimens is negatively correlated with specimen size. The peak shear strain is generally unaffected by shear velocities, but it increases initially and then decreases with increasing specimen size. As specimen size increases, the degree of specimen damage intensifies, and larger specimens are more prone to developing derived fractures. This study broadens the application of sand-powder 3DP technology in investigating the shear mechanical properties of soft rocks, offering novel insights into the study of size effects in rock mechanics. However, the current research does not encompass tests on 3D-printed rock specimens with varying printing directions, nor does it delve into the role of fractures in size effect analyses. Future investigations will aim to address these limitations, thereby advancing the applicability of 3D printing technology in rock mechanics research and enhancing its contributions to the field.

Research on the Mechanical Properties of EPS Lightweight Soil Mixed with Fly Ash

Expanded polystyrene (EPS) bead–lightweight soil composites are a new type of artificial geotechnical material with low density and high strength. We applied EPS bead–lightweight soil in this project, replacing partial cement with fly ash to reduce construction costs. EPS beads were used as a lightweight material and cement and fly ash as curing agents in the raw soil were used to make EPS lightweight soil mixed with fly ash. The EPS bead proportions were 0.5%, 1%, 1.5%, and 2%; the total curing agent contents were 10%, 15%, 20%, and 25%; and the proportions of fly ash replacing cement were 0%, 15%, 30%, 45%, and 60%, respectively. Unconfined compressive strength (UCS) and scanning electron microscopy (SEM) tests were conducted. The results showed that the EPS content, total curing agent content, and proportion of fly ash replacing cement had a significant impact on the UCS of the lightweight soil. This decreased with an increase in EPS content and decrease in total curing agent content and decreased with increased proportions of fly ash replacing cement. When the proportion of fly ash replacing cement was not too high, the strength of the lightweight soil decreased less, and its performance still met engineering needs. At the same time, the soil can also consume fly ash and reduce environmental pollution. EPS lightweight soil mixed with fly ash still has advantages, and it is recommended to keep the proportion of fly ash replacing cement less than 30%. The failure patterns for lightweight soil mainly include splitting failure, oblique shear failure, and bulging failure, which are related to the material mix ratio.

Investigation of Shear Behavior in High-Strength Bolt Connectors for Steel–Concrete Composite Beams

High-strength bolt connectors, known for their robust strength and ease of disassembly, are suitable not only for the construction of new steel–concrete composite beams but also for reinforcing existing composite or steel beams. Static push-out tests were performed on nine specimens to examine their shear behavior. The primary failure mode was observed at the steel–concrete interface, characterized by the tensile–shear failure of the bolt and localized crushing of the concrete beneath the bolt. The preload had no significant influence on the ultimate bearing capacity and ultimate slip displacement, while it had a substantial impact on the initial slip load. The failure process was divided into static friction at the interface, sliding at the interface, elastic deformation of the bolt, and plastic deformation of the bolt. The parametric analysis using the finite element method was performed to assess the impact of concrete strength, reserved hole diameter, interface friction coefficient, and bolt diameter and strength. It revealed that the ultimate bearing capacity is composed of interfacial friction and bolt shear capacity, which are not independent of each other. To decouple these components, a novel calculation method for determining the ultimate bearing capacity of high-strength bolt connectors was developed and validated using existing test data.

Experimental and numerical investigations on the performance of screw micropiles under pull-out and lateral loads

ABSTRACT Screw micro-piles are becoming progressively popular for inherent advantages and installation convenience. Their load-transfer technique and failure mechanism are complex and they bear more loads compared to the conventional piles, due to soil-thread interactive shear and bearing stresses. Available knowledge on performance of such piles is limited. The authors conducted a series of small-scale model tests on pull-out and lateral load-displacement characteristics of screw micro-piles embedded in soft cohesive soil. A sophisticated test set-up is developed to carry out the experiments under controlled conditions. The tests are followed by three-dimensional finite-element modeling by ABAQUS. The numerical output data closely matched with experimental results. Utilizing the finite-element model, interface shear stress distributions, load-transfer mechanism and failure envelop development are studied. The pile capacities are found to be influenced by different pile parameters, like pitch, embedded length, diameter, thread angle, etc. A set of important conclusions are drawn from the entire study.

