Stress-displacement Response Research Articles

The oedometric compression behavior of expanded clay including the crushability of single particles

The objective of this paper is to study the expanded clay material behaviour considering the crushability of its single grains. The probed material is characterized by a high internal porosity; randomly distributed inside its grains. The assembly behaviour was investigated considering the oedometric path and the fragmentation amount was quantified using the relative breakage coefficient (Br). Oedometric compression tests were performed on different loose samples with similar grain size distribution. In this paper, single grain indirect tensile tests were made to identify the microscopic parameters affecting the breakage occurring. The density effect was studied using a statistical approach and the calibration was made based on the indirect tensile tests results performed on individual grains. Authors further developed a crushing algorithm based on the experimental tests results performed on several dimensions ranges of single grains. The algorithm was implemented in the PFC3D software aiming to model the material’s macroscopic behaviour using discrete elements modelling (DEM). The analysis of the macroscopic response of a loose grain’s assembly was conducted using both the stress–displacement responses and the breakage factor. The comparison between experimental and numerical simulations shows the DEM’s potential, coupled with the fragmentation algorithm of the individual grains issued from the experimental single grains study, to reproduce the assembly’s mechanical behaviour. However, basing on the breakage factor Br, the DEM simulations performed herein are not in a good agreement with experimental ones; which will be discussed in this work.

Localization analysis of nonlocal models with damage-dependent nonlocal interaction

This paper systematically evaluates (in the one-dimensional setting) the performance of a new type of integral nonlocal averaging scheme, initially motivated by the idea of internal time that reflects the reduction of the elastic wave speed in a damaged material. The formulation dealing with internal time is replaced by the equivalent concept of a modified spatial metric leading to a damage-dependent interaction distance. This modification has a favorable effect on the evolution of the active part of damage zone and leads to its gradual shrinking, which naturally describes the transition from a thin process zone to a fully localized crack. However, when a pure damage model (with no permanent strain) is considered, the resulting load-displacement diagrams exhibit dramatic snapbacks and excessively brittle behavior in the final stages of failure. The concept of damage-dependent interaction distances is therefore extended to damage-plastic models and damage models with inelastic (permanent) strain. It is shown that, for formulations that consider a part of the strain as irreversible, the overall stress-displacement response becomes realistic for quasi-brittle materials such as concrete, for which the diagram typically exhibits a long tail.

Multiaxial crushing of open-cell foams

The paper investigates the crushing behavior of low-density foams under triaxial loadings. A micromechanically accurate cubical model of an Al-alloy random open-cell foam is surrounded by three pairs of orthogonal rigid planes and crushed simultaneously in three directions. The foams are crushed along radial displacement paths to volume changes of 70%. For all loading paths the foam traces initial stiff and stable branches during which it deforms as an elastic, isotropic solid. The branches terminate into stress maxima followed by localized crushing in narrow bands of cells. The crushing spreads with limited additional effort with crushed and relatively undeformed cells coexisting. The deformation becomes uniform again once the crushing has spread to the whole domain. Reported are stress-displacement responses in the three directions, and corresponding foam deformed configurations; the mean stress-volume change responses; and the energy absorbed as a function of the volume change. The results point to the need for constitutive models that can capture this partially unstable crushing behavior.

The similarity law and its verification of cylindrical lattice shell model under internal explosion

It is very important to ensure that the results of a scale model experiment of the large-span space steel structure under internal explosion could accurately show the characteristics of its original model. For this purpose, a one-way inclined bar type of single layer cylindrical lattice shell was taken as an example in this paper, and the similarity law between the scale models and the original model under the internal explosion was deduced by the dimension analysis principle. Then, the original model and the scale models under internal explosions were simulated by ANSYS/LS-DYNA, and the correctness and the feasibility of the similarity law were compared and confirmed. The research results showed that: if the scale models conform to the similarity law, the propagation law and the overpressure distribution of shock waves to the scale models subjected to internal explosions were consistent with the original model; the phenomena of critical destruction of the scale models were similar to the experimental results. But the accuracy of the loading effects of blast (such as the overpressure peak of shock waves, the positive pressure time and the impulse etc.) and the structural dynamic responses obtained by the scale models was affected by the geometric incomplete similarity of the scale models and the strain rate effect of steel respectively, which would reduce with the decrease of the scaling coefficient. Especially, the research results also indicated that if the scaling coefficient was not less than 0.1, the maximum deviations of the overpressure peak of shock waves, the positive pressure time after conversion and the specific impulse after conversion would be 10%, 4.78% and 29.8%, respectively, and if the scaling coefficient was less than 0.25, the stress responses and displacement responses of the scale model under internal explosion would not be reliable. Based on the research results, the reasonable scaling coefficients to the scale model experiment of cylindrical lattice shell under internal explosion are proposed.

