Plastic Stage Research Articles

Evaluation of the off‐axis and on‐axis tensile creep behavior of glass‐fiber reinforced polymer unidirectional lamina

AbstractThe off‐axis creep behavior of glass‐fiber reinforced polymer (GFRP) unidirectional lamina is crucial for engineering structural applications. A unified prediction model for time‐dependent off‐axis and on‐axis tensile creep strain in GFRP lamina is developed. The model, analogous in form to the one‐parameter plasticity model, accounts for off‐axis creep deformation separately for elastic and plastic response stages, incorporating a correction term to describe on‐axis creep deformation. The accuracy of the model is confirmed through tensile creep tests on unidirectional lamina at angles of 0, 15, 30, 45, 60, and 90 degrees. The model successfully predicts of creep deformation in the elastic and plastic response stages, respectively. The effects of creep stress, creep time, and off‐axis angle on the creep behavior are investigated. The optimal test angles for predicting off‐axis creep behavior using only two off‐axis angles' creep tests are analyzed. The dominant factors of creep behavior at various off‐axis angles are evaluated. The findings of the study include recommendations for lay‐up configuration and engineering applications of GFRP composites.Highlights A unified creep strain prediction model for GFRP lamina was developed. Creep strain was calculated separately in elastic and plastic response stages. The link between stress, time, angle and creep behavior was determined. The optimal test angles for predicting off‐axis creep behavior were evaluated.

In situ digital image correlation study on the mechanical properties of GH4169 at different temperatures

AbstractGH4169 is a precipitation‐strengthened nickel‐based high‐temperature alloy, and its mechanical behavior at different temperatures is of significant importance for practical applications. Here, an in situ study on the mechanical properties and microstructural evolution of the GH4169 alloy at different temperatures was conducted using scanning electron microscopy in combination with digital image correlation. The results demonstrate the existence of a linear relationship between the local average strain and the macroscopic strain. At temperatures of 20 °C, 100 °C, 180 °C, and 260 °C, the local average strain is 1.96, 2.13, 2.26, and 2.30 times the macroscopic strain, respectively, which shows a clear trend. This indicates that the temperature has a significant influence on the plasticity of the alloy. Additionally, the strain is concentrated below the notch, whereby at a macroscopic strain of 1.7 % at 20 °C and 260 °C, the local maximum strain at the notch is 52 % and 83 %, respectively. Furthermore, when the macroscopic strain reaches 2.12 %, the local maximum strain at the notch is 72 % and 103 %, respectively. At this point, cracks start to initiate and gradually propagate. In situ observations show that at the beginning of the tensile plasticity stage, the plastic strain rate in the grains is twice the rate at the grain boundaries; subsequently, both rates tend to stabilize, and a high‐strain zone appears inside the grains and is coordinately transferred to the surrounding grains through the grain boundaries. The final stage of damage of the alloy consists of perforation fracture, and the presence of a large number of cracked carbides at the fracture results in specimen cracking.

Unusual deformation mechanisms evoked by hetero-zone interaction in a heterostructured FCC high-entropy alloy

Understanding the synergistic mechanical effects of heterostructured materials remains challenging due to the complexities in the underlying deformation mechanisms, which are usually diverse, activated at different length scales and possibly interacting. Here, we unravel a deformation fundamental for heterostructures: in addition to the direct contribution on strength, hetero-zone interaction and the development of long-range internal stress could assist in evoking extra plastic mechanisms that are difficult to activate in their homogeneous counterparts. Specifically, the deformation of a heterostructure in Al0.1CoCrFeNi alloy, featuring nanostructured hard lamellae embedded in fine-grained soft matrix, is taken as an example for study. Drawing from experimental insights, the long-range internal stress buildup at elastoplastic transition stage due to intense dislocation pile-ups against hetero-zone boundary, which increases yield strength significantly, is theoretically analyzed. At the plastic stage, the high internal stress helps to activate phase transformation in the fine-grained zones, and the inter-zone constraint leads to form dispersed stable strain bands in the nanostructured zones. These extra mechanisms, together with enhanced deformation twinning, facilitate work hardening and coordinate the strain partitioning between zones, imparting improved ductility at high flow stress. These findings indicate a principle for heterostructural design: introducing strong hetero-zone interaction to enhance internal stress and thereby invoke new deformation mechanisms.

