Dynamic Stress State Research Articles

Prediction of resilient modulus and critical dynamic stress of recycled aggregates: Experimental study and machine learning methods

This paper aimed to investigate the feasibility of partially or completely replacing natural aggregates with recycled aggregates from construction and demolition wastes for low-carbon-emission use as coarse-grained embankment fill materials. The laboratory specimens were prepared by blending natural and recycled aggregates at varying proportions, and a series of laboratory repeated load triaxial compression tests were carried out to study the effects of material index properties and dynamic stress states on the resilient modulus and permanent strain characteristics. Based on the experimental results and by considering the main influencing parameters of the resilient modulus and permanent deformation, an artificial neural network (ANN) prediction model with optimal architecture was developed and optimized by the particle swarm optimization (PSO) algorithm, and its performance and accuracy were verified by supplementary analyses. A shakedown state classification method was proposed based on the unsupervised clustering algorithm, and a prediction model of critical dynamic stress was established based on the machine learning (ML) method and the shakedown state classification results. The research results indicate that the stress state has a greater influence on the resilient modulus and permanent deformation characteristics than other factors, and the shear stress ratio has a significant effect on the shakedown state. The resilient modulus and critical dynamic stress of such specimens vary linearly with confining pressure. The improved PSO-ANN prediction model exhibits high prediction accuracy and robustness, superior to several other commonly used ML regression prediction algorithms. The resilient modulus and critical dynamic stress prediction methods based on ML algorithms can provide technical guidance and theoretical basis for the design and in-service maintenance of similar unbound granular materials.

Critical speed of ballasted railway tracks: Influence of ballast and subgrade degradation

Track substructure layers such as ballast and subgrade involve progressive degradation with time in relation to the loading history. Degradation of track components directly influences the dynamic track response including the displacements and stresses. Determining the influence of real-world substructure degradation problems such as particle breakage, ballast fouling and mud pumping on the critical speed of a given track is essential to design ballasted tracks for high-speed passenger and freight trains. In this paper, a coupled rheological-continuum model is developed to predict the track dynamic response for different train speeds and axle loads. In this novel approach, field dispersion curves obtained from MASW (Multichannel Analysis of Surface Waves) testing can be adopted along with the proposed analytical model to predict the critical speed of track as well as the corresponding dynamic stress state in the substructure layers. Analytical modelling shows that problems such as ballast fouling and mud pumping lead to reduction in the computed critical speed, while also amplifying the dynamic stress response at lower train speeds. The influence of substructure degradation is found to be more prominent for heavy-haul (freight) trains imparting higher axle loads, when compared to passenger trains. Practical insights highlighting the influence of track improvements such as placing a thicker ballast layer and the addition of a capping layer on the critical speed are presented in this study with the objective of rejuvenating future track design.

Macro-micro numerical analysis of granular materials considering principal stress rotation based on DEM simulation of dynamic hollow cylinder test

The granular materials were subjected to the principal stress rotation effect induced by traffic loads, wave loads, and earthquake loads in geotechnical engineering practice. In order to reveal the influence of principal stress rotation on the macro-micro dynamic characteristics, this study proposed a numerical simulation method modeling physical dynamic hollow cylinder tests. Taking the dynamic stress state induced by traffic loading as an example, the numerical test model of the dynamic hollow cylinder was conducted on the subgrade soil, and the influence of principal stress rotation on the macro-micro evolution of dynamic characteristics was analyzed.The results showed that the principal stress rotation accelerated the development of vertical accumulative plastic strain and particle displacement, which aggravated micro-structure destruction of granular material. Moreover, the increase of confining stress declined the variations of normal contact numbers in each direction, which resisted the shear stress induced by principal stress rotation. This study provided a novel method to investigate the macro-micro dynamic characteristics of granular materials subjected to principal stress rotation, which made up for the inability of physical hollow cylinder tests to study microscopic evolution.

