Dynamic failure process of expanded polystyrene particle lightweight soil under cyclic loading using discrete element method

Expanded polystyrene(EPS)particle-based lightweight soil,which is a type of lightweight filler,is mainly used in road engineering.The stability of subgrades under dynamic loading is attracting increased research attention.The traditional method for studying the dynamic strength characteristics of soils is dynamic triaxial testing,and the discrete element simulation of lightweight soils under cyclic load has rarely been considered.To study the meso-mechanisms of the dynamic failure processes of EPS particle lightweight soils,a discrete element numerical model is established using the particle flow code(PFC)software.The contact force,displacement field,and velocity field of lightweight soil under different cumulative compressive strains are studied.The results show that the hysteresis curves of lightweight soil present characteristics of strain accumulation,which reflect the cyclic effects of the dynamic load.When the confining pressure increases,the contact force of the particles also increases.The confining pressure can restrain the motion of the particle system and increase the dynamic strength of the sample.When the confining pressure is held constant,an increase in compressive strain causes minimal change in the contact force between soil particles.However,the contact force between the EPS particles decreases,and their displacement direction points vertically toward the center of the sample.Under an increase in compressive strain,the velocity direction of the particle system changes from a random distribution and points vertically toward the center of the sample.When the compressive strain is 5%,the number of particles deflected in the particle velocity direction increases significantly,and the cumulative rate of deformation in the lightweight soil accelerates.Therefore,it is feasible to use 5%compressive strain as the dynamic strength standard for lightweight soil.Discrete element methods provide a new approach toward the dynamic performance evaluation of lightweight soil subgrades.

Stability and accuracy of central difference method for real-time dynamic substructure testing considering mass participation coefficient

For real-time dynamic substructure testing(RTDST),the influence of the inertia force of fluid specimens on the stability and accuracy of the integration algorithms has never been investigated.Therefore,this study proposes to investigate the stability and accuracy of the central difference method(CDM)for RTDST considering the specimen mass participation coefficient.First,the theory of the CDM for RTDST is presented.Next,the stability and accuracy of the CDM for RTDST considering the specimen mass participation coefficient are investigated.Finally,numerical simulations and experimental tests are conducted for verifying the effectiveness of the method.The study indicates that the stability of the algorithm is affected by the mass participation coefficient of the specimen,and the stability limit first increases and then decreases as the mass participation coefficient increases.In most cases,the mass participation coefficient will increase the stability limit of the algorithm,but in specific circumstances,the algorithm may lose its stability.The stability and accuracy of the CDM considering the mass participation coefficient are verified by numerical simulations and experimental tests on a three-story frame structure with a tuned liquid damper.

Dynamic mechanical characteristics of frozen subgrade soil subjected to freeze-thaw cycles

As a widely-applied engineering material in cold regions, the frozen subgrade soils are usually subjected to seismic loading, which are also dramatically influenced by the freeze-thaw(F-T)cycles due to the varying temperature. A series of dynamic cyclic triaxial experiments were conducted through a cryogenic triaxial apparatus for exploring the influences of F-T cycles on the dynamic mechanical properties of frozen subgrade clay.According to the experimental results of frozen clay at the temperature of-10℃, the dynamic responses and microstructure variation at different times of F-T cycles(0, 1, 5, and 20 cycles) were explored in detail.It is experimentally demonstrated that the dynamic stress-strain curves and dynamic volumetric strain curves of frozen clay are significantly sparse after 20F-T cycles. Meanwhile, the cyclic number at failure(Nf) of the frozen specimen reduces by 89% after 20freeze-thaw cycles at a low ratio of the dynamic stress amplitude. In addition, with the increasing F-T cycles,the axial accumulative strain, residual deformation,and the value of damage variable of frozen clay increase, while the dynamic resilient modulus and dynamic strength decrease. Finally, the influence of the F-T cycles on the failure mechanisms of frozen clay was discussed in terms of the microstructure variation. These studies contribute to a better understanding of the fundamental changes in the dynamic mechanical of frozen soils exposed to F-T cycles in cold and seismic regions.

