Simulations Of Compression Tests Research Articles

Specimen-specific beam models for fast and accurate prediction of human trabecular bone mechanical properties

Direct assessment of bone competence in vivo is not possible, hence, it is inevitable to predict it using appropriate simulation techniques. Although accurate estimates of bone competence can be obtained from micro-finite element models (μFE), it is at the expense of large computer efforts. In this study, we investigated the application of structural idealizations to represent individual trabeculae by single elements. The objective was to implement and validate this technique.We scanned 42 human vertebral bone samples (10 mm height, 8 mm diameter) with micro-computed tomography using a 20 μm resolution. After scanning, direct mechanical testing was performed. Topological classification and dilation-based algorithms were used to identify individual rods and plates. Two FE models were created for each specimen. In the first one, each rod-like trabecula was modeled with one thickness-matched beam; each plate-like trabecula was modeled with several beams. From a simulated compression test, assuming one isotropic tissue modulus for all elements, the apparent stiffness was calculated. After reducing the voxel size to 40 μm, a second FE model was created using a standard voxel conversion technique. Again, one tissue modulus was assumed for all elements in all models, and a compression test was simulated.Bone volume fraction ranged from 3.7% to 19.5%; Young's moduli from 43 MPa to 649 MPa. Both models predicted measured apparent moduli equally well (R2=0.85), and were in excellent agreement with each other (R2=0.97). Tissue modulus was estimated at 9.0 GPa and 10.7 GPa for the beam FE and voxel FE models, respectively. On average, the beam models were solved in 219 s, reducing CPU usage up to 1150-fold as compared to 40 μm voxel FE models. Relative to 20 μm voxel models 10,000-fold reductions can be expected. The presented beam FE model is an abstraction of the intricate real trabecular structure using simple cylindrical beam elements. Nevertheless, it enabled an accurate prediction of global mechanical properties of microstructural bone. The strong reduction in CPU time provides the means to increase throughput, to analyze multiple loading configuration and to increase sample size, without increasing computational costs. With upcoming in vivo high-resolution imaging systems, this model has the potential to become a standard for mechanical characterization of bone.

Multiaxial failure of low-cohesive frictional materials: fracture behaviour and size dependency

An experimental investigation concerning the size effect in quasi-brittle materials under multiaxial compressive loading was carried out. Laboratory results are presented concerning the size effect and fracture mechanisms in hollow cylinders using two cement-based model materials. Hollow cylinders in a size range 1 : 4 were tested under external hydrostatic pressures while monitoring their inner-hole deformations. A mild but notable size effect was observed during the tests with strength decrease as specimen size increases. Fracture mechanisms for hollow cylinders during failure were investigated using impregnation techniques and their results indicated the occurrence of splitting type, which encircled the inner holes in a rather uniform manner. Numerical simulations using the distinct element code PFC2D were performed in order to provide insight into the experimental findings. The model was capable of replicating the failure mechanisms and damage pattern in the hollow cylinder tests. In terms of size effect, the model showed similar trends as those observed during Brazilian splitting tests with a decreasing nominal strength as size increases. The simulation results suggested a statistical effect to be noteworthy for the observed size effects in tension, which was further demonstrated from a good agreement between these results and the predicted size effect from the Weibull theory. In compressive test simulations, a slight increase in strength with size was observed for confined and unconfined compression tests simulations on rectangular assemblies.

Superimposed ultrasonic oscillations in compression tests of aluminium

The application of ultrasonics in metal forming applications has been shown to reduce the forming load significantly in many research studies. The load reduction has been related to the stress superposition effect, rise in temperature and change in the friction condition between the specimen and die interfaces. This paper reports an investigation into the effects of superimposed ultrasonic oscillation of the die in compression tests on aluminium specimens. In particular, a finite element model is developed to simulate uniaxial compression and to model the effects of a change in the friction boundary condition when ultrasonic excitation is applied to the lower platen. The model predictions of the stress–strain relationship can be compared with test data to provide some insights into the effects of the interfacial condition. The paper shows how the analysis of the test data, combined with finite element models of ultrasonic compression test simulations, allows some initial conclusions to be drawn regarding the influence of the interfacial friction during ultrasonic compression.

個別要素法による凍結融解作用を受ける粒状体の力学挙動解析

This paper presentsa numerical experiment to reveal the effect of freeze-thaw action on the mechanical behavior of granular materials in tams of micmmechanics. The odjective is to propose a new analytical procedure to simulate the mechanical behavior of granular materials subjected to freeze-thaw sequence. Numerical simulations of freezing-thawing triaxial compression tests were performed with 3D DEM coupled with a thermal analysis, and the applicability of the DEM to the freezing-thawing behavior of granular materials was discussed As the results, it was showed that the proposed DEM model that puts many small elements which expand with dropping the temperature between soil particles can simulate the freezing-thawing behavior of granular materials to some extent though there is room for further investigation as to numerical modeling and setting analytical parameters.