Influence of water saturation on mechanical characteristics and fracture evolution of coal rock assemblage with rough interfaces

To comprehensively investigate the influence of water content on the mechanical and crack propagation characteristics of coal rock assemblage (CRA) with a rough interface, uniaxial compression tests were conducted on specimens with varying water content. Nuclear magnetic resonance (NMR) and acoustic emission (AE) techniques were employed to monitor the water content and AE signals throughout the experiment. The physical and mechanical properties, as well as the extent of crack development and acoustic emission (AE) parameters, were comprehensively investigated under conditions of water erosion. The results demonstrate that a rough interface contributes to an enhancement in the compressive strength of the composite material. Moreover, the moisture content exerts a significant influence on various aspects of the composite specimen, including its compressive strength, time b value, crack development, and crack propagation. With the increase in water content, the initial single slope shear failure of the composite specimen gradually transitions into a multi-section shear failure mechanism. Under the influence of water-rock interaction, sandstone within the formation undergoes a metamorphosis from a densely cemented structure to an irregular honeycomb-like configuration. This transformative process engenders novel porosity and fractures, ultimately compromising the rock’s mechanical strength. The analysis focuses on the relationship between the AE parameter b and uniaxial stress and water content, with emphasis on its relevance to damage theory. A damage model based on water immersion rate was established to elucidate the correlation between damage variables and water content. This was achieved by considering the characteristics of water-rock coupling AE and constructing a structural model of the water absorption process in different pore throats, thereby providing valuable insights for stability design and evaluation of roadway rock masses.

Effectiveness of UHPC Jackets in Pier Retrofitting for Lateral Load Resistance

Ultra-high-performance concrete (UHPC) is a recently emerged material with exceptional durability and ductility. While widely used in bridge retrofitting, particularly to replace expansion joints and deck overlays, UHPC has seen limited use in jacketing piers for the improvement of lateral load resistance. It presents superior mechanical properties and deformation resilience, enabled by the distributed fibers and the dense microstructure, providing corrosion resistance and a maintenance-free service life. The significant tensile strength and ductility establish UHPC as an attractive resilient jacketing system for structural members. The experimental literature documents the effectiveness of this solution in enhancing the strength and ductility of the retrofitted member, whereas premature modes of failure (i.e., lap splices and shear failure in lightly reinforced piers) are moderated. A comprehensive database of tests on UHPC-jacketed piers under lateral loads was compiled for the development of practical guidelines. Various UHPC jacket configurations were evaluated, and detailed procedures were developed for their implementation in bridge pier retrofitting. These procedures include constitutive models for UHPC, confined concrete, and the strengthening of lap splices, flexure, and shear resistance. The results are supported by the database, providing a solid foundation for the broader application of UHPC in improving the lateral load resistance of bridge piers.

Experimental Study on the Mechanical Properties and Damage Mechanism of Saturated Coal-Measure Sandstone in an Open Pit Mine Under the Freeze–Thaw Effect

The damage and degradation of coal-measure sandstone in cold-region open-pit mines due to freeze–thaw effects has become one of the significant factors inducing instability in the rock mass of open-pit mine slopes. This study conducts experiments on the physical and mechanical properties of saturated coal-measure sandstone under varying freeze–thaw cycle counts and freezing temperatures, revealing the intrinsic mechanisms of damage and degradation in saturated coal-measure sandstone due to freeze–thaw effects. The experimental results indicate that, with an increase in the number of freeze–thaw cycles and a decrease in the freezing temperature, the elastic modulus and peak compressive strength of the specimens exhibit an exponential decrease. In contrast, the peak strain shows an exponential increase. However, compared to the freezing temperature, the increase in the freeze–thaw cycle frequency leads to a more significant change in the mechanical parameters of the specimens, indicating that the frequency of freeze–thaw cycles has a more pronounced effect on the deformation resistance of saturated coal-measure sandstone than the freezing temperature. The failure mode of coal-measure sandstone specimens under uniaxial compressive loading primarily exhibits shear failure; however, as the number of freeze–thaw cycles increases and the freezing temperature decreases, the specimens begin to exhibit tensile failure modes, which gradually develop into a combined tensile and shear failure mode. Based on the experimental data, two sets of surface equations were fitted to characterize the relationship between the mechanical properties (peak compressive strength, elastic modulus) of the specimens and the experimental parameters (number of freeze–thaw cycles, freezing temperature). The research findings can provide references and insights for engineering disasters caused by the degradation of coal-bearing sandstone in cold-region open-pit mines.