Dynamic resilient modulus of subgrade silty clay for heavy-haul railway: an experimental investigation and the predicted method

A large-scale series of cyclic triaxial tests were conducted to explore the evolution of the dynamic resilient modulus of silty clay for the heavy-haul railway subgrade. A novel loading sequence for measuring the dynamic resilient modulus was established, which characterized the dynamic stress state of the subgrade induced by the heavy-haul train load. In the experimental investigation, the deviatoric stresses, confining stress, initial moisture content, and compaction degree were considered as variables, and the effects of the aforementioned variables were evaluated quantitatively. The experimental results showed that the dynamic resilient modulus was negatively related to deviatoric stresses and initial moisture content, where the average decreased rates were 14.65% and 27.79% with the increase in deviatoric stresses from 60 kPa to 150 kPa and increase in the initial moisture content from 9.8% to 15.8%, respectively. Furthermore, the dynamic resilient modulus was positively related to confining stress and compaction degree, where the average increased rates were 23.25% and 27.48% with the increase in confining stress from 20 kPa to 60 kPa and increase in compaction degree from 0.91 to 0.95. To provide a better application, the two high-accuracy predicted methods were established through the empirical model and artificial neural network approach including the aforementioned variables. This study can provide useful guidelines for the effective and safe design of the heavy-haul railway subgrade filled with silty clay.

Three-Dimensional Static and Dynamic Analyses of an Embedded Concrete-Face Rockfill Dam

Concrete-face rockfill dams have gradually become the preferred dam type in the engineering community. This study presents a hydropower station in China as a case study to introduce a new type of embedded concrete-face rockfill dam. The static and dynamic stress–strain characteristics of the proposed and conventional concrete-face rockfill dams were compared, and the optimal height of the embedded concrete body at the hydropower station was determined. The results indicate that, under static conditions, the embedded concrete body could reduce deformation upstream and downstream of the rockfill body, eliminate tensile stress along the concrete-face slab slope, reduce concrete-face slab deflection, and increase the maximum deflection area to 0.47 times the dam height. The inhibitory effect of the embedded concrete body on the stress and strain of the dam body became more evident as the size of the embedded body increased. Although the embedded concrete body did not enhance the dynamic and superposed static–dynamic stress states of the embedded concrete body and rockfill, the stress and strain increase in the dynamic state were within a controllable range. Through a sensitivity analysis and considering the terrain conditions and engineering cost of the hydropower station, the height of the embedded concrete body is recommended to be 0.4 times the dam height.

Can the geometry of an injection clastic dike swarm indicate a short-term process? Examining the Dead Sea Basin in Israel

Despite the importance of identifying dynamic stress fields, doing so is not straightforward. This is because most geological processes occur slowly and the formation of fractures is generally attributed to long-term processes.Within the seismically active Dead Sea Basin (DSB), radial and concentric injection clastic dikes were emplaced during earthquake events, but a satisfactory explanation for their geometric configuration is lacking. In this study, field observations were used to explore whether the geometry of the injection clastic dike swarm reflects a quasistatic state of stress, such as with a salt diapir, or if it reflects a dynamic short-term stress state associated with the passage of seismic waves. The results show that the radial clastic dikes are not concentrated toward one central point located approximately 3 km east of the Ami'az Plain, that the concentric clastic dikes are arranged by three subsets, and that the spatial density and strain of both fracture sets are focused mainly toward the center of the Ami'az Plain. This evidence does not fit with a quasistatic state-of-stress interpretation of the region that would be associated with a local stress state around a salt diapir. A more plausible explanation is that the geometry of the radial and concentric clastic dike sets was controlled by short-term seismic waves that were probably generated by earthquake events located along an adjacent fault and that were trapped by the sedimentary basin located below the Ami'az Plain itself.The reconstruction of the injection clastic dike pattern may provide an opportunity to more deeply understand the short-term process induced by paleoearthquakes occurring adjacent to sedimentary basins close to the surface.

