Dynamic Loading Process Research Articles

The analysis of damage mechanisms and vibration effects caused by cut blasting under various void shapes

The effect of the void shape on cut blasting is investigated by establishing a damage analysis model for a void using an improved calculation formula for stress on the wall of the void. Additionally, an optimization scheme is provided for the “square void cutting theoretical model.” The vibration effect of the rock mass outside the excavation area is simultaneously monitored, and its validity is substantiated through experimental and simulation-based verification. The findings indicate that the variation of q2sinθ over time aligns with the dynamic loading process of wall stress in holes. Under the influence of longitudinal waves, thin-walled circular damage occurs, and tensile effects from reflected waves are influenced by impedance. In a square void cutting model, two-stage stress waves cause tensile shear damage and inward collapse in the rock mass along hole walls. The explosive energy generated by square holes significantly contributes to fragmentation of rock masses. The circular empty-hole cutting model generates significant vibrations in central areas due to resistance. Vibration velocity in 45° direction for circular holes is lower than that for square voids outside cuts while vibration velocity remains equivalent between circular and square holes at 90° direction.

Investigation of dynamic three–point bending fracture properties of SCB sandstone

The stress intensity factor (SIF) is not only an important parameter in the field of fracture mechanics but also a key indicator reflecting the strength of the stress field at the crack tip. Therefore, this paper uses the Split–Hopkinson–Pressure–Bar (SHPB) device to conduct dynamic fracture experiments on semi–circular bend (SCB) sandstones of different fracture modes and then uses an experimental–numerical method to simulate the dynamic loading process to explore the changes law in SIF. Finally, combined with the high–speed digital image correlation (HS–DIC) technology, the fracture characteristics of SCB sandstones are studied. The results show that the rising rate of dissipated energy acting on the specimen fracture and damage increases rapidly when it is close to dynamic fracture, which makes the dynamic SIF also get a substantial increase, and the influence of dissipated energy on dynamic SIF is seen to be very strong. Moreover, prior to the occurrence of dynamic initiation fracture, the damage to the specimen is primarily triggered by microscopic tensile effects at the tip of the preset crack, which results in the value of KI always being greater than KII before the dynamic fracture occurs. Meanwhile, the distribution curve of dynamic SIF exhibits better symmetry or a trend towards symmetry compared to the distribution curve of static SIF. This suggests that the tip of the preset crack distributes with more uniform tensile and shear stresses during dynamic loading. Using the HS–DIC technique, when the specimen occurs dynamic pure mode I initiation fracture, there was a good symmetrical trend in both the vertical strain around the fracture crack and the horizontal displacement field at the specimen. However, when specimens occur dynamic mixed mode I/II initiation fracture, the horizontal displacement fields showed asymmetry, and the specimens all suffered varying degrees of shear damage.

3D meso‐scale numerical model and dynamic mechanical behavior of reinforced concrete

AbstractBased on the modeling method of 3D meso‐scale finite element model of plain concrete, a 3D meso‐scale finite element model of reinforced concrete is established considering the interface transition zone between reinforcement frame and mortar, and the reliability of the established meso‐scale finite element model is verified by split Hopkinson pressure bar (SHPB) test results. The failure process of reinforced concrete specimens under typical impact velocity and the influence of interface effect on specimen strength are analyzed by numerical simulation. The research results show that the 3D meso‐scale model of reinforced concrete considers the meso‐scale structure of reinforced concrete more realistically, and can better reflect the damage change process inside the material in the dynamic loading process. The weak interface effect caused by reinforcement can lead to the slight weakening of the dynamic strength of the material, but at the same time the weak interface effect of the reinforcement is also the main reason why the residual form of reinforced concrete remains well aligned with the shape of the reinforcement frame after the failure.