Application of computational fluid dynamics in design of viscous dampers-CFD modeling and full-scale dynamic testing

Computational fluid dynamics(CFD)provides a powerful tool for investigating complicated fluid flows.This paper aims to study the applicability of CFD in the preliminary design of linear and nonlinear fluid viscous dampers.Two fluid viscous dampers were designed based on CFD models.The first device was a linear viscous damper with straight orifices.The second was a nonlinear viscous damper containing a one-way pressure-responsive valve inside its orifices.Both dampers were detailed based on CFD simulations,and their internal fluid flows were investigated.Full-scale specimens of both dampers were manufactured and tested under dynamic loads.According to the tests results,both dampers demonstrate stable cyclic behaviors,and as expected,the nonlinear damper generally tends to dissipate more energy compared to its linear counterpart.Good compatibility was achieved between the experimentally measured damper force-velocity curves and those estimated from CFD analyses.Using a thermography camera,a rise in temperature of the dampers was measured during the tests.It was found that output force of the manufactured devices was virtually independent of temperature even during long duration loadings.Accordingly,temperature dependence can be ignored in CFD models,because a reliable temperature compensator mechanism was used(or intended to be used)by the damper manufacturer.

Correction of dynamic Brazilian disc tensile strength of rocks under preloading conditions considering the overload phenomenon and invoking the Griffith criterion

Dynamic tensile failure is a common phenomenon in deep rock practices,and thus accurately evaluating the dynamic tensile responses of rocks under triaxial pressures is of great significance.The Brazilian disc(BD)test is the suggested method by the International Society for Rock Mechanics and Rock Engineering(ISRM)for measuring both the static and dynamic tensile strengths of rock-like materials.However,due to the overload phenomenon and the complex preloading conditions,the dynamic tensile strengths of rocks measured by the BD tests tend to be overestimated.To address this issue,the dynamic BD tensile strength(BTS)of Fangshan marble(FM)under different preloading conditions were measured through a triaxial split Hopkinson pressure bar(SHPB).The fracture onset in BD specimen was captured through a strain gage around the disc center.The discrepancy between the traditional tensile strength(TTS,determined by the peak load P_(f) of the BD specimen)and the nominal tensile strength(NTS,obtained from the load P_(i) when the diametral fracture commences in the tested BD specimen)was applied to quantitatively evaluating the overload phenomenon.The Griffith criterion was used to rectify the calculation of the tensile stress at the disc center under triaxial stress states.The results demonstrate that the overload ratio(s)increases with the loading rate(σ)and decreases with the hydrostatic pressure(σ_(s)).The TTS corrected by the Griffith criterion is independent of theσ_(s)due to the overload phenomenon,while the NTS corrected by the Griffith criterion is sensitive to both the andσ.Therefore,it is essential to modify the tensile stress in dynamic confined BD tests using both the overload correction and the Griffith criterion rectification to obtain the accurate dynamic BTS of rocks.

Optimal design and dynamic impact tests of removable bollards

Anti-ram bollard systems, which are installed around buildings and infrastructure, can prevent unauthorized vehicles from entering, maintain distance from vehicle-borne improvised explosive devices （VBIED） and reduce the corresponding damage. Compared with a fixed bollard system, a removable bollard system provides more flexibility as it can be removed when needed. This paper first proposes a new type of K4-rated removable anti-ram bollard system. To simulate the collision of a vehicle hitting the bollard system, a finite element model was then built and verified through comparison of numerical simulation results and existing experimental results. Based on the orthogonal design method, the factors influencing the safety and economy of this proposed system were examined and sorted according to their importance. An optimal design scheme was then produced. Finally, to validate the effectiveness of the proposed design scheme, four dynamic impact tests, including two front impact tests and two side impact tests, have been conducted according to BSI Specifications. The residual rotation angles of the specimen are smaller than 30~ and satisfy the requirements of the BSI Specification.

Analysis of volatile organic compounds emitted from aircraft carpets:comparison using headspace and dynamic chamber tests

Volatile organic compounds(VOCs) emitted from three types of carpets used in aircrafts were compared by using headspace and dynamic chamber tests. The headspace samples contained many compounds that were not detected in the dynamic chamber test; in addition, the dominant VOCs found by these two methods were different. The findings indicate that for highly sorptive materials such as carpets, headspace analysis may give inaccurate indication of actual VOC emissions, and it is necessary to conduct dynamic chamber tests over a certain period of time in order to identify the true emission characteristics. From the dynamic chamber tests, 2-ethyl-1-hexanol was the main VOC emitted from all three carpets. The study also examined the emission characteristics of aircraft carpets. In all experiments, total VOC(TVOC) concentration peaked within a few hours after the start of the experiment and was followed by rapid decay. The emission parameters of TVOC emitted by all three carpets were calculated and the simulated data matched the measured data well.