Field evaluation of the three-dimensional dynamic stress state of the subgrade induced by the heavy-haul train load

The three-dimensional stress state of subgrade soil elements plays a crucial role in railway subgrade design and service life evaluation, especially in heavy-haul railways. To solve the complex problem of the field test of three-dimensional stress state in subgrade induced by train vehicles, a novel regular dodecahedron soil pressure sensor (RDSPS) was developed and applied to field tests, which can measure dynamic stress components, including normal stress and shear stress in ×,y and z directions. Then the field test of total stress components of subgrade was carried out on the Datong-Qinhuangdao heavy-haul railway. The dynamic stress, including three normal stresses and three shear stresses, was determined at seven points in the subgrade for trains with 25 t axle load at different speeds (45, 55, and 60 km/h). The result indicates that the distributions of dynamic stresses of railway subgrade show periodic fluctuation subjected to moving train load, and each bogie can be regarded as a loading point for the field test section. Meanwhile, the stress path of soil elements in the subgrade changes continuously during the passage of a heavy-haul train, and the maximum principal stress angle changes between -π/2 and π/2. The normal stress in the × and y directions decreases first slowly and then rapidly with the depth of subgrade, while the attenuation law of normal stress in the z-direction with the depth of subgrade is just the opposite. Due to the stress superposition effect, the normal stress in the y-direction of the measuring points below the track centerline is greater than that at other measuring points. A calculation model of vertical normal stress was proposed based on the measured data. The empirical model for the attenuation coefficient of normal stress in the ×, y, and z directions was established to predict the stress attenuation coefficient more accurately. In this paper, the total stress components of the railway subgrade were measured for the first time during the passage of the train, which can provide helpful guidance for a better understanding of the dynamic response of the subgrade.

Influence of Strain History on Dynamic Strain Localization and Stress State During High-Rate Tensile Loading of Titanium Alloys: Experiments, Modeling, and Analytical Methods

Abstract The determination of the mechanical response of engineering materials subjected to high loading rates plays an important role in determining their performance and application. The high strain-rate tensile response of metals is usually investigated by means of the split-Hopkinson tension bar (SHTB) apparatus. The interpretation of the obtained results is, however, subjected to analogous stress and strain uniformity challenges present during quasi-static tensile experiments. Beyond the onset of necking, strains cease to be uniform along the gauge length and localize around the necking zone. Consequently, the nominal strain rate underestimates the effective strain rate experienced by the material. The analysis of the effective strain rate and stress state beyond the onset of necking has received considerable attention in the literature. Several research efforts have focused on the optimization of the geometry of specimens to be employed for the characterization of the dynamic tensile response using the SHTB. The present work investigates, systematically, the effects of strain history and adiabatic heating on the stress state during dynamic loading. A series of monotonic and various strain history experiments were conducted and analyzed. The diameter evolution, effective strain rate, and temperature histories were measured for all conducted experiments. Numerical simulations were carried out to examine the stress state during strain localization and to accurately reproduce engineering and local thermos-mechanical variables. The effectiveness of existing postnecking corrections for high-rate experiments is assessed. A modified postnecking correlation taking into account the effects of adiabatically induced thermal softening is proposed.

Experimental study of dynamic biaxial compressive properties of full grade aggregate concrete after freeze thaw cycles

Most hydraulic concrete structures are in complex stress state, this study attempts to take some test of full grade aggregate concrete under biaxial compression after 0, 25, 50, 75, and 100 cycles of freeze-thaw. Five principle compression stress ratios of 1:0, 1:0.25, 1:0.5, 1:0.75 and 1:1 and four different strain rates of 10−5/s, 10−4/s, 10−3/s and 10−2/s were the main variables. The mass loss of concrete after freeze-thaw cycles has been studied. With the increase of freeze-thaw cycles, the quality loss is little. The failure pattern, ultimate strength, peak strain and stress-strain curve under different freeze-thaw cycles, stress ratios and strain rates were obtained. The dependence of ultimate strength on freeze-thaw cycles and ultimate strength, and the relationship between freeze-thaw cycles, stress ratio and strain rate were analyzed according to the testing data. The unified failure criterion of full grade aggregate concrete after freeze-thaw cycles under dynamic biaxial compressive stress state was established in principal stress space and octahedral stress space, respectively. The comparative analysis of experimental results of different types of concrete(large aggregate concrete, ordinary concrete, lightweight aggregate concrete, wet screened concrete) under biaxial compression after freeze-thaw cycles are made. With the increase of freeze-thaw cycles, the ultimate compressive strength of common concrete, wet screened concrete, lightweight aggregate concrete and large aggregate concrete gradually decreases. The results may serve as an experimental and theoretical reference for the design and maintenance of hydraulic structures.