Effect of Extreme Environments on Adhesive Joint Performance

The presented research on adhesives was conducted with the aim of supporting the design of composite repairs for composite aircraft structures that can withstand specific environmental conditions. Double-sided strap joint specimens of epoxy-based CFRP adherents and straps were bonded by two types of adhesives. Room-temperature curing epoxy adhesives EC-9323 and EA-9395 were used for bonding. The specimens’ shear strength and failure modes were evaluated under four different environmental conditions from −72 °C up to 70 °C unconditioned and at 70 °C after humidity conditioning. The results show that EC-9323 performed excellently at room temperature, but very poorly at elevated temperatures after hot–wet conditioning. Adhesive EA-9395 performed consistently well across all tested conditions. The failure mode analysis explained the performance trends and the effect of the environment on the fractured surface. This study will support proper repair design and verification of numerical simulations. The novelty of this article lies in its combined analysis of multiple environmental factors, providing a more realistic assessment of joint performance.

Enhancement Effect of Phragmites australis Roots on Soil Shear Strength in the Yellow River Delta

Soil erosion is one of the causes of ecosystem fragility in the Yellow River Delta. Plant roots can improve soil shear strength and effectively prevent soil erosion. However, there are no studies on soil shear strength in the Yellow River Delta. In this study, Phragmites australis (PA) root–soil composites with different root area ratios (RARs) (RARs = 0%, 0.06%, 0.14%, 0.17%, 0.19%, 0.24%, 0.36%) were prototypically sampled from the Yellow River Delta. Direct shear tests of root–soil composites were performed by a ZJ-type (three-speed) strain-controlled direct shear apparatus. The normal stresses were 25, 50, 100, and 200 kPa, and the shear rate was 1.2 mm/min. The results showed that PA roots significantly increased soil shear strength and cohesion with maximum growth rates of 219.0% and 440.1%, respectively. An optimal RAR of 0.14% in the range of 0~0.36% maximized the shear strength and cohesion of the root–soil composites. The internal friction angles of root–soil composites with different RARs did not differ significantly from those of the rootless soil. This indicates that the increase in shear strength was mainly due to an increase in cohesion. In addition, overall shear failure was the primary failure mode of rootless soil, with the roots pulled out of the soil in the root–soil composite failure mode. It is important to note that the root is deflected during shear in the direction opposite to the direction of the shear stress. These findings deepen our understanding of the effect of vegetation roots on soil shear characteristics and provide a scientific basis for the protection of bank slopes, soil and water conservation, and vegetation restoration in the Yellow River Delta.

Development and experimental study of a replaceable double stage coupling beam damper

A double stage coupling beam damper (DSCBD) was proposed and tested in this paper. The DSCBD comprises a viscoelastic damper and a metallic damper connected in series. Deformation of the DSCBD in the first stage was concentrated on the viscoelastic damper, which provided initial stiffness and energy dissipation capacity. With the increase in displacement and the initiation of the second stage, the deformation of the viscoelastic damper was constrained by the limiting mechanism and triggered the deformation concentrating in the metallic damper. These further increased the stiffness and energy dissipation capacity and the DSCBD exhibits the double stage behavior. The DSCBD proposed in this paper can simultaneously satisfy the seismic requirements of the main structure under different levels of seismic excitation, without installing different types of traditional dampers to achieve the same effect. A performance-based design method was proposed. Two full-scale DSCBD specimens were subjected to quasi-static multi-stage cyclic loading tests to investigate the effects of different aspect ratios. The test results showed that DSCBD with an aspect ratio of 3.2 exhibited ideal shear failure mode, and the minimum equivalent viscoelastic damping ratio in the second stage was 22.58 %, while the DSCBD with an aspect ratio of 4.4 exhibited unexpected bending failure mode, and the maximum equivalent viscoelastic damping ratio in the second stage was 19.42 %. In addition, the DSCBD with an aspect ratio of 3.2 showed better low-cycle fatigue capacity, whose degradation of bearing and energy dissipation capacities can be controlled within 5 % after 30 additional loading cycles. The accuracy and the correction scheme of the design method were verified by the test results.

