Three-dimensional Finite Element Approach Research Articles

Additional eddy current losses in transformer working at static VAr compensation station

The article discusses the impact of reactive power load on additional power losses in a high-power transformer operating at a VAr static compensation station. The presence of an almost pure capacitive load on the secondary side together with the thyristor switches is the cause of high-frequency components in the phase currents. The additional effect of electrical resonance between the transformer inductance and the load capacitance is also taken into account. The analysis is achieved using a three-dimensional finite element approach.

Nonlinear Static Stiffness of Rail Pads for Different Materials and Thicknesses using Finite Element Method

The rail pad located in between the steel rail and base plate provides flexibility to the track and cushions from shocks and vibrations resulting from train operation. This polymeric material has nonlinear behaviour. In this study, the behaviour of the rail pad for different polymeric materials and various thicknesses was investigated. The three different types of rail pads were ethylene propylene diene monomer (EPDM), thermoplastic elastomers (TPEs) and ethylene vinyl acetate (EVA). Three-dimensional (3D) finite element (FE) approach was used to model the fastening system under the static load. The Abaqus software was used for the FE analysis. The thicker rail pad deformed more than the thinner for the same load value. Comparing the thickness, EPDM pad had the highest amount of deformation which had an average difference of 40%, followed by TPE and then EVA with 33% and 23% respectively. The static stiffness of EVA material was the highest followed by TPE and EPDM. For all materials, the rail pad became stiffer as thickness decreased. The reaction force decreased as the thickness of the rail pad increased. EVA, TPE and EPDM showed 15%, 9% and 5% reduction of reaction force respectively as the rail pad thickness increased from 5 mm to 10 mm. Despite that, in term of material, EVA had highest capability to reduce reaction force compared to EPDM and TPE.

Modeling impact fracture in a quasi-brittle solids using a 3D nonlocal graph-based finite element analysis: Theory, finite element simulations, and experimental verification

In this work, the authors have developed and implemented a novel nonlocal three-dimensional graph-based finite element approach for simulating fracture in quasi-brittle solids as an extension of their previous work in two dimensions. In order to validate the nonlocal aspects of the model, the authors have also fabricated a gypsum-based particulate composite with silica particles of specific dimensions and mass fractions, thus the length scale of the material is fixed by the particulate media. The GraFEA fracture model is implemented in a graphics processing unit (GPU) parallel computing environment that allows substantial speed-up of the simulations in both cases of impact and quasi-static loading conditions. The improvement in computational performance is especially essential for carrying out the simulation of parametric study. Comparison of the physical response of this specially designed composite with the three-dimensional nonlocal GraFEA shows that the model is capable of simulating fracture in such materials. Finally, the efficacy of simulating impact response of concrete including crack closure behavior is tested by simulating hammer drop test for the concrete beam sample and cyclic shear loading on the circumferentially-notched concrete cylinder sample.

Condensed finite element scheme for symmetric multi-layer structures including dilatational motion

Condensed models act as useful tools to reduce the computational time in the finite element simulations by describing the dynamic behaviour of the multi-layer structure with only one equivalent homogeneous layer. Many condensed plate models in the literature assume constant velocity through the thickness which allows only anti-symmetric motions like bending and shear, but do not include symmetric motions like dilatational or compressional behaviour. The recently developed condensed model by Marchetti et al. (2021) overcomes this setback by including the symmetric motion through three dynamic intrinsic parameters of the multi-layer structure: a dynamic bending stiffness and two dynamic mass densities. This paper presents a finite element scheme to compute vibro-acoustic quantities from the condensed layer with these three dynamic parameters. It is further showed that the proposed scheme has high computational gain over the conventional full three-dimensional finite element approach.