Modelling and investigation of the stress state in a finite cylinder with a circular crack under sudden torsional loading

This paper is devoted to the analysis of the dynamic stress state of a finite cylinder with an internal circular crack. One of the cylinder’s bases is fixed, and a time-dependent torque is applied to the other base. For solving this problem, a modified finite difference method was applied. This method consists in the difference approximation of only the time derivative. As a result, the original problem is reduced to a set of sequentially solved homogeneous boundary value problems. Their solutions contain an unknown displacement jump on the crack. The integral equation for the unknown jump is reduced to the Fredholm integral equation of the second kind, which was solved numerically. This numerical solution made it possible to obtain an approximate formula for calculating the stress intensity factor (SIF).

Identifying the Types of Loading Mode for Rock Fracture via Convolutional Neural Networks

AbstractFor decades, monitoring and identification of the dynamic stress states of rock masses in reservoirs have been challenging tasks owing to the lack of effective observation and analysis methods. In this study, we used deep learning methods to identify whether acoustic emission (AE) signals of rock fractures are created by different loading conditions. We performed Brazilian split and uniaxial compressive experiments on six different engineering materials, and we obtained the AE waveforms. To take advantage of the powerful image processing capabilities of convolutional neural networks (CNNs), we transformed the AE waveforms into time–frequency images. We used five types of CNN to identify the time–frequency images of the AE signals created in the Brazilian split and uniaxial compressive experiments. As a result, we found that Xception model had the highest recognition accuracy. We analyzed the basis of the CNN models to recognize the AE signals using local interpretable model‐agnostic explanations and found that the Xception model mainly used the patterns of the low‐energy region of the time–frequency images to determine the loading modes of the rock fractures. Furthermore, the high‐energy time–frequency region had little effect on recognition. The findings of this work can aid in automatically monitoring the dynamic stress states of fracture areas in reservoir formations and ensure the safety of oil and gas resource exploitation.

On the dynamic response of flexible dual-row retaining walls in dry sand

Dual-row retaining walls can be utilised to form embankment structures for protection against tsunamis or as port structures. The dynamic response of these walls involves a complex interaction between the soil and structural elements which is not well understood. In this paper, for the first time a combination of centrifuge and finite-element modelling is used to better understand the mechanical response of the combined wall–soil system. The measured variation of the horizontal stresses leads to bending moment distributions featuring large, singly outward bending or double curvature with significant inward bending of the wall near the ground level. The numerical analyses are used to understand the stress state and highlight the dynamic variations of the vertical effective stress that drive this previously unexplained behaviour. The shear stresses that can develop at the wall–soil interface govern the mechanism by which the vertical effective stresses can vary. Further consideration of the dynamic soil stress state suggests a purer interpretation of the limiting loads in soil–structure interaction problems relative to the traditionally defined dynamic earth pressure coefficients. Combining the altered vertical soil stresses with earth pressure coefficients depending only on the soil friction angle adequately bounds the horizontal stresses that develop in the soil.

Study on dynamic multiaxial tension–compression properties of different types of concrete

The mechanical properties of four types of concrete, tested under multiaxial (biaxial tension–compression, triaxial tension–compression–compression) dynamic (strain rates of 10−5, 10−4, 10−3 and 10−2 s−1) complex stress states with different stress ratios, were compared and analysed. The same test apparatus was used in order to avoid test errors caused by differences of the triaxial testing machine. The aim of this work was mainly to determine the dynamic multiaxial (DM) strengths of the four concrete types. It was found that the mechanical behaviour of the four concretes was similar to a certain extent. The DM strengths of the concretes increased with an increase in strain rate and their DM strengths in any direction were lower than the concrete static strength under uniaxial tension and compression. The influence of strain rate and stress ratio on the DM strengths of the concretes were compared and analysed. A unified failure criterion was developed, expressed by the octahedral stress space criterion, of the various concrete types under DM tension and compression. This investigation provides a theoretical foundation for the design and checking of structures subjected to DM load conditions.