The seismic displacement of a block sliding on an inclined plane with resistance varying with the distance moved

The paper considers a rigid block on an inclined plane. First, the typical response measured along slip surfaces of sands and clays under both drained and undrained conditions is identified. Then, for any expression simulating this change in shear stress with displacement along the boundary between the block and the plane, the paper derives an equation which predicts the equivalent friction angle which, when applied, computes the same seismic displacement of the block. The paper also proposes, validates and calibrates a simple constitutive equation which simulates the typical shear stress-displacement response measured in ring shear tests for both sands and clays sheared under both drained and undrained conditions. Next, the paper for the constitutive law above simulating the change in shear stress with displacement along the slip surface, derives equations which predict the equivalent friction angle which, when applied, computes the same seismic displacement of the block. Finally, based on these equations, the paper proposes an easy-to-apply methodology predicting the seismic displacement of slopes along slip surfaces with strain softening with the aid of the seismic displacement predicted by the conventional sliding-block model and applies this methodology in order (a) to validate it against results of elaborate numerical analyses and (b) to modify the Ambraseys-Menu expressions predicting the seismic displacement of the conventional sliding-block model for soils exhibiting strain softening.

Comparison of direct shear and simple shear responses of municipal solid waste in USA

Although large-size simple shear (SS) testing of municipal solid waste (MSW) may arguably provide a more realistic estimate of the shear strength (τ ) of MSW than the most commonly used direct shear (DS) testing, a systematic comparison between the shear responses of MSW obtained from the two testing methods is lacking. In this study, a large-size shear device was used to test identical MSW specimens sampled in USA in DS and SS. Eight DS tests and 11 SS tests were conducted at vertical effective stresses of 50–500 kPa. The stress–displacement response of MSW in SS testing was hyperbolic and a maximum shear stress was reached, whereas a maximum shear stress was not reached in most DS tests. The τ, effective friction angle (ϕ ′) and cohesion (c ′) of MSW were obtained from DS and SS tests by using a displacement failure criterion of 40 mm. τ in SS testing was found to be equal to or lower than τ in DS testing with ratios of τ between 73 and 101%. SS testing resulted in higher ϕ ′ but lower c ′ than DS testing. The shear strength parameters were lower than those obtained in previous studies from DS tests at 55 mm displacement.

Surface topographical analysis of geomembranes and sands using a 3D optical profilometer

Understanding the stress-displacement response of granular materials and their interfaces with geosynthetics through surface topographical analysis is evolving as a new area of potential interest. Recent developments in optical techniques coupled with digital image recording have enabled the measurement of the surface topographical features of sand particles and geosynthetics, which can be applied to measure the shear-induced surface changes in geomembranes. This paper presents one of the latest, advanced surface measurement techniques based on coherence correlation interferometry using a 3D optical profilometer. An experimental technique was developed to measure the surface topography of sand particles, and the shear-induced surface changes in a smooth geomembrane caused by sand particles of different mean sizes, in interface direct shear tests. As the measured surfaces consisted of form and waviness along with roughness, appropriate filters were applied to remove form and waviness and measure the roughness of the surfaces. The selection of these filters for different cases and the spatial variability of surface roughness measurements are discussed. The results of the surface topographical analysis of sand particles of three different sizes, geomembranes sheared by sands of three different mean sizes, and their repeatability, are presented. The stress-displacement response of the sand–geomembrane interfaces is correlated to the surface changes on sheared geomembranes through visual observations and roughness quantifications.

An integrated method for analyzing load transfer in geosynthetic-reinforced and pile-supported embankment

The load transfer mechanism of a geosynthetic-reinforced and pile-supported embankmentis is influenced by the interactions among embankment fill, bridging layer, piles and foundation soil. In this paper, an integrated method for calculating the loads carried by the pile and subsoil is developed based on the displacement compatibility principle, which takes the soil arching effect, tensioned membrane effect and pile-soil interaction into account. In the presented method, the magnitude of shear stress due to the arching action depends on the relative displacement within the embankment fill and the elastic-perfectly plastic stress-displacement response is considered, the tension and strain in the geosynthetic are obtained using the parabolic method, and the hyperbolic model is adopted to relate the negative and positive skin friction with differential settlement between pile and surrounding soil. The proposed method of determining the stress on the pile and subsoil provides a better agreement with the field measurement comparing to the existing analytical methods. In addition, a parametric study is conducted to evaluate the effects of several factors on the load transfer behavior, such as the mobilization coefficient of shear strength, cohesion and internal friction angle of embankment fill, pile spacing, and compressive modulus of subsoil. The results indict that the cohesion plays a more important role in transferring of load from soft subsoil to rigid piles than the internal friction angle of embankment fill. Additionally, different optimal pile spacings which lead to the maximum stress concentration ratio are found for different embankment heights.