Control of complex dynamic nonlinear loading process for electromagnetic mill

Control of complex dynamic nonlinear loading process for electromagnetic mill

Dynamic behaviors of frozen silt under repeated traffic loading

The temperature state of warm permafrost is in the negative temperature near-phase-transition interval and thus is extremely sensitive to small fluctuations of the heat and stress environment. The dynamic load induced by the vehicle operation not only changes its magnitude cyclically, but also its principal stress direction (PSD) consistently changes rotationally. However, the existing conventional dynamic research methods can only simulate the cyclic stress environment with constant PSD, which is very different from the stress environment induced by locomotive operation, and thus the conventional dynamic research results are bound to be difficult to accurately reflect the influence of the stress field induced by vehicle operation on the development law of warm permafrost permanent settlement deformation. Therefore, in this paper, a series of dynamic studies were carried out for the Qinghai-Tibet silt at −2 °C by simulating the cardioid-shaped stress path induced during the operation of a heavy-duty locomotive using a frozen hollow cylinder apparatus. It is found that the rotational effect of the principal stress direction under the conditions of the cardioid-shaped stress path (CSSP) action accelerates the development of vertical permanent deformation in the frozen soil. The main reason that caused the axial strain increases advantage is the rotation of principal stress direction can accelerate the viscous energy dissipation during dynamic loading process. According to the law of axial strain development in the high-temperature frozen soil under the conditions of different CSSP action, an empirical model of permanent deformation development in high-temperature frozen soil considering the PSD rotation effect was established, and it was found by this model calculation that under the general condition of heavy-load locomotive operation, compared with the conventional research method without considering the PSD rotation, the axial cumulated strain in high-temperature frozen soil increased by 33.3% after considering the rotation effect in the PSD.

Mechanical behavior and optimization of constitutive prediction model for Epoxy/Al energetic composite materials considering temperature and strain rate effects

Epoxy/Al as a functional structural energetic composite material that can be used as an adhesive, energetic fragments and warhead shells due to its high energy density, low density, and high strength. It can release a large amount of chemical energy (about 21.36 kJ/g) through chemical reactions when subjected to high-speed impact and thermal stimulation. Whether used as a matrix or auxiliary component, it has enormous development potential. Therefore, it is particularly important to study the mechanical properties of Epoxy/Al energetic composite materials under different temperature environments and strain rates of loading. This article describes the preparation of Epoxy/Al specimens with a mass percentage of 70%–30% using vacuum curing after 24 h under 0.8 Bar and 50 °C. The mechanical properties of Epoxy/Al specimens under different loading conditions were characterized using a high and low temperature universal testing machine and a separate Hopkinson experimental system. The modulus prediction method of binary composite materials was improved by combining SEM (scanning electron microscopy) and microstructure, and an adaptive constitutive model was developed combined with the K Srinivas model and neural network model based on the Sherwood Frost empirical constitutive relationship. The results show that Epoxy/Al energetic composite material has a significant temperature effect. When the temperature exceeds 100 °C, Epoxy/Al energetic composite material will be a significant physical and chemical property transition (the specimen exhibits viscous fluid characteristics and gradually begins to slowly decompose). As the temperature decreases, the specimen gradually exhibits a certain degree of brittleness and strength is improved. Due to the deformation, displacement, and interfacial debonding of internal particles during the loading process, Epoxy/Al materials exhibit excellent impact energy absorption effects, with energy absorption reaching 15.30 MJ/m3 at room temperature, and unit mass cost much lower than popular CFRP (carbon fiber-reinforced polymer) materials (CFRP energy absorption cost is 0.268 J/g/£, while Epoxy/Al materials do not exceed 0.013 J/g/£). During the dynamic loading process, Epoxy/Al energetic composite materials exhibit a phenomenon of structural reconstruction and enhancement throughout the entire loading process. The maximum strength at room temperature can reach 240.70 MPa, which is superior to all existing energetic materials of the same type. By introducing interface effects and quantifying them in the Halpin Tsai model, the existing prediction method for the Young's modulus of binary particle added composite materials has been effectively improved, reducing the prediction error of the Young's modulus of Epoxy/Al energetic composite materials from 14.2% to 2.4%. The numerical simulation results indicate that the newly developed constitutive model has high accuracy and can better reflect the mechanical properties of Epoxy/Al materials. The development method of this constitutive model has a positive contribution to the development of composite materials.