Effect of high strain rate and confinement on the compressive properties of autoclaved aerated concrete

Autoclaved aerated concrete (AAC), also known as autoclaved cellular concrete, is a highly porous and light-weight material with a huge amount of closed air bubbles. It has been widely used in over 40 countries around the world to make masonry units such as infill walls or other nonstructural elements such as wall panels. Some structures during their service life might be exposed to blast and impact loads. Such extreme dynamic loads owing to their specific characteristics such as high amplitude and short duration induce very different structural responses and damage as compared to static and low-rate dynamic load. Understanding the properties of AAC material under dynamic and complex stress states is essential for accurate prediction of the response of structural components made of AAC subjected to highly dynamic loads. This study investigates the compressive behavior of AAC material under uniaxial quasi-static and impact and quasi-static triaxial loading tests. AAC material with nominal density of 500 kg/m3 was used to prepare specimens in all tests. For uniaxial compressive tests, the strain rate ranged from 3.3 × 10−5s−1 to 100 s−1 by using electric universal testing machine and INSTRON VHS 160/100–20 high-strain-rate testing system. A high-speed camera was used to capture the failure processes of AAC specimens during the tests. Based on testing data and high-speed camera images, the failure mechanism was investigated. Dynamic stress-strain curves under different strain rates were derived, and rate effects on the compressive strength were discussed. The corresponding empirical DIF (dynamic increase factor) relations were proposed. For quasi-static pseudo triaxial compressive tests, the deviatoric stress-strain curves of AAC specimens under different levels of confining pressure were obtained and compared. The damage induced by hydrostatic pressure and the corresponding failure modes were investigated. The compressive meridian and the empirical EOS (equation of state) relations were proposed based on the test data.

Soft body impact on composites: Delamination experiments and advanced numerical modelling

Cohesive interface elements have become commonly used for modelling composites delamination. However, a limitation of this technique is the fine mesh size required. Here, a novel cohesive element formulation is proposed and demonstrated for modelling the numerical cohesive zone with equal fidelity but fewer elements in comparison to a linear cohesive element formulation. The newly proposed formulation has additional degrees of freedom in the form of nodal rotations which when combined with the use of multiple integration points per cohesive element, allows for delamination propagation to be modelled with increased stability. This element formulation is introduced with an adaptive modelling method, termed Adaptive Mesh Segmentation (AMS). To demonstrate its effectiveness under impact loading the new model is applied to a soft body beam bending test. This test, containing a delamination pre-crack, uses inertial constraints and results in a dynamic stress state when impacted by a gelatin cylinder.

Experimental Study on Biaxial Dynamic Compressive Properties of ECC.

An improved hydraulic servo structure testing machine has been used to conduct biaxial dynamic compression tests on eight types of engineered cementitious composites (ECC) with lateral pressure levels of 0, 0.125, 0.25, 0.5, 0.7, 0.8, 0.9, 1.0 (the ratio of the compressive strength applied laterally to the static compressive strength of the specimen), and three strain rates of 10−4, 10−3 and 10−2 s−1. The failure mode, peak stress, peak strain, deformation modulus, stress-strain curve, and compressive toughness index of ECC under biaxial dynamic compressive stress state are obtained. The test results show that the lateral pressure affects the direction of ECC cracking, while the strain rate has little effect on the failure morphology of ECC. The growth of lateral pressure level and strain rate upgrades the limit failure strength and peak strain of ECC, and the small improvement is achieved in elastic modulus. A two-stage ECC biaxial failure strength standard was established, and the influence of the lateral pressure level and peak strain was quantitatively evaluated through the fitting curve of the peak stress, peak strain, and deformation modulus of ECC under various strain rates and lateral pressure levels. ECC’s compressive stress-strain curve can be divided into four stages, and a normalized biaxial dynamic ECC constitutive relationship is established. The toughness index of ECC can be increased with the increase of lateral pressure level, while the increase of strain rate can reduce the toughness index of ECC. Under the effect of biaxial dynamic load, the ultimate strength of ECC is increased higher than that of plain concrete.