Synergistic densification mechanism of irregular Ti powder during CIP: 3D MPFEM simulation with real-shape particles

Achieving high green density is essential for ensuring the mechanical properties of powder metallurgy workpieces. However, the densification behavior of irregular ductile powders remains unclear due to oversimplification in current numerical simulations. To address this, a three-dimensional multi-particle finite element method model incorporating realistic powder morphology, size distribution, and stacking structure is constructed. It is found that a pivotal motion-deformation synergistic stage exists between the conventional particle rearrangement and elastic–plastic deformation stages. In this stage, plastic deformation is driven by the spheroidization of coarse powders, whereas the resulting vacancies in the stacking structure facilitate slight particle motion. Thereafter, plastic deformation is dominated by the flattening of fine particles, and the pore-filling capacity decreases due to the reduction in non-contact surface area. This synergistic and complementary interaction between the spheroidization of coarse powders and the flattening of fine powders enhances mechanical interlocking and promotes micropore closure. As a result, the micropores exhibit a tendency of downsizing and homogenization, substantially boosting the potential for achieving full densification during sintering. Based on these findings, a method for determining the optimal forming pressure is proposed, considering the manufacturing costs of powder compacts and the characteristics of the micropores in both green and sintered bodies.

Molecular dynamics simulations of thermomechanical properties of silicone-modified phenolic polymer

The silicone-phenolic multicomponent polymers are typically employed as the matrix of fiber-reinforced nanocomposites developed for reentry vehicles due to their excellent thermal and mechanical properties. The thermomechanical properties of the silicone-phenolic multicomponent polymer system, which are greatly related to the processing and microstructures, were studied using molecular dynamics (MD) simulations combined with experiments. A multistep dynamic polymerization approach was utilized to form the crosslinked polymer model, which was also validated in terms of both microstructures and properties. The thermomechanical properties of the crosslinked polymer system were established as a function of crosslinking degree, component ratio, temperature, strain rate, and cooling rate, and the influence mechanisms of the processing parameters were revealed. The crosslinking degree can greatly influence the glass transition temperature and volumetric coefficient of thermal expansion, which is attributed to the constrained chain mobility. The crosslinking degree and the component ratio have a significant effect on the morphologies and vibrational density of states of the polymer system, respectively, which in turn affects the thermal conductivity. The failure mode during uniaxial tensile was investigated in terms of the atomic energy distribution through MD simulations. The elastic and plastic deformation stages are dominated by intermolecular non-bonding interactions, but less contributed by the bonding interactions. This work can guide the design of polymeric nanocomposites by establishing the relationship of processing-microstructure-thermomechanical properties.

Atomistic investigation on the anisotropic elastic and plastic responses of nanotwinned metals

Introducing nanotwins into materials is one of the important strengthening methods to achieve the synergistic improvement of strength and ductility. The anisotropic mechanical behaviors of nanotwinned materials have been widely studied by experimental and computational methods. The dominant deformation mechanisms about dislocation slippages can be effectively switched among three modes, and controlled by twin spacing and the angle between twin boundaries (TBs) orientation and loading direction. Particularly, most of previous researches mainly focused on the deformation mechanisms during the plastic flow stage and researchers paid little attention on the anisotropic characteristics of TBs at the elastic stage which are also essential to manufacture the high-performance materials. Therefore, this study is aiming to systematically investigate the anisotropic effect of TBs both at the elastic and plastic stages within the single crystalline or polycrystalline systems by molecular dynamics (MD) simulations. It is revealed that, when the loading direction is parallel to twin planes, the introduction of TBs in single crystalline models will significantly affect the characteristics of atomic bond rotation and elongation dominated elastic deformation, which can alter the Poisson’s ratio of materials, generate elastic-softening behavior and inhomogeneous elastic deformation. At the plastic flow stage, the deformation mechanism transforms from trans-twin dislocation slippage into the coexistence of Hard Mode I and threading dislocation slippage (Hard Mode II) when the loading direction changes from parallel to perpendicular direction with respect to TBs. Moreover, the dislocation segments at the conjunction of trans-twin dislocation play a momentous role in enhancing material strength. The results for polycrystalline models are consistent with that of single crystalline ones. These findings are expected to be beneficial for the development of high-performance nanostructured materials for structural and functional applications by strain engineering and defect regulation.