Modelling geogrid-reinforced railway ballast using the discrete element method

Abstract Rail ballast is an unbounded granular material that spreads laterally when subjected to train loading. Railroads can be reinforced by geogrids to reduce lateral movement and to optimize track performance. This paper presents a study of the behaviour of geogrid-reinforced ballast subjected to monotonic and cyclic loading using a large-scale direct shear box and a novel Track Process Simulation Apparatus (TPSA). The shear stress–strain response of fresh and fouled ballast reinforced by geogrid was investigated using large-scale direct shear tests subjected to normal stresses from 15 kPa to 75 kPa, where the levels of fouling varied from 20% to 95% Void Contamination Index (VCI). Cyclic tests for fresh and fouled ballast were conducted using the TPSA to realistically simulate real track conditions. The experimental results showed that a geogrid provides extra internal confinement and interlocks the aggregates in its apertures, hence reduces ballast deformation. The discrete element method (DEM) was used to model geogrid-reinforced fresh and fouled ballast subjected to monotonic and cyclic loading. Irregularly-shaped particles and geogird were simulated by clumping spherical balls together, while the coal fines were simulated by adding 1.5 mm diameter spheres into the pore spaces of ballast. The predicted stress-displacement responses obtained from the DEM were in good agreement with those measured in the laboratory, where the peak shear stress of fouled ballast decreased and the dilation of fouled ballast increased with an increasing level of fouling. The contact force distributions and the orientations of normal and shear force were analyzed to provide more insight into the behaviour of ballast subjected to shearing.

Load-Displacement and Moment-Rotation Responses of Foundations Subjected to Moment Loading

AbstractThis study investigated the response of rigid surface foundations to moment loading based on nonlinear (hyperbolic) soil response. The response of a rectangular foundation on the surface of the ground was modeled as a nonlinear Winkler model. The nonlinear Winkler-based shallow foundation model was evaluated for its dependence on eccentricity of load, subgrade properties, etc. The key subgrade properties on which the proposed nonlinear model depends are the initial tangent subgrade modulus and ultimate bearing stress of the soil. The model was validated by comparing the predicted response of stress versus normalized displacement based on relevant soil properties with the response from the published data of field footing load tests. A parametric study quantified the effects of different parameters on the stress-displacement and moment-rotation responses of the foundation. The proposed analysis captured the inherent nonlinear deformations of the soil and the coupled nature of displacements and rotat...

Phase Transformations and Fatigue of NiTi

Tensile fatigue of superelastic medical graded NiTi wires was analysed in stress control regime with special attention paid to the deformation/transformation processes involved. Concave shaped samples were machined from NiTi wires allowing us to confine deformation processes into the centre of the sample. The localization of phase transformations within the centre was confirmed using in-situ infrared imaging. Characteristic stress-displacement responses for amplitudes inducing either R-phase only or both R-phase and martensite were identified for these samples. A limited number of pull-pull fatigue tests in force-control regime were performed. The obtained S-N curve and associated stress-displacement responses allowed for identification of three fatigue regimes differing in the fatigue life-time by orders of magnitude. Particularly, fatigue life-time deteriorating effect of R-phase transformation was identified.

Monte Carlo simulations of mesoscale fracture modelling of concrete with random aggregates and pores

A procedure for generating two-dimensional heterogeneous meso-scale concrete samples is developed, in which the multi-phasic features including the shape, size, volume fraction and spatial distribution of aggregates and pores are randomised. Zero-thickness cohesive interface elements with softening traction–separation relations are pre-inserted within solid element meshes to simulate complex crack initiation and propagation. Extensive Monte Carlo simulations (MCS) of uniaxial tension tests were carried out to investigate the effects of key multi-phasic features on the fracture patterns and load-carrying capacities. It is found that the fracture behaviour and stress-displacement responses of the numerical specimens are highly dependent on the random mesostructures, especially the post-peak softening responses. The specimens fail with either one or two macro-cracks, regardless of the shapes and volume fractions of aggregates and pores. Assuming that the aggregate–mortar interface is weaker than the mortar, using polygonal rather than circular or elliptical aggregates, or increasing the aggregate volume fraction will reduce the tensile strength of specimens. The porosity is found to have severely adverse effects on the specimen strength and cannot be neglected in mesoscale fracture modelling of concrete.