Wave Theory of Seismic Resistance of Underground Pipelines

The object of the research is an underground straight horizontal pipeline subjected to seismic impact. The research method was analytical. The results were compared with the experimental results of other authors and computer calculations. It was shown that the main disadvantage of the dynamic theory of seismic resistance of underground pipelines is the neglect of the dynamic stress state in soil under seismic wave propagation. The next drawback of the dynamic theory is an inaccurate, approximate accounting for the displacement of the soil medium to which the underground pipeline is embedded. The complete interaction process includes the stages of nonlinear changes in the interaction force (the friction force) by manifesting its peak value and the Coulomb friction. The contact layer of soil undergoes shear deformations until complete structural destruction of the soil contact layer. The interaction force is the friction force, and its peak value does not appear. The seismic resistance of underground pipelines should be considered based on the theory of propagating seismic waves in a soil medium and the interaction of seismic waves with underground pipelines, i.e., based on the wave theory of seismic resistance of underground pipelines. A one-dimensional coupled problem of seismic resistance of underground pipelines under seismic impacts was posed based on the wave theory. An algorithm and a program for the numerical solution of the stated wave problems were developed using the method of characteristics and the method of finite differences. An analysis of the laws of interaction of underground pipelines with soil under seismic influences shows that it is necessary to use in the calculations the laws of interaction that account for the complete interaction processes observed in experiments. The analysis of the obtained numerical solutions and the posed coupled problems of the wave theory of seismic resistance of underground pipelines show the occurrence mechanisms of longitudinal stresses in underground pipelines under seismic influences. The results of calculations stated that an account for the dynamic stress normal to the underground pipeline’s outer surface leads to multiple increases in longitudinal stress in the underground pipeline. This multiple increase is due to the transformation of the interaction force into an active frictional force, resulting from a greater strain in soil than the one in the underground pipeline. Based on the analysis results, a theory of the seismic wave propagation process in an underground pipeline and surrounding soil was proposed.

Study of the Dynamic Stress State of Microporous Media Within the Framework of the Cosserat Pseudocontinuum

We develop an analytic-numerical method for the investigation of the dynamic stress state of an unbounded layered element containing a hole within the framework of Cosserat pseudocontinuum. The method is based on the application of the Fourier transformation with respect to the time and the method of boundary integral equations. On the basis of the proposed method, we study the concentrations of the dynamic circumferential and radial stresses in biomaterials near circular and elliptic holes.

Influence of geostress field on dynamic mechanical behavior of intact rock

The high magnitude of in situ geostress under deep underground conditions and the intermediate principal stress under complex stress conditions have a significant influence on the static or dynamic mechanical behavior and strength characteristics of intact rock material. The constitutive relationship for quasi-brittle materials (e.g., intact rock or concrete) subjected to large strains, high strain rates, high pressures, and complex stress states was investigated, but the mechanical behavior and strength characteristics for rock under high pressures and complex stress states have not yet been elucidated. This study mainly focuses on constitutive modeling and theoretical prediction of the dynamic mechanical behavior and strength characteristics for rock under high dynamic pressures and complex stress states. A damaged elastoplastic constitutive model for intact rock is established in this study by taking the influence of a high strain rate, high pressure, and complex stress state into account. The failure mechanism of deeply buried rock engineering under high in situ stress and an additional impact pressure from a high-velocity underground impact was analyzed in this paper. The influence of the high pressure and intermediate principal stress on the quasi-static or dynamic mechanical behavior and strength characteristics of the deeply buried rock was investigated under a complex stress state.

Dynamic Behaviour of Bridge Girders with Trapezoidal Profiled Webs Subjected to Moving Loads.

The aim of this study is to find out the degradation of dynamic behaviour of bridge girders with trapezoidal profiled webs when subjected to different vehicle moving loads. Finite element modelling based parametric analysis is demonstrated to be desirable in capturing the dynamic deflection and stress state of critical structural details of girders. The model is validated in the modal analysis through a comparison with theoretical eigenfrequencies. The vibration characteristics are shown to be significantly affected by the corrugation details. The structural service life results of analysed bridge girders are in close agreement with experimental data. It is shown that the dynamic nodal velocity and deflection of analysed bridge girders are greatly affected by the magnitude of the load corresponding to the overload of the vehicle in contrast to the vehicle travel speed. Similar observations can be made for the fatigue life prediction analysis related to the crack initiation when unfavourable effects of the overload vehicle are concerned. Presented analytical results using a fracture mechanics approach could be taken as a good basis for the service life assessment of related bridges with the desired level of performance or functionality.