Molecular Dynamics Simulations of the Mechanical Response of Metallic glass/BCC Nanocomposite Materials

AbstractBulk metallic glass (BMG) has been of intense interest recently because of its unique combination of excellent mechanical properties including high strength. One of the major drawbacks of monolithic BMG materials is the limited ductility due to strain localization. Except a few recent reports on monolithic ductile BMG, the most popular method to enhance the ductility of the BMG samples is to introduce a crystalline or quasicrystalline phase by precipitation or mixing. The composite material usually exhibits some degree of strain hardening along with significantly higher impact resistance and fracture toughness. However, the mechanism of this strain hardening is not well understood. We present simulated uniaxial compression tests on a monatomic model amorphous system embedded with body-center cubic (BCC) nanocrystals. The advantage of this model system is that intimate amorphous-crystal interfaces can be obtained. We observe that when comparing to monolithic glassy samples where a single shear band normally dominates, multiple shear bands appear in the BCC-amorphous composite samples. The plastic deformation initiates at the interfaces between nanocrystals and the glassy phase due to stress concentration. Furthermore, we demonstrate that shear along the bands results in growth of the nanocrystals.

Modelling a geo-composite cell using discrete analysis

Reinforced soil structures can be used to protect infrastructures against rockfalls. An innovative way to implement such protection is to construct the front face using a cellular assembly. This paper focuses on the cellular scale, with the cell a complex heterogeneous material, composed of a wire netting box filled with rocky particles. The behaviour of a single cell was modelled using the discrete element method, thus accounting for the interaction between the rocky particles and the interaction between the box and the rocky particles. A constitutive model was developed and calibrated along confined compression loading paths, then preliminary elements of validation were obtained from the simulation of unconfined compression tests.

Modelling approaches of the in-plane shear behaviour of unreinforced and FRP strengthened masonry panels

The paper presents different finite element modelling approaches, developed with a commercial software, for the analysis of the behaviour of unreinforced and FRP strengthened masonry walls when their are subjected to a predominant shear load. Three models are analyzed, having different complexity levels. These models are used for the simulation of diagonal compression tests on masonry panels. The numerical simulations are compared with experimental results and the reliability of the different finite element models is discussed.

Strain localization in soft rock—a typical rate‐dependent solid: experimental and numerical studies

AbstractStrain localization developing inside soft rock specimens is examined through experimental observation and numerical simulation. In the experimental study, soft rock specimens are sheared at different strain rates under plane strain conditions and deformation and strain localization characteristics are analysed. Transition of localization mode from highly localized mode for higher strain rate to distributed and diffused mode of strain localization for lower strain rates was observed. In the numerical study, simulations of plane strain compression tests are carried out at different strain rates by using an overstressed‐type elasto‐viscoplastic model in finite element computations. The role of strain rates on setting gradients of strain fields across shear band is clarified. The probable mechanism for transition of localization mode is discussed. Copyright © 2005 John Wiley & Sons, Ltd.

DEM simulation of diametrical compression test on particle compounds

A 2 Dimensional discrete element analysis is carried out with diametrical stressing condition to understand the fracture behaviour of particle compounds. The new surface generation and particle size distributions are also analysed to study an efficiency of the crushing system. Concrete spheres of 150 mm diameter with properties of B35 (35 N/mm2 compressive strength) are chosen to represent particle compounds. The paper discusses the discrete element approach for crack propagation analysis and their correlations in particle compounds.

Poroelastic damage rheology: Dilation, compaction, and failure of rocks

We present three‐dimensional numerical simulations for the behavior of porous rocks under loading based on the theoretical formulation recently developed by Y. Hamiel et al. This theoretical formulation combines the classic Biot poroelastic theory with a damage rheology model. The numerical simulations of triaxial compression tests reproduce the gradual transition from localized brittle failure to distributed cataclastic flow with increasing pressure at high‐porosity rocks. Under relatively low confining pressures (approximately lower than 100 MPa for Berea sandstone samples), porous rocks fail in a brittle mode with sharp localization of damage in a narrow deformation zone and dilatancy preceding the total failure. At these pressures the yield stress increases with confining pressure (positive slope for yield curve). In simulations with higher confining pressures, instead of dilatancy, the deformation zone has a reduced porosity due to compaction. The porosity reduction plays an essential role in strengthening the deformation zone, and therefore continuous loading of the sample leads to the progressive development of a wide deformation zone. Under relatively high confining pressures (approximately higher than 300 MPa for Berea sandstone samples), damage is nonlocalized, and the macroscopic deformation of the model corresponds to experimentally observed cataclastic flow. At these pressures the yield stress decreases with confining pressure (negative slope for yield curve). We found good agreement between the measured and calculated yield curve for Berea sandstone.