Evolution of mesoscopic granular clusters in comminution systems: a structural mechanics model of grain breakage and force chain buckling

A major scientific challenge in establishing a micromechanics theory for complex materials is the characterisation and modelling of emergent mesoscopic phenomena. This study demonstrates the key elements of a structural mechanics approach to the modelling of mesoscopic dissipative phenomena in comminution systems where grain breakage and force chain buckling coexist. Given the many degrees of freedom in these systems, there are multitude of possible configurations and configurational transitions accessible even for a small particle cluster (e.g. a particle and its immediate neighbours). Here, we develop a model of the evolution of a 6-particle cluster undergoing breakage and force chain buckling, in sequence. The analysis lays bare the intricate connections between the contact topology, the relative kinematics arising from the interactions of particles at the bonded versus non-bonded contacts, and the collective dynamics of these interactions as the cluster is monotonically compressed under confinement. The stress-displacement response profiles at the cluster scale exhibit qualitatively similar properties to those seen in macroscopic assemblies under confined compression. A parametric analysis is undertaken to explore the effects of grain-scale resistances to breakage and buckling with respect to the overall force-displacement behaviour of the granular cluster. The study casts light on open problems for future research into the micromechanics of emergent cluster behaviour germane to comminution systems.

DEM Simulation of Direct Shear: 2. Grain Boundary and Mineral Grain Strength Component Influence on Shear Rupture

The influence of mineral grain and grain boundary strength is investigated using a calibrated intact (non-jointed) brittle rock specimen subjected to direct shear with a particle-based distinct element method and its embedded grain-based method. The adopted numerical approach allows one to independently control the grain boundary and mineral grain strength. The investigation reveals that, in direct shear, the normal stress (σn) applied to a rock specimen relative to its uniaxial compressive strength (UCS) determines the resulting rupture mechanism, the ultimate rupture zone geometry, and thus its shear stress versus horizontal displacement response. This allows one to develop a rupture matrix based on this controlling parameter (i.e., σn/UCS). Mineral grain strength reductions result in the lowering of the apparent cohesion intercept of the peak linear Coulomb strength envelope, while grain boundary strength reductions change the peak linear Coulomb strength envelope to a bi-linear or curved shape. The impact of grain boundary strength is only relevant at σn/UCS ratios 0.17), the influence of weakened grain boundaries is minimized and strength is controlled by that of the mineral grains.

DEM Simulation of Direct Shear: 1. Rupture Under Constant Normal Stress Boundary Conditions

A particle-based distinct element method and its grain-based method are used to generate and simulate a synthetic specimen calibrated to the rupture characteristics of an intact (non-jointed) low-porosity brittle rock deformed in direct shear. The simulations are compared to the laboratory-generated ruptures and used to investigate rupture at various normal stress magnitudes. The fracturing processes leading to shear rupture zone creation and the rupture mechanism are found to be normal stress dependent (progressing from tensile splitting to shear rupture) and show partial confirmation of rupture zone creation in nature and in experiments from other materials. The normal stress dependent change is found to be due to the orientation of the major principal stress and local stress concentrations internal to the synthetic specimens being deformed. The normal stress dependent rupture creation process results in a change to the rupture zone’s geometry, shear stress versus horizontal displacement response, and thus ultimate strength.

Ductile failure under combined shear and tension

The aim of this paper is to establish the extent to which materials can be deformed under shear-dominant loadings. Custom Al-6061-T6 tubular specimens are loaded under radial paths of tension and shear to failure. During the experiments, the deformation is monitored in a test section designed to have nearly uniform stress and deformation at large strains while providing minimum constraint to the development of localization that precedes failure. The recorded shear stress-rotation and axial stress-displacement responses exhibit maxima beyond which deformation localizes in a narrow band that is of the order of the 1mm wall thickness of the test section. For the mainly shear dominated stress paths followed, deformation remained nearly planar allowing for the establishment of both the true stresses and the local deformation strictly from measurements. Results from thirteen radial path experiments show the strain at failure to monotonically increase as the mean stress decreases, a result that is in contrast with previously reported results for Al alloys. In addition, the measured failure strains are significantly larger than previously reported values. Furthermore, statistical grain-level strain estimates revealed a significant variation in strain across the macroscopically observed localization zone, with strains reaching 25–100% higher levels than the macroscopic values. This indicates that localization also occurs at a smaller scale than hitherto understood. Scanning electron microscopic evaluation of the failure zones showed that for this material, void formation and coalescence is delayed until the very end of the life of the deforming material.