Effects of dynamic flow rates on degradation deposition behavior of Mg scaffold

Degradability of bone tissue engineering scaffold that matching the regeneration rate could allow a complete replacement of host tissue.However,the porous structure of biodegradable Mg scaffolds certainly generated high specific surface area,and the three-dimensional interconnected pores provided fast pervasive invasion entrance for the corrosive medium,rising concern of the structural integrity during the degradation.To clarify the structural evolution of the three-dimensional(3D)porous structure,semi-static immersion tests were carried out to evaluate the degradation performance in our previous study.Nevertheless,dynamic immersion tests mimicking the in vivo circulatory fluid through the interconnected porous structure have yet been investigated.Moreover,the effects of dynamic flow rates on the degradation deposition behavior of 3D porous Mg scaffolds were rarely reported.In this study,Mg scaffolds degraded at three flow rates exhibited different degradation rates and deposition process.A flow rate of 0.5 m L/min introduced maximum drop of porosity by accumulated deposition products.The deposition products provided limited protection against the degradation process at a flow rate of 1.0 m L/min.The three-dimensional interconnected porous structure of Mg scaffold degraded at 2.0 m L/min well retained after 14 days showing the best interconnectivity resistance to the degradation deposition process.The dynamic immersion tests disclosed the reason for the different degradation rates on account of flow rates,which may bring insight into understanding of varied in vivo degradation rates related to implantation sites.

Dynamic Testing of Elastic Modulus,Shear Modulus,and Poisson’s Ratio of Bamboo Scrimber

The bamboo scrimber is an anisotropic material.The elastic constant values of the bamboo scrimber specimens measured by the dynamic and static methods are consistent,and the dynamic test method has the advantages of rapidity,simplicity,good repeatability,and high precision.Bamboo scrimber has strong potential as a building material,and its elastic constant is an important index to measure its mechanical properties.To quickly,simply,non-destructively,and accurately detect the elastic constant of the bamboo scrimber,they were dynamically tested by the free plate transient excitation method and cantilever plate torsional vibration method.The static four-point bending method was used to verify the accuracy and reliability of the dynamic elastic modulus,shear modulus,and Poisson’s ratio of the bamboo scrimber.The mechanism analysis and evaluation of the quality grade,homogeneity,and size effect of the bamboo scrimber whole board were carried out.The main results show that the dynamic elastic modulus,shear modulus,and Poisson’s ratio of the bamboo scrimber are 12 GPa,1500 MPa,and 0.31,respectively,which meet the requirements of GB/T 40247-2021 for structural bamboo scrimber.

Research on damage failure mechanism and dynamic mechanical behavior of layered shale with different angles under confining pressure

The hydrostatic or confining pressure of deep rocks has a significant impact on the mechanical behavior of brittle materials.Especially when confining pressure is applied,the mechanical properties of rock materials will undergo significant changes.Considering that the process of shale sample subjected to impact load is in a closed container in the dynamic triaxial SHPB test,the failure process of the sample cannot be observed.Meanwhile,the activation volume of the shale sample would be large and local failure would occur in the test under the high strain rate loading.Therefore,thefinite element model of shale considering the bedding effect under confining pressure was established in this study.Taking shale materials with different bedding dip angles as simulation objects,the dynamic failure characteristics of shale were studied using the dynamic analysis software ANSYS/LS‐DYNA from three aspects:stress‐strain curve,failure growth process,and failure morphology.The research results obtained can serve as the key technical parameters for deep resource extraction.

Method of Testing of Dynamic Forces on Digital Jet Elements

Method of testing for dynamic output forces from jet elements is studied, the handwidth is large in testing with this method. By establishing a model of the test system and simulating it, principles of how inherent features of the test system affect the dynamic force test are found out. Thus a theoretical foundation is given for the design and error modification to the actual test system.