A FEM‐DEM Coupling Analysis on Bearing Capacity of Sandy Soil Foundation Considering Fabric Anisotropy

ABSTRACTWith the acceleration of urbanization, the stability of the foundation is being more crucial to the performance and service of the superstructure. As our understanding of the factors influencing soil's physical and mechanical behavior deepens, it becomes increasingly challenging for traditional limit equilibrium and limit analysis methods to accurately consider the complex factors affecting foundation stability, such as initial fabric anisotropy caused by the particle morphology and geological deposition in sand. Although some scholars had used advanced constitutive models in the finite element method (FEM) to investigate the influence of initial fabric anisotropy on mechanical responses of foundations, this approach failed to reveal the microscopic information underlying the shear failure of sandy soil foundations. In this study, the influence of the initial fabric anisotropy of sandy soil on the ultimate bearing capacity and shear failure mode of shallow foundation is studied using the hierarchical FEM and discrete element method (DEM) coupling analysis method. Four representative volume elements (RVEs) with varying initial bedding plane angles are constructed in DEM for characterizing different initial fabric anisotropies, and the specific stress–strain information of DEM RVEs is directly passed into the corresponding Gauss points in FEM to replace the conventional constitutive model. Numerical results show that the initial fabric anisotropy affects the ultimate bearing capacity and shear failure mode of shallow foundations significantly, and the corresponding micromechanical behaviors at different local Gauss points have been explored, which advances our understanding of the micromechanisms underlying the progressive shear failure of sandy soil foundations significantly.

Investigation of temperature effect on thermo-mechanical property of carbon fiber/PEEK composites

Abstract Considering the temperature sensitivity and two-phase incompatibility of the fiber matrix interface, carbon fiber-reinforced thermoplastic composites suffer from weak interfacial bonding strength. To investigate the delamination damage and shear failure mechanism of T700/PEEK composites at varying temperatures, interfacial shear strength (IFSS) and interlaminar shear strength (ILSS) were performed by employing micro debonding experiments and three-point bending. The results indicated that at temperatures below the glass transition, the IFSS of T700/PEEK composites was positively correlated with temperature, and the average strength recorded was 52 ± 6 MPa. When the glass transition temperature was exceeded, the bonding state between polyether ether ketone material and fiber surface became tighter, resulting in a slight increase in IFSS, reaching 60 ± 4 MPa. Further, the ILSS of T700/PEEK composites was tested, and the results indicated a negative correlation between ILSS and temperature, with a maximum ILSS of 40 ± 2 MPa observed at 23°C, and a minimum of 11 ± 1 MPa recorded at 230°C. The decline in bending strength observed with increasing temperature was attributed to the separation of the fiber and matrix interface at high temperatures, which reduced the mechanical properties of T700/PEEK composites. By conducting temperature related mechanical performance tests on T700/PEEK composite materials, the obtained test results will help researchers expand the application scenarios of this material, deepen the relationship between the temperature and its performance, and thus more quickly explore the mechanism of temperature action.

Shear Failure Assessment of a G+3 RC Framed Building via Nonlinear Static Pushover Analysis using SAP2000

Accurately modelling shear behaviour in reinforced concrete (RC) structures is essential for seismic evaluation, as shear failure can lead to catastrophic outcomes. Current industry practices often emphasize nonlinear flexural analysis while underestimating the critical role of shear behaviour. This study aims to develop a nonlinear force-deformation model specifically for shear in RC sections to bridge the gap in existing literature. Standards like IS-456:2000 and ACI-318:2008 provide ultimate strength estimates but may inadequately consider the contribution of concrete under seismic loads. Insights from models by Priestley et al. (1996) and Park and Pauley (1975) are integrated to improve the calculation of shear hinge properties. A comparative nonlinear static (pushover) analysis of an RC framed building, with and without shear hinges, demonstrates the impact of shear modelling. Results show that neglecting shear hinges overestimates base shear and roof displacement capacity, masking non-ductile failure modes. Incorporating shear hinges provides a more realistic assessment of structural strength and ductility, emphasizing their necessity in seismic analysis and design. Keywords: Shear behaviour, Reinforced concrete (RC) structures, Seismic evaluation, Shear failure, Nonlinear analysis, Force-deformation model, Shear hinge.