Temperature increase in STT-MRAM at writing: A fully three-dimensional finite element approach

The writing process in spin transfer torque magnetoresistive random access memories (STT-MRAM) is facilitated by elevated temperatures. In this work we investigate the temperature distribution and development in the free layer (FL) of an STT-MRAM during switching. With our fully three-dimensional (3D) finite element method simulation approach, we numerically solve the heat transport equation coupled to the electron, spin, and magnetization dynamics and demonstrate that the FL temperature is highly inhomogeneous due to the non-uniform magnetization of the FL during switching for an STT-MRAM with large magnetic tunnel junction (MTJ) diameter. While the average temperature in the FL can be obtained based on an average current density and an averaged potential drop across the tunnel barrier, a fully 3D model is required to evaluate the large local temperature variations. The temperature variations in the FL further increase, when realistic device structures with an MTJ coated by a passivation layer and contacted to broad electrodes are considered. Lowering the FL thermal conductivity increases the temperature variations even further. However, the variations are strongly reduced with the decrease of the MTJ diameter.

Pile group in clay under cyclic lateral loading with emphasis on bending moment: Numerical modelling

Pile foundations supporting major structures are often founded in soft compressible clays. Apart from usual super-structural loading, these piles are subjected to cyclic lateral loads originating from actions of waves, ship impacts, winds or moving vehicles. Such repetitive loading induces stress reversal in adjacent soft clay initiating progressive degradation in soil strength and stiffness. This not only deteriorates the pile capacity with unacceptable displacements, the bending moments also increase. Although past studies investigated the response of single pile under lateral cyclic loading, a detailed study on pile group in clay under cyclic lateral loading with emphasis on bending moment is of immense practical interest. This paper focuses on detailed study of the response of pile group in clay under cyclic lateral loading, with emphasis on bending moment, through numerical modelling via a three-dimensional dynamic finite element (FE) approach and simplified boundary element modelling (BEM). Comparisons of computed results with available test data imply that the results obtained by 3 D dynamic FE model are better than the BEM. Extensive parametric studies with field data indicate that pile bending moment has been significantly influenced by cyclic loading parameters (number of cycles, frequency and amplitude). Relevant conclusions are drawn from the entire study.

Experimental and Numerical Study of Reinforcement Strains in RC Ties

The importance of interaction between concrete and reinforcement for reinforced concrete (RC) mechanics is a known issue, yet its complexity enforces one to perform in-depth investigations at the microscopic level. In this paper, short RC ties with lengths less than transfer length of bond stresses were chosen to investigate contact zones between concrete and reinforcement. A three-dimensional finite element approach (Model-3D) with simplified geometry of ribs (rib-scale model) is modelled. Its effectiveness is checked against the results yielded by a number of double pull-out tests on RC prisms, where the strain distribution of the bars was measured with strain gauges. As it turns out, the considered model shows a good correlation with experimental data at different loading levels, and the most important factor describing the effectiveness is the geometry of ribs.

Dynamic Behavior in Transition Zones and Long-Term Railway Track Performance

Transition zones between embankments and bridges or tunnels are examples of critical assets of the railway infrastructure. These locations often exhibit higher degradations rates, mostly due to the development of differential settlements, which amplify the dynamic train-track interaction, thus further accelerating the development of settlements and deteriorating track components and vehicles. Despite the technical and scientific interest in predicting the long-term behavior of transition zones, few studies have been able to develop a robust approach that could accurately simulate this complex structural response. To address this topic, this work presents a three-dimensional finite element (3D FEM) approach to simulate the long-term behavior of railway tracks at transition zones. The approach considers both plastic deformation of the ballast layer using a high-cycle strain accumulation model and the non-linearity of the dynamic vehicle-track interaction that results from the evolution of the deformed states of the track itself. The results shed some light into the behavior of transition zones and evidence the complex long-term response of this structures and its interdependency with the transient response of the train-track interaction. Aspects that are critical when assessing the performance of these systems are analyzed in detail, which might be of relevance for researchers and practitioners in the design, construction, and maintenance processes.