Performance analysis and design method of strengthening beam-column welded connections against progressive collapse

To address the insufficient collapse resistance of the beam-column welded connection (BWC), this study proposes a strengthening beam-column welded connection with truss elements (BWCTE). A static test on a BWCTE specimen with a composite slab was conducted to investigate its failure pattern, final deflection, and the evolution of axial force, bending moment, and collapse resistance. The results indicate that the addition of truss elements increases the peak load and corresponding deformation of BWCTE by 108.6 % and 27.3 %, respectively, compared to BWC. Moreover, the axial force in the composite beam of the BWCTE specimen achieved the yield axial force, facilitating the complete development of catenary action and optimizing beam performance. Subsequently, primary parameters of the truss element were analyzed using refined numerical models. It is recommended that the projected length of the inside leaning limb is within 0.5 to 0.7 times the height of the steel beam, and the thickness of each limb is within 0.6 to 1.2 times the thickness of the beam flange. Then, the working mechanism of BWCTE was explored through theoretical analysis. The resistance process of BWCTE can be divided into elastic, elastic-plastic, plastic, transition, and catenary stages. The loading and deformation synergies between truss elements and composite beams enable the formation of double plastic zones and double strengthening zones at the beam root, effectively delaying the rupture of the stretched beam flange. Finally, a design method for BWCTE is proposed according to theoretical and numerical analysis.

Monitoring of damage evolution in carbon fiber reinforced polymer composites by electrical impedance tomography

Electrical impedance tomography (EIT) has been widely investigated as a nondestructive testing method for carbon fiber reinforced polymer (CFRP) composites. However, the performance of EIT method on the damage process monitoring of the composites is lack of investigation. Herein, quasi-static indentation tests were utilized to introduce damage on CFRP laminates. The damage evolution process was monitored by EIT, while the potential of distinguishing the elastic deformation stage from the plastic stage was analyzed with the aid of acoustic emission. It was found that the changes in conductivity first appeared in the non-central region. With the accumulation of damage, the conductivity changes gradually extended to the center region. The reconstructed damage images were in the crossover shape, which was consist with the micro-CT results that showed the fracture of fibers in ±45° direction. This work further promotes the application of EIT in damage process monitoring of CFRP components.

Pengelolaan Limbah Plastik menjadi Ecobrick Sebagai Upaya Kolaborasi Mahasiswa KKN UIN Walisongo dengan SDN 2 Rowosari dalam Membangun Taman Ecobrick

This study examines the management of plastic waste into ecobricks through the collaboration of UIN Walisongo Real Work Lecture (KKN) students with SDN 2 Rowosari. The purpose of this study is to describe the process of managing plastic waste into ecobricks and analyze the impact of collaboration on environmental awareness at SDN 2 Rowosari. The research method used is descriptive qualitative with data collection techniques through observation, interviews, and documentation. The results show that the process of managing plastic waste into ecobricks involves the stages of collecting, washing, drying, and packaging plastic in bottles. The collaboration between KKN students and SDN 2 Rowosari proved effective in increasing the environmental awareness of students and teachers, as well as encouraging active participation in plastic waste management. The development of the Ecobrick Garden as a result of this collaboration is expected to be an innovative and sustainable model of environmental learning.