Modelling the Kaiser effect and deformation rate analysis in sandstone using the discrete element method

Non-destructive methods for rock in-situ stress estimation have been extensively investigated in recent years. Such techniques as deformation rate analysis and the acoustic emission method, which are based on ‘rock memory’ recovery utilising the Kaiser effect, have become popular among many practitioners. The physical nature of this phenomenon has not however been universally established. In this study the primary cause of the Kaiser effect, the interaction of microcracks, was investigated. The numerical modelling package PFC 2D was employed for creation of a synthetic cored specimen and simulation of uniaxial compression tests. The ability of the numerical model to reproduce the Kaiser effect and the deformation rate analysis phenomenon was confirmed, and a direct comparison was made between laboratory and numerical observations. The link between the Kaiser effect/deformation rate analysis and development of microcracks was established. The realistic behaviour of the numerical model can be used for exploration of other mechanical properties of rocks and also to further study the characteristics of the deformation rate analysis curve. The results of the modelling work may provide assistance for a more accurate interpretation of deformation rate analysis curves.

Simulation of Mg Sheet Press Cell Materials

Major characteristics of Magnesium alloy are low density (= 1.8 g/cm3) and excellent recyclability; therefore Mg is considered as one of low environmental loading materials. The low-density materials, such as Mg, are suitable for the transportation system to reduce the emission of Co2, save energy resource and increase the safety for accidents like corrosions. In the place, cellular materials like aluminum foams are also low-density materials and can be controlled the stress-strain relation. Combination of Mg alloy and cellular materials, that is Mg cell, is one of the most excellent materials for transportation system, because of its ultra low density. safeties, and recycle ability. To make the Mg cellular materials. there are some problems to solve. One is how to make them uniformly to supply them as same perfonnance materials. One is how to make them inexpensively. Most of cellular materials are very expensive because of there are many processes or special fabrication system to make them. To solve these problems. we investigated the press cell materials. The press cell material consists sheets pressed as cell shape and wall. Therefore, it is very easy to make cellular materials and control validation of the performance of each foam materials. In this paper, we simulated compression tests of this new type of Mg alloy cellular materials under dynamic loading to investigate the relation between the compression speed and the compression behavior to show the shock absorbing capability of this new foam material. It is very important to understand their mechanical properties relatcd with cell shapes and wall to be applied widely.

Microscale and mesoscale discrete models for dynamic fracture of structures built of brittle material

In this work we present the discrete models for dynamic fracture of structures built of brittle materials. The models construction is based on Voronoi cell representation of the heterogeneous structure, with the beam lattice network used to model the cohesive and compressive forces between the neighboring cells. Each lattice component is a geometrically exact shear deformable beam which can describe large rigid body motion and the most salient fracture mechanisms. The latter can be represented through the corresponding form of the beam constitutive equations, which are derived either at microscale with random distribution of material properties or at a mesoscale with average deterministic values. The proposed models are also placed within the framework of dynamics, where special attention is paid to constructing the lattice network mass matrix as well as the corresponding time-stepping schemes. Numerical simulations of compression and bending tests is given to illustrate the models performance.

５． 離散体の力学 マルチメカニズムを考慮した粒状材料の構成則に関する研究

This paper presents a constitutive model based on the multiple plastic mechanisms which result in the incrementally nonlinear behavior and it also discusses the consequent instability. The constitutive equation is derived through 3D-GEM simulations of tri-axial compression and extension tests as well as a series of stress probe tests. The test results show that two types of plastic mechanism govern the plastic behavior of granular materials. The first one is called the basic mechanism, which obeys the non-associative flow rule. The other one is called the additive mechanism, which is generated in each stress probe. The last part is dedicated to a bifurcation analysis performed in terms of the presented constitutive model.

Simulation of sedimentary rock deformation: Lab‐scale model calibration and parameterization

Understanding the mechanical behavior of rock is critical for researchers and decision‐makers in fields from petroleum recovery to hazardous waste disposal. Traditional continuum‐based numerical models are hampered by inadequate constitutive relationships governing fracture initiation and growth. To overcome limits associated with continuum models we employed a discrete model based on the fundamental laws of contact physics to calibrate triaxial tests. Results from simulations of triaxial compression tests on a suite of sedimentary rocks indicate that the basic physics of rock behavior are clearly captured. Evidence for this conclusion lie in the fact that one set of model parameters describes rock behavior at many confining pressures. The use of both inelastic and elastic parameters for comparison yields insight concerning the uniqueness of these models. These tests will facilitate development and calibration of larger scale discrete element models, which may be applied to a wide range of geological problems.