Fracture analysis of composite pressure vessel using FEM

A finite element-based numerical method is applied to predict the possibility of propagation of existing delamination/patch in a filament wound composite pressure vessel in this work. Strain energy release rates (SERRs’) in the three principal modes are predicted along the circumference of delamination for two different load cases, i.e., internal pressure load when two diametrically opposite patches are existing in the nozzle end side dome portion, and combined load due to thrust and bending when a patch is located in the nozzle end side skirt region of the composite pressure vessel (CPV). The problem is modeled in ANSYS software and a three-dimensional finite element approach in association with virtual crack closure technique (VCCT) is used to analyze the fracture behavior of the CPV under two load cases as stated. In both the load cases, SERR is found to be maximum at the bottom side of the patch, Mode-I being dominant under pressure load, and Mode-II in axial load. Delamination growth is observed at 26.5% of applied pressure in Case-1 and 57.5% of axial load in Case-2. This fracture analysis approach can be extended to composite structural components in defense and aerospace applications.

Non-conforming interface coupling and symmetric iterative solution in isogeometric FE–BE​ analysis

In this paper, a three-dimensional (3D) isogeometric finite element (FE) and boundary element (BE) coupling approach based on non-uniform rational B-splines (NURBS) is developed to simulate the deformed body. Based on the geometric exactness and higher smoothness of isogeometric analysis (IGA), NURBS discretization is applied to the approximation of geometric representation, displacement field and traction field simultaneously. An improved power series expansion method (M-PSEM) is used to evaluate the singular integral and is suitable for high order (≥3) NURBS basis functions, which is the limitation of the traditional method. Adaptive integration scheme based on quadtree subdivision technique is adopted to enable the non-singular integration computation more flexible and efficient at optimal computational cost. The relationship between the tractions and nodal external forces on the interface is established by using the virtual work principle. On the other hand, displacement coupling constraints among the non-conforming (or incomplete coincident) surface meshes are constructed by a virtual knot insertion technique. The main advantages of this method are simplicity and robustness, as it is problem-independent and only depends on the NURBS meshes on both sides of the interface. Introducing an enhanced symmetric matrix related to the BE subdomain, the symmetric iterative coupling method (SICM) is extended to solve the final system coupling equation. The final coupling matrix obtained by this iterative method is symmetric, and its dimension is the same as that of the stiffness matrix of IGA finite element method (IGAFEM). Numerical examples are investigated to assess the accuracy and efficiency of the presented method.

Effects of Tunneling-Induced Ground Movements on Stability of Piled Raft Foundation: Three-Dimensional Finite-Element Approach

Abstract Across the globe, rapid urbanization demands the construction of tunnels within the vicinity of high-rise buildings supported on piled raft foundations. As a consequence, ground movements ...

Simulating reactive soil and substructure interaction using a simplified hydro-mechanical finite element model dependent on soil saturation, suction and moisture-swelling relationship

Lightweight structures are susceptible to damage caused by cyclic shrink-swell movements of clayey soils amounting to significant aggregated global financial loss for repair costs. Due to this issue, the main aim of this study is to develop an improved method for a deeper understanding of the soil-structure behaviour and for a more practical design process of substructures through a coupled hydro-mechanical finite element model. The developed model uses the relationship between the degree of saturation, soil suction and volumetric strain using a three-dimensional finite element approach. The developed model was validated using three case studies. The first case study verified its ability to simulate movements of a ground and deformation of a substructure due to changes in soil saturation and suction using field data from the literature. The second case study verified the ability of the developed model to simulate cracking of a substructure induced by soil movements using field data from the literature. The third case study compared simulations of hypothetical stiffened and waffle rafts using the developed coupled model and a decoupled traditional method being used in practice. The developed model was observed to have a satisfactory performance, which overcome most limitations of traditional substructure design methods.

Influence of vertical misfit in screw fatigue behavior in dental implants: A three-dimensional finite element approach.