Impact of prototype scale effect on the dynamic responses and damage of steel plates under explosions

Scaling model was widely used in engineering to reveal the responses of full-scale prototype model. This study provided insight into the impact of the scale effect on the dynamic response and damage of stiffened and unstiffened steel plates under explosions. A numerical model consider fluid-structure-interaction (FSI) was established using LS-DYNA platform, and the numerical model was verified by the blast test results. Moreover, the scaling factors (S) equal to 1 (prototype), 1/2, and 1/4, and 1/8, and 1/12 were considered in the study, the scaling effects on the breach size, plastic area, and the deformation shape were analyzed. The results showed that the breach size and plastic area of the scaled model deviated from the ideal results based on the replica law. However, the deformation shapes of scaled-down plates accurately reflected the deformation characteristics of the full prototype plate. The scaling factor significantly influenced the dynamic responses of stiffened and unstiffened steel plates; as the scaling factor (S) increased, the dimensionless displacement decreased. The results indicated that a smaller S led to a larger deviation in response. The influencing factors of the scaling effect were investigated, and it was found that stiffened steel plates were more sensitive to the scaling factor than unstiffened steel plates. By comparing the detonation of cuboid, cylindrical, and spherical explosives blast-loaded onto stiffened steel plates, it was determined that the detonation of cubic explosives caused the most severe scaling effect on stiffened steel plates under blast loads. The study also found that explosive mass and standoff distance influenced the scaling effect of stiffened steel plates under blast loads. Additionally, steel plates in the plastic stage and the critical state between the elastic and plastic stages were particularly sensitive to the scaling effect. Due to the strain rate effect, which can lead to a deviation from the ideal scale law, different steel materials exhibit varying scale sensitivities. Comparing stiffened plates made from Q235, Q345, DH36, and L907A, the order of sensitivity to the scaling factor was: Q235 > Q345 ≈ DH36 > L907A.

Dynamic responses of steep bedding slope-tunnel system under coupled rainfall-seismicity: Shaking table test

The coupling effects of rainfall, earthquake, and complex topographic and geological conditions complicate the dynamic responses and disasters of slope-tunnel systems. For this, the large-scale shaking table tests were carried out to explore the dynamic responses of steep bedding slope-tunnel system under the coupling effect of rainfall and earthquake. Results show that the slope surface and elevation amplification effect exhibit pronounced nonlinear change caused by the tunnel and weak interlayers. When seismic wave propagates to tunnels, the weak interlayers and rock intersecting areas present complex wave field distribution characteristics. The dynamic responses of the slope are influenced by the frequency, amplitude, and direction of seismic waves. The acceleration amplification coefficient initially rises and then falls as increasing seismic frequency, peaking at 20 Hz. Additionally, the seismic damage process of slope is categorized into elastic (2-3 m/s2), elastoplastic (4-5 m/s2) and plastic damage stages (≥ 6.5m/s2). In elastic stage, ΔMPGA (ratio of acceleration amplification factor) increases with increasing seismic intensity, without obvious strain distribution change. In plastic stage, ΔMPGA begins to gradually plummet, and the strain is mainly distributed in the damaged area. The modes of seismic damage in the slope-tunnel system are mainly of tensile failure of the weak interlayer, cracking failure of tunnel lining, formation of persistent cracks on the slope crest and waist, development and outward shearing of the sliding mass, and buckling failure at the slope foot under extrusion of the upper rock body. This study can serve as a reference for predicting the failure modes of tunnel-slope system in strong seismic regions.

Acoustic and thermal response characteristics and failure mode of gas-bearing coal–rock composite structure under loading