Shear behavior of high-strength concrete beams reinforced with carbon fiber-reinforced polymer bars

This study experimentally investigates the shear performance of high-strength concrete (HSC) beams reinforced with carbon fiber-reinforced polymer (CFRP) bars. Thirteen HSC beams were tested under four-point monotonic loading until failure, focusing on parameters such as the shear span-to-depth (a/d) ratio, longitudinal reinforcement ratio, transverse reinforcement ratio, and the type of longitudinal reinforcement (CFRP, steel, and hybrid CFRP-steel). The study analyzed failure modes, shear cracking patterns, cracking load, ultimate shear capacity, and load-deflection responses. Results indicated that shear failures in CFRP-reinforced HSC beams can occur suddenly due to the brittle nature of both FRP and HSC. Adding longitudinal steel bars alongside CFRP bars significantly enhanced the ultimate shear capacity while balancing both ductility and durability. The ultimate shear capacity was primarily influenced by the longitudinal reinforcement ratio and the a/d ratio. Specifically, the ultimate shear capacity decreased by approximately 72 % as the a/d ratio increased from 1.0 to 3.0. Furthermore, the maximum midspan deflection increased by 135 %, rising from 4.0 mm to 9.4 mm with the same change in the a/d ratio. Conversely, increasing the longitudinal reinforcement ratio from 0.6 % to 1.7 % led to a 43 % increase in ultimate shear strength due to enhanced dowel action mechanism and improved crack control, which contributed to a higher shear resistance in the concrete beam. Both CFRP and steel-reinforced beams exhibited similar shear capacity, indicating comparable shear transfer mechanisms. Additionally, a proposed shear model based on regression analysis of 139 FRP-reinforced HSC beams demonstrated better predictive accuracy than existing design codes.

Experimental and numerical studies on low velocity impact behaviors of curved prestressed concrete shells

This study investigates the behavior of curved prestressed concrete (CPC) shells under low-velocity impact loads. The dynamic response of CPC shells under impact loading was studied by conducting impact tests and numerical simulations, with the impact process divided into three stages. The failure modes, impact force, displacement response, and energy dissipation of the specimens were analyzed to reveal the effects of prestress level, curvature, and initial impact momentum on the impact performance of CPC shells. Shear and flexural failure modes were observed in the six specimens under impact loading, along with three types of local damage: concrete indentation, crushing, and spalling. As curvature decreases, the failure mode transitions from shear to flexural failure. However, the prestress level and impact momentum did not change the failure mode of the specimens. Prestress induced both pressure strengthening and flexural weakening effects on the specimens, which collectively influenced their impact performance. Additionally, it was observed that prestress and curvature interactively affect the impact performance of the specimens. Furthermore, under higher initial impact momentum, the CPC shell experienced more severe shear damage.

Optimization of preparation parameters and testing verification of carbon nanotube suspensions used in concrete

Abstract Carbon nanotubes (CNTs) have received extensive attention due to their exceptional properties and wide range of applications. However, the agglomeration of CNTs in aqueous solutions and organic solvents significantly limits their large-scale application. In this study, the microscopic morphology and dispersion stability of the CNT suspensions were analyzed, and the most suitable surfactant in this study was selected. The preparation parameters of the CNT suspensions were optimized, and uniaxial compression tests were conducted on carbon nanotube concrete (CNTC) prepared using the optimized parameters. Scanning electron microscope analysis was used to investigate the improvement in the microstructure of the concrete by CNTs. Transmission electron microscope micrographs of the polyvinyl pyrrolidone (PVP)-CNT suspensions exhibited a uniformly distributed CNT cross-linked network. The absorbance reduction ratio of PVP-CNT suspensions after standing for 90 days was 13.75 and 22.41%, respectively. The absorbance reduction ratio of the suspensions first increased and then decreased with increasing dispersant ratio and ultrasonic dispersion time and increased with increasing ultrasonic power ratio. Compared with that of plain concrete, the uniaxial compressive strength of CNTC significantly improved, with a maximum increase of 18.15% when the content was 0.10%, and the failure mode exhibited typical shear failure characteristics. The optimized preparation parameters for the CNT suspensions were a PVP-to-multiwalled carbon nanotube mass ratio of 4:1, an ultrasonic dispersion time of 20 min, and an ultrasonic power of 60%. These optimized parameters are ideal choices for preparing CNT cement-based composite suspensions.