Research on the Collaborative Optimization of the Power Distribution Network and Traffic Network Based on Dynamic Traffic Allocation

With the increasing penetration rate of electric vehicles, the spatiotemporal coupling relationship between the power distribution network and traffic network is stronger than ever before. Under the dynamic wireless charging mode, traffic jam charging is introduced and the dynamic loading process of traffic flow is described using a cellular transmission model. The charging load is related to traffic flow and serves as a bond between the power distribution network and traffic network. The traffic flow achieves balanced allocation under dynamic user equilibrium conditions, and cooperatively optimizes the power flow of the power distribution network in conjunction with charging loads. Numerical analysis shows that this model can accurately depict the congestion situation during peak travel periods, and alleviate traffic congestion and distribution network voltage out of range.

Study of structural factors of structure-resolved filter media on the particle loading performance with microscale simulation

Microstructure strongly affects the macroscopic properties of filter media. Many previous works studied the filter loading behavior with the single fiber or macroscale model using unresolved fiber networks, but the influence of pore-scale structure was not thoroughly investigated. In this paper, the dynamic dust loading process in structure-resolved filter media was studied with microscale simulations. The distribution of trapped particles in the filter media in depth and cake filtration regimes was recorded, and the evolution of pressure drop and filtration efficiency was analyzed in depth. This work systematically explored how structural factors such as filter thickness, solidity (i.e., packing density), bulk, and fiber orientation affect the dust holding capacity (DHC) of the filter media. We also investigated better composite filters based on various designed filters with multilayer, uniform, or gradient structures. The results showed that the filter L-bot with a finer fiber layer at the bottom had the highest dust loading capacity, 63.98% higher than the filter L-top with an inverted structure, followed by the gradient filter GI-lin and GI-exp with open pore structure near the inlet. Our work presents a strategy for designing innovative depth filter media with effective and prolonged service life. Our detailed simulation with structure-resolved fiber network and particle deposits provides a fundamental understanding of the dust loading process.

Laser Shock-Induced Nano-Twist of Transition Metal Dichalcogenides.

Mechanical strain, such as stretching, compression, bending, and rotation, significantly alters the photonic and electronic properties of 2D materials. The laser shock process, which allows 2D materials to deform at an ultrahigh strain rate, is a promising technology for alleviating the low strain transfer efficiency caused by the low interfacial bonding strength of the layered 2D materials. However, the mechanical strain introduced by shock waves is currently limited to uniaxial compression or bending deformation, and the monotonic strain patterns constrain the strain diversity and performance expansion space of 2D materials. This work proposed a novel strategy for nano-twist manufacturing using laser shock processing, based on partial interfacial decoupling behavior. Apart from the conventional uniaxial strain, we demonstrated experimentally and theoretically that the manufacturing of nano-twist allows the introduction of interlayer tensile and rotational strains in TMDCs. The microstructure and properties of the strained 2D materials were investigated. Furthermore, the dynamic deformation response of WSe2 during the shock process was studied using molecular dynamics simulations. The correlation between the laser shock-induced dynamic loading process, interfacial behavior, and deformation behavior of 2D materials was comprehensively explored. The primary contribution of this study lies in the introduction of diversified strain modes through nano-twist manufacturing by the laser shock process, which is expected to provide a convenient nano-twist fabrication process for the strain engineering and twistronics fields.

Determination of fibre tension fracture toughness of composite laminates at high loading rate

The dynamic fracture toughness of composite laminates in fibre tension failure mode is characterized here with compact tension (CT) sample, at loading rate of 25 m/s using bi-directional electromagnetic Hopkinson bars. Digital image correlation (DIC) with high-speed imaging is employed for obtaining the full-field strain fields. A dynamic J-integral method is proposed to analyse the dynamic fracture toughness, the kinetic energy and inertia effects involved in dynamic loading and crack propagation process has been analysed and calibrated. The fracture toughness of fibre tension failure at high loading rate is found to be about 25% lower than that in quasi-static load regime, and the R-curve effect under high loading rate is not significant than that in quasi-static load regime. It is suggested from X-ray CT and SEM images that the fibre pull-out length and size of failure process zone is reduced with increase in loading rate.