Exploring the characteristics of the instability and failure processes of gas-bearing coal and rock is crucial for monitoring and predicting mine gas accidents. Thus, a real gas environment was simulated based on a self-developed gas–solid coupling infrared observation system. The acoustic–thermal response characteristics and failure mode of the gas-bearing coal–rock composite structure were studied. The results showed the following: (1) From the plastic stage, the average infrared radiation temperature of the coal increased significantly. The variances of differential infrared temperature (VDIRT) of the combination and coal started to mutate approximately 30 s before the peak, and the b value of the combination began to fluctuate frequently, while the VDIRT of rock remained approximately 2.128 × 10−4 throughout the process. (2) When stress was about to peak, a clear temperature boundary formed between coal and rock. Acoustic emissions with high energy were mainly concentrated at the interface and inside the coal. (3) The early plastic stage was dominated by high-frequency, low-amplitude events. In the post-peak stage and late plastic stage, the proportion of events with 80–90 dB amplitude rose, and there was a significant increase in low-frequency, high-amplitude events. (4) As the loading proceeded, the density and area gradually increased and tended to move toward the shear crack region. The distribution range of the rise time/amplitude expanded from 0–12 ms/V at the beginning of the loading to the range of 0–60 ms/V in the post-peak stage.

Mechanical properties of teguline clay pellet mixtures during continuous oedometric compression

Clay pellet mixtures are generally compressed to improve their engineering performance. Deepening the comprehension of the mechanical properties of these mixtures in the complete compression process facilitates the benefit to the engineering design and their utilization. In this study, the effects of soil grain size distribution, water content and dry density on the mechanical properties and microstructure of Teguline clay pellet mixtures during a continuous oedometric compression process are explored. Three types of soil pellet mixtures, including mixture A (grain size ≤5 mm), mixture B (≤ 0.4 mm) and mixture C (2–5 mm), were prepared with different water contents of 7%, 8% and 12% respectively. Subsequently, continuous oedometeric compression was undertaken to explore their mechanical behaviours of the soil pellet mixtures. After that, the microstropic structures of the compacted pellet mixtures were investigated using mercury intrusion porosimetry (MIP). The results indicated that mixture A has a minimal initial packing density of pellet mixtures, while mixture C has a maximum one at the initial compression stage. After completion of compression, the compression curves of the pellet mixtures tended to converge uniformity at a semilogarithm coordinate as the vertical stress increased. All of the compression curves presented a concave shape at the plastic compression stage, which is significantly influenced by grain size distribution and water content. In contrast, the elastic compression and rebound behaviours are little affected by the grain size distribution and water content. As far as the microstructure is concerned, compacted samples prepared by mixture A or C presented a unimodal pore structure, while those by mixture B showcased a bimodal pore structure. In comparison with the unimodal pore distribution of the undisturbed stiff clay, the compacted samples displayed a pseudo-unimodal pore distribution because the inter-aggregate pores still existed. A double tangent method was proposed to determine the delimiting pore diameter of the pseudo-unimodal pore distribution curves and found that the delimiting pore diameter decreased with the increase of dry density and water content. Moreover, the inflexion point for the pore diameter of compacted samples prepared by coarse soil was larger than that of fine soil. Combining this work with previous research, it was found that the high compression of coarse soil easily causes the pseudo-unimodal shape, which is also impacted by water content and particle properties. This work could help deepen the understanding of the mechanical characteristics and microstructure of the stiff clay pellet mixtures during continuous oedometric compression.

The multi-stage plasticity in the aggression circuit underlying the winner effect.

Winning increases the readiness to attack and the probability of winning, a widespread phenomenon known as the "winner effect". Here, we reveal a transition from target-specific to generalized aggression enhancement over 10 days of winning in male mice, which is supported by three stages of plasticity in the ventrolateral part of the ventromedial hypothalamus (VMHvl), a critical node for aggression. Over 10-day winning, VMHvl cells experience monotonic potentiation of long-range excitatory inputs, a transient local connectivity strengthening, and a delayed excitability increase. These plasticity events are causally linked. Optogenetically coactivating the posterior amygdala (PA) terminals and VMHvl cells potentiates the PA-VMHvl pathway and triggers the cascade of plasticity events as those during repeated winning. Optogenetically blocking PA-VMHvl synaptic potentiation eliminates all winning-induced plasticity. These results reveal the complex Hebbian synaptic and excitability plasticity in the aggression circuit during winning that ultimately leads to an increase in "aggressiveness" in repeated winners.