Creep Behavior of Geocell-Reinforced Recycled Asphalt Pavement Bases

AbstractThe use of recycled asphalt pavement (RAP) materials for roadway construction is a sustainable approach and has received more attention in the recent years. However, permanent deformation, or rutting, is one of the concerns in the use of RAP as a base course due to traffic loading and creep deformation. To minimize the creep deformation, use of a novel polymeric alloy (NPA) geocell was proposed to confine the RAP. One tensile creep test was conducted at a room temperature of approximately 25°C to investigate the creep behavior of the geocell. Nineteen static plate-loading tests were conducted at a room temperature using a medium-sized loading system to evaluate the applied vertical stress-displacement response, the creep strain—time relationship, and the factors influencing the creep deformations of RAP bases. In order to verify the repeatability of the test method, two repeated creep tests were conducted. Well-graded aggregate cover was used in place of the RAP cover to investigate the cover effe...

3D real-time micromechanical compressive behaviour of bone–cement interface: Experimental and finite element studies

The integrity of bone–cement interface is essential for the long-term stability of cemented total joint arthroplasty. Although several studies have been carried out on bone–cement interface at continuum level, micromechanics of the interface has been studied only recently for tensile and shear loading cases. Fundamental studies of bone–cement interface at microstructural level are critical to the understanding of the failure processes of the interface, where multiple factors may contribute to failure. Here we present a micromechanical study of bone–cement interface under compression, which utilised in situ mechanical testing, time-lapsed microcomputed tomography (CT) and finite element (FE) modelling. Bovine trabecular bone was used to interdigitate with bone cement to obtain bone–cement interface samples, which were tested in step-wise compression using a custom-made loading stage within the μCT chamber. A finite element model was built from the CT images of one of the tested samples and loaded similarly as in the experiment. The simulated stress-displacement response fell within the range of the experimental responses, and the predicted local strain distribution correlated well with the failure pattern in the subject-specific experimental model. Damage evolution with load in the samples was monitored both experimentally and numerically. The results from the FE simulations further revealed the development of damage in the regions of interest during compression, which may be useful towards a micromechanics understanding of the failure processes at bone–cement interface.

Performance of quasilinear elastic constitutive models in simulation of geosynthetic encased columns

Past numerical simulations of geosynthetic encased columns (GECs) using different versions of the quasilinear elastic hyperbolic model for the encased granular material have, in certain cases, yielded unrealistic results. In this paper the cause of such results is investigated by performing three-dimensional finite element analyses of GECs in soft clay, utilizing three common functional forms of the hyperbolic model for the encased granular material. Results indicate that one form of the hyperbolic model can predict an unrealistic lateral response for GECs during application of load to the column. In addition, the inability of hyperbolic models to properly account for soil behavior near failure compromises their ability to realistically capture the behavior of encased granular soil in GECs. Modeling the behavior of soil near failure is essential for properly simulating the behavior of GECs, as soil shear failure is necessary to mobilize the tensile stresses in the encasement and improve the stress–displacement response of the GEC. Although this type of hyperbolic model behavior was demonstrated for the specific case of encased soil in a GEC, the limitations of the hyperbolic model described herein apply equally to other geotechnical problems in which some portion of the soil mass is at or near failure.

Linear Static Response of Suspension Arm Based on Artificial Neural Network Technique

Modeling and simulation are indispensable when dealing with complex engineering systems. This study deals with intelligent techniques modeling for linear response of suspension arm. The finite element analysis and Radial Basis Function Neural Network (RBFNN) technique is used to predict the response of suspension arm. The linear static analysis was performed utilizing the finite element analysis code. The neural network model has 3 inputs representing the load, mesh size and material while 4 output representing the maximum displacement, maximum Principal stress, von Mises and Tresca. Finally, regression analysis between finite element results and values predicted by the neural network model was made. It can be seen that the RBFNN proposed approach was found to be highly effective with least error in identification of stress-displacement of suspension arm. Simulated results show that RBF can be very successively used for reduction of the effort and time required to predict the stress-displacement response of suspension arm as FE methods usually deal with only a single problem for each run.