Experimental Research on the Dynamic Characteristics and Voltage Uniformity of a PEMFC Stack under Subzero Temperatures

The working life of a stack is basically determined by the behavior of its weakest monomer; therefore, during a dynamic loading process, high-voltage uniformity plays a key role in stack durability. This work experimentally investigated dynamic responses and voltage uniformity with different loading strategies from an initial state (open-circuit state) to a target current at subzero temperatures. The results presented that the maximum voltage coefficient variation Cv values, representing the relative standard deviation of the monomer voltage, increased from 19.75 to 35.65 and the voltage uniformity sharply decreased with the increase in loading sizes at the stack temperature of −4 °C. It was possible that this could result in the instability of the single cell voltage output and a voltage uniformity fluctuation with a high step loading amplitude. The voltage uniformity became worse as the stack temperature decreased from −2 °C to −8 °C, the voltage uniformity between single cells of the stack continued to deteriorate under a step current density of 0.15 A/cm2 at a relatively low temperature of −8 °C. By comparing several constant loading rate strategies, it could be ascertained that reducing the loading rate was conducive to voltage uniformity in the whole investigated process. A comparison of the different loading rates indicated that the square increasing loading rate strategy is the best strategy among them and a Cv threshold of 5.5 can be obtained during the whole load process at a temperature of −4 °C, which might be a guideline and provide a reference in actual engineering applications for PEMFC dynamic loading responses.

Topology optimization for enhanced dynamic fracture resistance of structures

A topology optimization framework for improving the dynamic fracture resistance of structures is proposed. The phase field method for fracture is combined with Solid Isotropic Material with Penalization (SIMP) topology optimization. The topology optimization problem is defined as minimizing the fracture energy during the whole dynamic loading process, from initiation of cracks to full failure of the structure, under volume and compliance constraints. Semi analytical expressions of sensitivities in a dynamic context are provided to solve the topology optimization problem efficiently. Numerical examples involving structures subjected to impact loading are investigated. It is shown that the present framework allows a significant reduction of the fracture energy as compared to designs obtained by static optimization.

Research on Shear Behavior of Sand–Structure Interface Based on Monotonic and Cyclic Tests

In order to study the shear behavior of the interface between sand and structure, a series of shear tests were carried out using an HJ-1 ring shear apparatus (Nanjing, China). First, through the monotonic shear tests, the loose sand and dense sand were sheared at the steel interface with different roughnesses. The results showed that when the interface was relatively smooth, the shear stress–shear displacement curves of loose sand and dense sand both exhibit strain hardening characteristics. When the interface was rough, the dense sand showed strain softening. The initial shear stiffness of the sand–steel interface increased with the increase in normal stress, interface roughness, or sand relative density. Then, considering the influence of initial shear stress, through the cyclic shear test, this work analyzed the shape of the loading and unloading curves and the development law of cumulative normal deformation, and discussed the change of loading and unloading shear stiffness under different stress level amplitudes and the residual deformation generated during the cycle. The research results showed that loose sand and dense sand generally shrunk in volume during the cycle. The initial loading process was similar to the case of static loading. In the later dynamic loading process, the shear shrinkage per cycle was relatively small and continued to develop. Additionally, it was found that the unloading stiffness of the sand–steel interface is always greater than the initial loading stiffness. As the number of cycles increases, the loading stiffness increases, and it may eventually approach the unloading stiffness.

Numerical analysis of ground support systems subjected to dynamic loading

Herein, a new numerical approach is proposed to simulate the dynamic loading processes in mines. To present a realistic seismic phenomenon, a parallel two-dimensional seismic source is introduced by adjusting the classic point seismic source. The seismic source can be easily integrated into the universal distinct element code numerical model, and it can adjust its bearings and strength. The numerical model assumes an elastic rock mass with potential fractures because the dynamic-related bulking surrounding excavations is mainly caused by the separation and sliding of fracture planes; cable bolts are installed on the sides of roadways. The numerical simulations can accurately reproduce the full dynamic loading process. After axial forces build up, numerical results from a case analysis show that cable bolts considerably restrict the movement of the surrounding rock mass towards the roadway, preventing potential fractures from developing into the large opening cracks. The cable bolts do not considerably change the local stresses, although they can maintain local horizontal stress compressive near the roadway surface. The monitored cable element experiences multiple loading and unloading processes; it is found that cable bolts initially cause rapid attenuation of seismic velocities and then dissipate dynamic energy in a fluctuating manner.