Networked collaborative pseudo-dynamic testing of a multi-span bridge based on NetSLab

Modem dynamic tests such as networked collaborative pseudo-dynamic testing （PDT） provide new tools to study the dynamic performance of large and complex structures. In this paper, several networked collaborative PDT systems established in China and abroad are introduced, including a detailed description of the first networked collaborative platform that involved the construction of a standardized demonstration procedure for networked collaborative PDT. The example is a multi-span bridge with RC piers retrofitted by FRP, and a networked structural laboratory （NetSLab） platform is used to link distributed laboratories located at several universities together. Substructure technology is also used in the testing. The characteristics, resource sharing and collaborative work of NetSLab are described, and the results illustrate that use of the NetSLab is feasible for studying the dynamic performance of multi-span bridge structures.

Energy-based dynamic parameter identification for Pasternak foundation model

Parameter identification of Pasternak foundation models(PFM)is never satisfactory,which discourages the application and popularization of PFM.In the present study,an energy-based model to predict the dynamic foundation coefficients was proposed using the vibration kinetic energy and potential energy of a Pasternak foundation-rigid plate system.On the basis of the Pasternak foundation,the relationship among the natural frequency,dynamic foundation coefficients,rigid plate configuration,and vibrating soil equivalent mass per unit area was considered.To obtain the natural frequencies of the Pasternak foundation-rigid plate system,dynamic tests were performed.Using two or more dynamic test results of various rigid plates on a foundation,a set of equations of dynamic foundation coefficients was set up to directly identify the foundation coefficients and equivalent mass per unit area of vibrating soil.The feasibility of the proposed method was verified by comparing it with the outdoor and indoor test results and finite element analysis results.When the proposed method is used to obtain the dynamic parameters,PFM can be generalized and applied more widely in engineering practice.

Research on the Process of Yield Formation Based on Dynamic Design in Winter Wheat

[Objective]The aim was to research on construction of yield formation model of winter wheat.[Method]In the case of variety Shijiazhuang 8,the process of yield trait formation was studied by the dynamic ideal and uniform experimental design;the differences between plant dry weight and population indexes were analyzed by using multiple comparison analysis,and the yield formation model was developed by multiple regression analysis.[Result]The results showed that multiple correlation coefficients of yield formation model ranged from 0.91 to 0.97.[Conclusion]The model was significant which provide certain theoretical base for high yield and high efficiency cultivation of winter wheat.

Delay-dependent stability and added damping of SDOF real-time dynamic hybrid testing

It is well-recognized that a transfer system response delay that reduces the test stability inevitably exists in real-time dynamic hybrid testing （RTDHT）. This paper focuses on the delay-dependent stability and added damping of SDOF systems in RTDHT. The exponential delay term is transferred into a rational fraction by the Pad6 approximation, and the delay-dependent stability conditions and instability mechanism of SDOF RTDHT systems are investigated by the root locus technique. First, the stability conditions are discussed separately for the cases of stiffness, mass, and damping experimental substructure. The use of root locus plots shows that the added damping effect and instability mechanism for mass are different from those for stiffness. For the stiffness experimental substructure case, the instability results from the inherent mode because of an obvious negative damping effect of the delay. For the mass case, the delay introduces an equivalent positive damping into the inherent mode, and instability occurs at an added high frequency mode. Then, the compound stability condition is investigated for a general case and the results show that the mass ratio may have both upper and lower limits to remain stable. Finally, a high-emulational virtual shaking table model is built to validate the stability conclusions.

Dynamic mechanical characteristics and application of constant resistance energy-absorbing supporting material

In deep underground engineering,rock burst and other dynamic disasters are prone to occur due to stress concentration and energy accumulation in surrounding rock.The control of dynamic disasters requires bolts and cables with high strength,high elongation,and high energy-absorbing capacity.Therefore,a constant resistance energy-absorbing(CREA)material is developed.In this study,the dynamic characteristics of the new material are obtained via the drop hammer tests and the Split Hopkinson Pressure Bar(SHPB)tests of the new material and two common bolt(CB)materials widely used in the field.The test results of drop hammer test and SHPB test show that the percentage elongation of CREA material is more than 2.64 and 3.22 times those of the CB material,and the total impact energy acting on CREA material is more than 18.50 and 21.84 times,respectively,indicating that the new material has high elongation and high energy-absorbing capacity.Subsequently,the CREA bolts and cables using the new material are developed,which are applied in roadways with high stress and strong dynamic disturbance.The field monitoring results show that CREA bolts and cables can effectively control the surrounding rock deformation and ensure engineering safety.