Calculation Method of New Assembled Corrugated Steel Initial Support Structure of Highway Tunnel

The assembled corrugated steel initial support structure is a new prefabricated structure in highway tunnel engineering, achieving a balance between economy and safety. This study proposes a simplified calculation method and elucidates the mechanical mechanisms of assembled corrugated steel initial support structures. Firstly, the stiffness characteristics of corrugated steel plates were studied based on full-scale tests. A general equivalent stiffness coefficient table was established. Numerical simulations of corrugated steel flange joints were conducted to explore their bending mechanical properties. A two-stage rotational stiffness model for corrugated steel flange joints was proposed. Finally, a plane strain-spring simplified calculation method for the assembled corrugated steel initial support structure was developed, and the monitoring data from the Qipanshan Tunnel validated the correctness and reliability of the proposed method. The results demonstrate that (1) the plane strain-spring simplified model consists of the planar strain equivalent calculation method for corrugated steel plates and the two-line stiffness equivalent spring of the corrugated steel flange joint. The simplified model was validated as effective by monitoring data. (2) Corrugated steel plates exhibit two stages under loading, namely gap elimination and elastic stages. The elastic stage stiffness of corrugated steel plates decreases with increasing ratio of depth to pitch (RDP), positively correlating with plate thickness when the RDP exceeds 0.333 and otherwise negatively correlated. (3) Corrugated steel lining flange joints exhibit distinct elastic and plastic stages in their linear moment–rotation curves under loading.

Quantitative calculation of rock strain concentration and corresponding damage evolution analysis

Understanding the critical strain and damage evolution process of sandstone can help engineers more accurately predict the failure modes and critical states of sandstone in actual engineering structures. Therefore, this article quantitatively analyzes the strain field data obtained through digital image correlation (DIC), and for the first time establishes a strain concentration calculation model (SCCM) to analyze the critical strain, compaction, and damage evolution process of sandstone under uniaxial compression conditions. The experimental results show that both elastic and plastic strain zones exist on the specimen surface. The proportion curve of strain concentration calculated by SCCM indicates that the strain field ε1 generally undergoes two stages: the elastic strain fluctuation stage and the plastic strain development stage. In contrast, the strain field ε2 exhibits roughly three stages: a rapid change phase, a slow change phase, and a stability phase. Microscopic strain analysis reveals an overlap between the compaction and damage processes of the specimen, and the critical strain value εmd for micro-damage in the specimen is significantly smaller than values obtained by traditional discrimination methods. Specifically, when the defect width of the specimen is 15 mm and 40 mm, the mean εmd vaules are approximately 0.006016 and 0.00539, respectively. In contrast, the mean critical strain values determined by traditional discrimination methods are 0.0180 and 0.0178, respectively. The above research results provide a new method for analyzing the strain field data of geotechnical materials, serving the optimization of engineering structure design.

Simulation and Algorithmic Optimization of the Cutting Process for the Green Machining of PM Green Compacts.

Powder metallurgy (PM) technology is extensively employed in the manufacturing sector, yet its processing presents numerous challenges. To alleviate these difficulties, green machining of PM green compacts has emerged as an effective approach. The aim of this research is to explore the deformation features of green compacts and assess the impact of various machining parameters on the force of cutting. The cutting variables for compacts of PM green were modeled, and the cutting process was analyzed using Abaqus (2022) software. Subsequently, the orthogonal test ANOVA method was utilized to evaluate the significance of each parameter for the cutting force. Optimization of the machining parameters was then achieved through a genetic algorithm for neural network optimization. The investigation revealed that PM green compacts, which are brittle, undergo a plastic deformation stage during cutting and deviate from the traditional model for brittle materials. The findings indicate that cutting thickness exerts the most substantial influence on the cutting force, whereas the speed of cutting, the tool rake angle, and the radius of the rounded edge exert minimal influence. The optimal parameter combination for the cutting of PM green compacts was determined via a genetic algorithm for neural network optimization, yielding a cutting force of 174.998 N at a cutting thickness of 0.15 mm, a cutting speed of 20 m/min, a tool rake angle of 10°, and a radius of the rounded edge of 25 μm, with a discrepancy of 4.05% from the actual measurement.