Misfit is unavoidable in dental implant-supported prostheses due to machining process or inappropriate assembling, and the definition of an admissible misfit is still a controversial issue. This work aims to understand the behavior of the screws in dental implant-supported prostheses to estimate an admissible vertical misfit value in terms of screw fatigue failure. For that purpose, a finite element model of a dental implant-supported prosthesis was created and analyzed. Vertical misfits were introduced in different positions, the lower and upper screws were tightened to the bolting force values recommended by the manufacturer, and two different occlusal loads were analyzed. In addition, two different prosthesis materials were studied. Screw load variations were reported and a fatigue analysis was performed. As a result, it was observed that the screw tightening sequence closed small vertical misfits (equal to or less than 40 µm), whereas larger misfits (more than 40 µm) remained open. If the vertical misfit is closed by the end of the tightening sequence, it may be considered equivalent to the ideal fit situation in regard to screw fatigue failure. The prosthesis material had no significant influence on the fatigue behavior.

Assessing delamination initiation of angle-ply laminates from fiber and matrix properties

As a laminate is essentially bonded together from laminae by the matrix, any delamination of it must be initiated from a matrix failure. This article predicts the initiation of a laminate delamination through analyzing the matrix failure. A three-dimensional finite element approach is used to calculate the stress field of an angle-plied laminate under a uniaxial tension. The stresses in each lamina of the laminate are averaged with respect to the lamina thickness to eliminate a weak singularity near a free edge. Homogenized stresses in the matrix are then obtained through bridging model, which are further converted into true values by virtue of stress concentration factors of the matrix in the composite. Influence of any interface crack between the fiber and the matrix on the true stresses has been taken into account. A criterion for detecting the delamination initiation is proposed, which only involves the true stresses and the original strength data of the matrix. To assess the efficiency, comparisons between the predictions and available experiment data are made for a number of angle-plied laminates. Reasonable correlation has been found.

Shrink-fitted joint behavior using three-dimensional solid finite elements in rotor dynamics with inclusion of stress-stiffening effect

In this article, the contact behavior of a shrink-fitted joint in a rotor assembly at different spin speeds is studied, using the three-dimensional solid finite element approach. A custom frictionless contact model is proposed and extensively tested by means of simulation. The theory for solving pre-stressed damped eigenvalue problem is presented and implemented, and the results are presented in Campbell diagram form. Two sample rotor assemblies are studied. In addition, experimental modal analysis results of a shrink-fitted joint with various interferences are presented and numerically studied. The reduction of the interference of the shrink-fitted joint due to centrifugal forces at high-speed operation as well as the contact status are updated at each rotational speed step. The inclusion of stress-stiffening effect is studied in detail. The comparison of the Campbell diagram results with the results obtained using a commercial finite element software (Ansys) shows a good agreement.

Effects of asphalt overlay on XCC pile-supported embankment vibration from a moving vehicle

Vibration behaviors of the asphalt pavement-embankment-X-shaped cast-in-situ concrete (XCC) piles reinforced ground system under a moving vehicle are simulated through three-dimensional finite element approach. The influences of asphalt overlays on displacement pattern on the top of pavement, the asymmetric movement of surrounding soil around XCC piles, and the concentration coefficient after laying the overlay are investigated. The obtained results show that the dynamic displacement decreases as a consequence of the increasing stiffness of this system after laying asphalt overlays. It is also shown that the vertical displacement of soil around piles decreases as the thickness of overlays increases. Meanwhile, the asymmetric extent, in which there is an obvious differential displacement between the flat and concave area, is presented. In addition, the concentration coefficient for the embankment's top flat is significantly larger than that for the slope surface, and its increasing is caused by asphalt overlays.

Behavior and Three-Dimensional Finite Element Modeling of Circular Concrete Columns Partially Wrapped with FRP Strips.