Compression failure and energy absorption characterization of mortise and tenon modular cellular structures

Abstract Based on the modular structure, effects of different module filling methods and dovetail geometry parameters of module connection on the failure modes and energy absorption characteristics of the overall structure are investigated under uniaxial compressive loading. The results show that when the model filling modes are hexagonal and auxetic cell elements filling, the upper-left, lower-middle, and upper-right parts start deforming first, and the connecting parts are all subjected to shear failures. When the overall filling mode is half-filling, the growth rate of the force-displacement curve is more stable, and the efficiency of the compression force is larger. Energy absorption is better when the overall filling pattern of the model is hexagonal unit element filling. The efficiency of the compression force can be improved when the angle α of the connection part is 60° and the width of the connection part is 12mm. The energy absorption effect can also be improved when the angle α of the connecting part is 65°.

Anisotropy in shear failure of shale: An insight from microcracking to macrorupture

Anisotropic shear failure in shale is crucial to wellbore instability and hydraulic fracturing. While previous studies have examined bedding angles ranging from 0° to 90°, the intermediate angles have not been thoroughly investigated, with limited quantitative analysis from microcracking to macrorupture. To address this gap, we conducted direct shear tests on cubic shale samples with seven bedding angles (α = 0°, 15°, 30°, 45°, 60°, 75°, and 90°) using an innovative combination of acoustic emission (AE) and digital image correlation (DIC) techniques. This approach allowed us to gain a comprehensive understanding of the cracking damage process from the interior to the surface. Under various normal stresses, the minimum peak shear strengths occurred at α = 0°, while the maximum values were observed at α = 45° or 60°, rather than remaining constant. The internal friction angle peaked at α = 45°, not at α = 30° as previously reported. Furthermore, the shear failure pattern was influenced by complex interactions among nominal shear plane, bedding structure, and normal stress. The primary strain concentration zones and microfractures formed not only along the nominal shear plane but also along the bedding planes. Notably, microcracking damage initiated most readily at α = 0°, where the b value in AE events was higher and the fractal dimension was lower. At α = 30°–60°, the b value reached its minimum, while the fractal dimension peaked. The more random distribution of larger-scale microcracks was associated with an increase in the shear strength. These findings provide critical insights into the anisotropy of shear failure in shale and demonstrate the potential of microcracking analysis in predicting critical macrorupture and shear resistance. The identification of three distinct shear failure mechanisms further enhances our understanding of shale anisotropy.

Riveting damage behavior and mechanical performance investigation of CFRP/CFRP thin-walled single-lap blind riveted joints

Riveting has a wide range of advantages in improving the performance of composite connections, however, it is highly likely to cause damage to the composite structure. Therefore, an experimental study was conducted to assess the damage behaviors of the carbon fiber reinforced polymer (CFRP)/CFRP blind riveted joint. Mechanical performance of joints was also evaluated experimentally. Furthermore, failure modes were exhaustively discussed. The influences of the rivet shank length on both the riveting damage and the mechanical performance were also investigated. The results demonstrated that: the surface bearing damage occurred on the first ply and was mainly caused by the pressure of forming the riveted head along the rivet shank axial. Serious damage occurred in the upper plies near the riveted head due to the radius expansion of the rivet shank. The damage modes were affected by the angles of the fiber laying direction and the rivet shank. The grooved blind riveting formed a localized protruding area around the periphery of the holes and caused less riveting damage. The failure modes of the tensile specimens were all shear failure, and the failure modes of the pull-off specimens were characterized by laminate fracture around the rivet holes, which subsequently led to rivet pull-off failure. It is worth emphasizing that the pull-off damage load and failure load of grooved blind riveted specimens were higher than those of general blind riveted specimens, while the grooved blind riveting reduced the tensile resistance.