Experimental study on the effect of defect curvature on the impact fracture behavior of structures using the real-virtual caustics method.

Defects have significant influence on the impact fracture behavior of structures. In this paper, the real-virtual caustics method is used to study the impact fracture behavior of structures with elliptical arc defects under impact loading of the drop hammer, and the impact loading process is simultaneously analyzed. The research results show that impact loading of the drop hammer in this experiment is a multi-period dynamic loading process, and the fracture of specimens under impact loading of the drop hammer is an energy-controlled process. The running crack initiates under impact loading and propagates toward the elliptical arc defect. After reaching the end of the elliptical arc defect, the running crack arrests and accumulates energy, and then it initiates again and propagates toward the loading position. The greater the end curvature of the elliptical arc defect, the shorter the time for the running crack to stagnate and accumulate energy at the defect end, and the earlier the time for the running crack to initiate again at the defect end, the smaller the impact loading stress of the drop hammer and the dynamic stress intensity factor of running crack tip when the running crack initiates again at the defect end.

In situ capture of impact-induced progressive damage and delamination in fiberglass composite laminate with a high-speed optical imaging method

Delamination is one of the severest failure modes for laminated composite materials during applications, especially when the composites are subjected to dynamic loading like impact. The impact-induced delamination patterns in laminates have been studied before but the progressive delamination evolution during the impact process has rarely been revealed through experimental testing. Capturing the real-time delamination progression is of great significance for understanding the fundamentals of the composite performance and damage resistance during the dynamic loading process. This study investigates the feasibility of using a high-speed optical imaging method for in-situ capturing the impact-induced progressive damage and delamination evolution in fiberglass composite laminate [0/90/0/…]13. The progressive delamination areas are further quantified through a sequential series of captured images using a custom algorithm. The relations between the progressive delamination evolution and the real-time absorbed energy, as well as the impact force and deflection, have been investigated. This study provides insights into the experimental testing method of dynamic damage and failure process in materials and structures.

Macro and meso dynamic response of granular materials in ballastless track subgrade for high-speed railway

Abstract Considering the discrete and stochastic properties of granular materials, a two-dimensional numerical model was established to study the dynamic behavior of high-speed railway (HSR) ballastless track subgrade by using finite difference-discrete element (FDM-DEM) coupling method. Combined with on-site vibration test results in macroscopic scale, the mesoscopic vibration response of subgrade granular materials was analyzed under different speed levels. Some unusual phenomena were observed as follow: 1). In the surface layer (i.e., the top area of the subgrade, thickness = 0.4 m), particles showed a very wide response band in the frequency domain (up to 130 Hz), even beyond the maximum frequency of train loading (60 Hz). It was found that some particles in the surface layer produced not only forced vibration, but also local random vibration. However, the bottom layer particles (i.e., the area under the surface layer, thickness = 2.3 m) were only active within 40 Hz. 2) Force chains in the granular aggregate were disappeared and rebuilt synchronously in the dynamic loading process. A typical mode of force chain weakening was described in meso scale. 3) Particles at interface region between the surface and bottom layer exhibited severe local vibration amplification when the train speed level was higher than 300 km/h.

A novel technique for determining the dynamic Bauschinger effect by electromagnetic Hopkinson bar

Dynamic mechanical behavior of metallic materials under complicated loading conditions has attracted much attention. However, it is hard to obtain the dynamic Bauschinger effect of metallic materials due to the limitation of loading equipment. In order to investigate the relationship between the Bauschinger effect and strain rate effect of metallic materials, this paper proposes an asynchronous loading technique based on electromagnetic split Hopkinson bar system, which could provide an effective way to study the Bauschinger effect of metallic materials under high strain rate loading. We first introduce the main characteristics of the asynchronous loading device, that is, the specimen can be loaded by one cycle of continuous dynamic tension-compression loading pulse in which the two separate stress waves are created by electromagnetic pulse generators and prove to maintain their consistency. The propagation of stress waves was analyzed to ensure the continuity of the loading process. Then the dynamic loading process and the methods of data processing and stress wave separation are presented. Stress equilibrium was also analyzed in order to demonstrate the reliability of the equipment. Finally, the Bauschinger effect of 6061 aluminum alloy at 5% pre-strain during the process of dynamic compression to dynamic tension loading was studied using this method, and the corresponding quasi-static tests were also conducted for comparison. It was found that the material shows less strain-rate sensitivity under axial compression loading, while its Bauschinger stress parameter increases from 0.07 in quasi-static loading to 0.17 in dynamic loading. The results indicate that the Bauschinger effect of 6061 aluminum alloys depends on the strain rate and can be significantly enhanced under dynamic loading. This conclusion presents a challenge to the traditional conception that aluminum alloys are insensitive to strain rate.