Fiber-reinforced polymer (FRP) jacketing/wrapping has become an attractive strengthening technique for concrete columns. Wrapping an existing concrete column with continuous FRP jackets with the fiber in the jacket being oriented in the hoop direction is referred to as FRP full wrapping strengthening technique. In practice, however, strengthening concrete columns with vertically discontinuous FRP strips is also favored and this technique is referred to as FRP partial wrapping strengthening technique. Existing research has demonstrated that FRP partial wrapping strengthening technique is a promising and economical alternative to the FRP full wrapping strengthening technique. Although extensive experimental investigations have hitherto been conducted on partially FRP-confined concrete columns, the confinement mechanics of confined concrete in partially FRP-confined circular columns remains unclear. In this paper, an experimental program consisting of fifteen column specimens was conducted and the test results were presented. A reliable three-dimensional (3D) finite element (FE) approach for modeling of partially FRP-confined circular columns was established. In the proposed FE approach, an accurate plastic-damage model for concrete under multiaxial compression is employed. The accuracy of the proposed FE approach was verified by comparisons between the numerical results and the test results. Numerical results from the verified FE approach were then presented to gain an improved understanding of the behavior of confined concrete in partially FRP-confined concrete columns.

Modeling the lateral response of pile groups in cohesionless and cohesive soils

A three-dimensional finite element approach was used to assess the lateral pile and pile group response subjected to pure lateral load. The study evaluated three pile group configurations (i.e. 2 × 1, 2 × 2 and 3 × 2 pile groups) with four values of pile spacing (i.e. 2D, 4D, 6D and 8D, where D is the pile diameter). The results of the influence of load intensities, group configuration, pile spacing are discussed in terms of response of load vs. lateral displacement, load vs. soil resistance and corresponding p–y curves. The improved plots can be used for laterally loaded pile design and also to produce the group action design p-multiplier curves and equations. As a result, design curves were developed and applied in the actual case studies and similar expected cases for assessment of pile group behavior using improved p-multiplier. A design equation was derived from predicted design curves to be used in the evaluation of the lateral pile group action. The equation was used with the previous results to predict the expected design curve take in the account different source of p-multiplier. It was found that the group interaction effect led to reduced lateral resistance for the pile in the group relative to that for the single pile. In addition, the present study was compatible with the results of previous results for the first and second trailing row and was less compatible with the result from the previous works in the case of piles in leading row.

Three-dimensional finite element modeling of inorganic-matrix composite materials using a mesoscale approach

Fiber reinforced cementitious matrix (FRCM) composites represent a promising alternative to the use of fiber reinforced polymer (FRP) composites for strengthening existing structures. FRCM composites are comprised of a high-strength fiber net (textile) embedded within an inorganic matrix, which is responsible of the stress-transfer between the composite and the substrate. FRCM composites comprising one layer of fiber are generally reported to fail due to debonding of the fibers from the embedding matrix. However, depending on the FRCM material employed, failure at the matrix-substrate interface, interlaminar (delamination) of the matrix, and fiber rupture may also occur. When stitch-bonded textiles are employed, i.e. textiles where longitudinal and transversal bundles are firmly connected, the interlocking between fibers and matrix plays a key role in the matrix-fiber stress-transfer mechanism and failure generally occurs due to cracking of the matrix. In this paper, the bond behavior of FRCM-masonry joints comprising one layer of a carbon stitch-bonded textile embedded within a lime-based matrix is studied by means of single-lap direct-shear tests of FRCM-masonry joints. A mesoscale three-dimensional finite element approach, which accounts for the matrix-fiber bond behavior and matrix-textile interlocking, is adopted to study the formation and propagation of the matrix cracks in the FRCM strip that eventually led to failure of the FRCM-masonry joints. The finite element analysis was carried out by using a dynamic explicit approach, which allowed for overcoming convergence difficulties associated with severe nonlinearities of the model.

A fully coupled three-dimensional hydro-mechanical finite discrete element approach with real porous seepage for simulating 3D hydraulic fracturing

A fully coupled 3D hydro-mechanical model with real porous seepage is presented for simulating hydraulic fracturing. In this model, fluid flow in a fracture is expressed by 2D fracture seepage in the broken joint elements based on the Cubic law, while fluid flow in the rock matrix is represented by 3D porous seepage in the tetrahedral elements based on Darcy's law. Several problems that have closed-form solutions and a 3D fracturing problem are given to verify the model. The simulation results show that the model can capture crack initiation and propagation, and the fluid pressure evolution during hydraulic fracturing.