Mechanical Properties and Cooperation Mechanism of Corroded Steel Plates Retrofitted by Laser Cladding Additive Manufacturing under Tension.

As a metal additive manufacturing process, laser cladding (LC) is employed as a novel and beneficial repair technology for damaged steel structures. This study employed LC technology with 316 L stainless steel powder to repair locally corroded steel plates. The influences of interface slope and scanning pattern on the mechanical properties of repaired specimens were investigated through tensile tests and finite element analysis. By comparing the tensile properties of the repaired specimens with those of the intact and corroded specimens, the effectiveness of LC repair technology was assessed. An analysis of strain variations in the LC sheet and substrate during the load was carried out to obtain the cooperation mechanism between the LC sheet and substrate. The experimental results showed that the decrease in interface slope slightly improved the mechanical properties of repaired specimens. The repaired specimens have similar yield strength and ultimate strength to the intact specimens and better ductility as compared to the corroded specimen. The stress-strain curve of repaired specimens can be divided into four stages: elastic stage, substrate yield-LC sheet elastic stage, substrate hardening-LC sheet elastic stage, and plastic stage. These findings suggest that the LC technology with 316 L stainless steel powder is effective in repairing damaged steel plates in civil engineering structures and that an interface slope of 1:2.5 with the transverse scanning pattern is suitable for the repair process.

Investigation of quantitative precursory indicators for rock failure using spatio-temporal characteristics of velocity

Stress redistribution and crack development are prominent features in rock failure. However, these processes are complex and challenging to analyze, making early warning of instability in rock engineering difficult. To characterize the rock failure process and obtain reliable precursory indicators, the imaging method that combines active and passive sources was employed to construct the velocity field during the rock failure process in this study. The micro-crack propagation law and its potential connection with the stress environment were investigated by mapping the velocity field, the acoustic emission (AE) density field, and the energy field. Moreover, the Mutation Trend Coefficient (MTC) and Deformation Coefficient (DC) were proposed as precursory indicators of instability based on the spatio-temporal correlation and directional differences of velocity evolution. Results show that the nucleation of AE sources and the release of energy accumulation are key to the formation of the velocity gradient, and progressive rock failure is accompanied by the intensification of velocity anisotropy. Additionally, low-velocity regions usually preferentially extend in the direction parallel to the principal stress, while expansion in the direction vertical to the principal stress is dominant in the plastic stage, indicating that the stress environment controls the spatial heterogeneity of the velocity field. The evolution trend of precursor indicators shows that the increase of MTC from a low value and the continuous decrease of DC can be regarded as the initiation of rock secondary cracks. The proposed indicators and method not only provide beneficial complements to the understanding of the evolution mechanism of rock damage but also have potential in the early warning and identification of the early stage of crack propagation, offering references for the safety prevention and control of rock engineering disasters.

Self-sensing performance of concrete columns using Multi-layer Graphene filled cement-based sensors with different arrangements

The Multi-layer Graphene (MLGs) filled cement-based sensor (GCBS) was developed and applied in the concrete columns. The strain-sensing performance of GCBS was investigated under cyclic and monotonic loading after being embedded in the column. Meanwhile, the influence of embedded location and adjacent sensor distance on the piezoresistive performance of the sensor was discussed. The strain measured by the embedded sensor agreed well with that obtained by LVDTs within the elastic regime. However, the embedded sensor underestimated the concrete strain after stepping into the plastic stage because of the increasing transverse deformation coefficient. Moreover, improper arrangement of the sensor resulted in uneven deformation, generating significant strain differences between the sensor with concrete and increasing prediction deviation. To guarantee the predicting accuracy of the sensor, it is recommended that the embedded position of the sensor from the superficial layer of the column should exceed 25 mm, and the distance between adjacent sensors should not be lower than 107.24 mm.