Experimental investigation on the voltage uniformity for a PEMFC stack with different dynamic loading strategies

The operating life of the proton exchange membrane fuel cell stack is mainly decided by performances of its weakest single cell because of the “Buckets effect”, thus high voltage uniformity during a dynamic loading process is key to the stack durability. In this work, a 3-kW stack is examined experimentally on its voltage uniformity (voltage coefficient variation (Cv)) under conditions of loading from open-circuit state (0 A) to nominal current (165 A) and stack temperatures of 30 °C, 45 °C and 65 °C. Different dynamic loading strategies, namely constant loading rate strategy, decreasing loading rate strategy, and increasing loading rate (square/cube increasing loading rate) strategy, are examined and compared. Results display that during the loading process, (a) the voltage uniformity rises abruptly and goes down quickly when the loading current is small (e.g. from 0 A to 22 A), (b) the voltage uniformity under a small loading current is better than that under the open-circuit state, and (c) voltage uniformity decreases as the loading current increases from a small value to the nominal current. Comparisons of different current loading strategies show that as the stack temperature rises from 30 °C to 65 °C, the stack Cv value under the open-circuit state increases from 1.12 to 1.84 and decreases from 3.85 to 2.45 in the nominal current state. The maximum Cv for the decreasing loading rate strategy decreases from 16.25 to 9.49 and that of the constant loading rate strategy also decreases from 5.85 to 4.96. Cv values of the square current increasing loading rate strategy keep below 3.85 under conditions of the three stack temperatures and display a slight fluctuation during the whole current loading process, which indicates that the strategy can effectively make the stack being of an excellent voltage uniformity during the instantaneous response process.

A mixed integer programming formulation and scalable solution algorithms for traffic control coordination across multiple intersections based on vehicle space-time trajectories

Thanks to the development of mobile computing, novel traffic data sources are emerging as the promising building blocks for more effective traffic control strategies. It is expected that the vehicle space-time trajectories will become ubiquitously available in foreseeable future. Real-time trajectory data will provide full-spectrum pattern of traffic dynamics among multiple intersections. In this paper, we present a new traffic control representation for multiple intersections. A new multi-intersection phase (MI-phase) is proposed to represent safe vehicle movements across a few tightly connected intersections. All the intersections are also viewed as one integral “super intersection” within which vehicles move according to their planned paths. Through scheduling the sequence and durations of MI-phases over time, the vehicles will be crossing intersections with minimal delays. This approach can provide more flexibilities for traffic control coordination than the traditional Cycle-Split-Offset approach. A linear integer programming formulation is presented for joint optimization of vehicle space-time trajectories and traffic control. We also design a scalable optimization frame for real-world traffic control optimization, referred to as “Lagrangian decomposition with subproblem approximation” approaches. In this new framework, we construct the dynamic network loading based lower bound estimator (DNL-LBE) in which the relaxed constraints and sensitivity to the Lagrangian multiplier prices are explicitly considered while vehicular flows are being loaded. By doing so, the complex controlled dynamic network loading process can be represented through Lagrangian multipliers interfacing with the MI-phase optimization module (then solved by Dynamic Programming). This approach can facilitate price-based search heuristics to find high quality solutions for both vehicular space-time trajectories and traffic control plans without increasing the overall computing complexity. The efficiency of the proposed optimization framework is further improved through multiple advanced computing techniques. In the end, one demonstrative and one real-world example are provided to show the performance of the new approach.