Brittle destruction of carbon based materials under off-normal ITER-FEAT conditions

A numerical simulation model for brittle destruction of carbon based materials (CBMs) based on crack generation by thermal shocks was developed. Crack generation in heated graphite samples depends on the grain anisotropy, the grain size, the failure stress distribution of the bonds connecting adjacent grains and on the temperature and the temperature gradient in the sample. A realistic value for the mean failure stress was obtained from a numerical simulation of destructive compression tests and from a comparison of calculated and measured onset of brittle destruction for volumetric heating. Graphite predamaging due to crack propagation into the depth of the sample or due to volumetric heating might occur, resulting in an enhancement of brittle destruction under cyclic heat loads for typical ITER-FEAT off-normal events. Results of a consistent analysis of brittle destruction and of target screening by dust particles and of enhancement of brittle destruction under cyclic heat loads are presented.

363 EFGMによる三軸圧縮試験シミュレーション(OS24-3 EFGM)(OS24 メッシュレス/粒子法)

363 EFGMによる三軸圧縮試験シミュレーション(OS24-3 EFGM)(OS24 メッシュレス/粒子法)

Compression Test Simulation of Controlled Cell Shape Open Cellular Magnesium Alloy under Dynamic Loading

This paper deals with compression test simulations of open cellular magnesium alloy under dynamic loading to analyze the characteristics of energy absorption, Metallic foams are new lightweight materials that have some excellent mechanical and chemical properties. The major characteristics of open cellular Mg alloy are ultra low density (about 0.05 kg/m 3 ) and energy absorption ability. Therefore this material can he used as suitable material for transportation systems. Most studies of metallic foams have been done using the Al close-cell foams so far and there are few studies for Mg open-cell foams. To make the open-cell metallic foams, the polyurethane form is used as the base shapes of metallic foams. As a result, it is very difficult to manufacture foams into the required shapes. If shapes of each cell can be controlled. the mechanical properties of the foams can be designed as expected. In this paper, we simulated the compression test of the shape controlled open-cell materials under dynamic loading made of Mg alloy by Finite Element Method (LS-DYNA). The analyzed sample is 14 mm cubic which includes 27 cell units. The strain rates of simulations are 10 2 . 10 3 and 10 4 s -1 . The sample is compressed between static and moving rigid walls. Simulation results show that the difference of strain rate affects the compression behavior and the total amount of absorbed energy.

Numerical simulation of plastic deformation and fracture in polysynthetically twinned (PST) crystals of TiAl

Plastic deformation and fracture in polysynthetically twinned (PST) crystals of TiAl have been simulated by using periodic unit cells representing the relaxed-constraint model recently proposed by Lebensohn et al. [Acta Mater. 46 (1998) 4701–4709] for the co-deformation of the lamellar compound of PST-TiAl. The unit cells contain both intermetallic phases, α 2-(Ti 3Al) and γ-(TiAl). Furthermore, the six orientation variants of the γ-phase are also considered. The constitutive behaviour of both phases is described by crystal plasticity, and the damage behaviour has been implemented by means of cohesive elements. The unit cells have been used as submodels for multi-scale finite element simulations of compression tests and fracture mechanics tests of notched micro-bend specimens. It is shown that the anisotropy of plastic deformation and damage in PST-TiAl can be well represented.

Numerical simulations of deviatoric shear deformation of granular media

Quasi-static shear deformation of granular media is examined by performing numerical simulations on polydisperse systems of elastic spheres in a periodic cell. Results of axisymmetric compression test simulations are reported for both a dense and a loose system. At the macroscopic scale, the simulated stress–strain–dilation responses are in excellent qualitative agreement with the mechanical behaviour of sand observed in real experiments. Numerical simulation permits a detailed examination of the evolution of internal variables associated with the micromechanical processes occurring at the particle scale. In this context, the evolution of the induced structural anisotropy, the percentage of sliding contacts, the average coordination number and the normal and tangential contact force contributions to the stress tensor are presented and discussed. Further simulations are reported which illustrate the effect of interparticle friction on both the macrocopic and microscopic mechanical behaviour. Finally, the versatility of numerical simulation is demonstrated by presenting results obtained from simulations of general radial deviatoric loading, a constant deviatoric strain test and a multi-axial plane strain test.