Mechanical properties of rock under coupled static-dynamic loads

Rock drilling machine,INSTRON testing system,and SHPB device are updated to investigate the characteristics of rocks at great depth,with high loads from overburden,tectonic stresses and dynamic impacts due to blasting and boring.It is verified that these testing systems can be used to study the mechanical properties of rock material under coupled static and dynamic loading condition and give useful guidance for the deep mining and underground cavern excavation.Various tests to determine the rock strength,fragmentation behavior,and energy absorption were conducted using the updated testing systems.It is shown that under coupled static-dynamic loads,if the axial prestress is lower than its elastic limit,the rock strength is higher than the individual static or dynamic strength.At the same axial prestress,rock strength under coupled loads rises with the increasing strain rates.Under coupled static and dynamic loads,rock is observed to fail with tensile mode.While shear failure may exist if axial prestress is high enough.In addition,it is shown that the percentage of small particles increases with the increasing axial prestress and impact load based on the analysis of the particle-size distribution of fragments.It is also suggested that the energy absorption ratio of a specimen varies with coupled loads,and the maximum energy absorption ratio for a rock can be obtained with an appropriate combination of static and dynamic loads.

Experimental study of dynamic resilient modulus of subgrade soils under coupling of freeze–thaw cycles and dynamic load

Although the dynamic properties of subgrade soils in seasonally frozen areas have already been studied, few researchers have considered the influence of shallow groundwater during the freeze–thaw(F–T) cycles. So a multifunctional F–T cycle system was developed to imitate the groundwater recharge in the subgrade during the freezing process and a large number of dynamic triaxial experiments were conducted after the F–T cycles. Some significant factors including the F–T cycle number, compaction degree, confining pressure, cyclic deviator stress, loading frequency, and water content were investigated for the resilient modulus of soils. The experimental results indicated that the dynamic resilient modulus of the subgrade was negatively correlated with the cyclic deviator stress, F–T cycle number, and initial water content, whereas the degree of compaction, confining pressure, and loading frequency could enhance the resilient modulus. Furthermore, a modified model considering the F–T cycle number and stress state was established to predict the dynamic resilient modulus. The calculated results of this modified model were very close to the experimental results. Consequently, calculation of the resilient modulus for F–T cycles considering the dynamic load was appropriate. This study provides reference for research focusing on F–T cycles with groundwater supply and the dynamic resilient moduli of subgrade soils in seasonally frozen areas.

Simulation of large-scale numerical substructure in real-time dynamic hybrid testing

A solution scheme is proposed in this paper for an existing RTDHT system to simulate large-scale finite element （FE） numerical substructures. The analysis of the FE numerical substructure is split into response analysis and signal generation tasks, and executed in two different target computers in real-time. One target computer implements the response analysis task, wherein a large time-step is used to solve the FE substructure, and another target computer implements the signal generation task, wherein an interpolation program is used to generate control signals in a small time-step to meet the input demand of the controller. By using this strategy, the scale of the FE numerical substructure simulation may be increased significantly. The proposed scheme is initially verified by two FE numerical substructure models with 98 and 1240 degrees of freedom （DOFs）. Thereafter, RTDHTs of a single frame-foundation structure are implemented where the foundation, considered as the numerical substructure, is simulated by the FE model with 1240 DOFs. Good agreements between the results of the RTDHT and those from the FE analysis in ABAQUS are obtained.

Effect of Temperature and Strain Rate on Dynamic Properties of Low Silicon TRIP Steel

The dynamic tensile test of 0.11C-0.62Si-1.65Mn TRIP steel was carried out at different strain rates and test temperatures. The results show that both temperature and strain rate affect the retained austenite transformation. At high strain rates, the uniform elongation decreases, whereas the total elongation and energy absorption increase. The tensile strength is less strain rate sensitive. With raising test temperature, the tensile strength is reduced and the mechanical properties generally deteriorate, especially at 110℃,However, excellent mechanical properties were obtained at 50℃ and 75℃.