Finite Element Computer Code Research Articles

Seismic behavior of pile foundations in unsaturated soils

Earthquakes have caused significant damage to civil engineering structures worldwide due to inadequate lateral load capacity and excessive deformation of pile foundations supporting these structures. The seismic performance of pile foundations interacting with unsaturated soils could be affected by changes in matric suction due to the moisture content variation induced by seasonal weather changes or water table fluctuations. Hence, the main objective of this study is to investigate the effects of unsaturated soil conditions on the seismic response of a pile-soil system in silty clay soils. This study utilized a stand-alone finite element computer code called DYPAC (Dynamic Piles Analysis Code) developed using the Beams on Nonlinear Winkler Foundation (BNWF) approach. Free field soil displacements and p-y curve parameters, inputs needed for DYPAC analyses, were updated based on the soil suction variations. This study found that soil suction can significantly influence the seismic performance of piles interacting with unsaturated silty clay soils, especially as the soil becomes drier in the transition zone. The best seismic performance of the pile, which is the minimum lateral pile displacement, happened in the transition zone between fully saturated and nearly dry conditions.

Ultra high performance concrete and C-FRP tension Re-bars: A unique combinations of materials for slabs subjected to low-velocity drop impact loading

In this research work, different combinations of normal strength concrete (NSC), ultra-high-performance concrete (UHPC), and steel fiber-reinforced UHPC (SFR-UHPC) concrete with re-bars of conventional steel and of carbon fiber-reinforced polymer (C-FRP) are used in a two-way square slab of size 1000mm x 1000mm x 75mm subjected to 2500 mm free-fall impact loading. Experimental arrangement consisting of 105 kg dropping weight with the circular flat impacting face of 40 mm diameter used for carrying out impact test is modeled using a high-fidelity physics-based finite element computer code, ABAQUS/Explicit-v.6.15. After validating the experimental results of the NSC slab with steel bars, analyses are extended by replacing NSC and steel bars with UHPC/SFR-UHPC and C-FRP bars, respectively, under the same dropping weight. Only the remote face (tension face) of the slabs is provided with the re-bars. Widely employed and available with the ABAQUS, the Concrete Damage Plasticity model with strain-rate effects has been entrusted for simulating the concrete plastic response. Re-bars of steel are idealized with the Johnson-Cook plasticity damage model. C-FRP re-bars are defined with the classical plasticity model following the elastic-plastic constitutive laws. The impact responses of the slabs consisting of NSC/UHPC/SFR-UHPC concrete with re-bars of steel, and C-FRP combinations considered are discussed and compared. Slabs made of UHPC/SFR-UHPC concrete with the C-FRP re-bars are found to offer a promising combination of materials to withstand low-velocity impact load with little damage and extraordinary impact performance.

Analysis of shear mock-up tests in a canister-clay system for spent nuclear fuel isolation

The aim of this paper is to simulate a canister-buffer mock-up test using the finite element computer code CODE_BRIGHT. The response of a copper canister put inside clay barriers that undergo hydration and swelling till complete saturation was investigated. Although the copper is very stiff and resistant and will be emplaced in a clay volume, shear deformations occurring in the clay barrier may have an impact on the canister. A Mock-up test was carried out in a steel cell at a scale of about 1:10 for assessing the consequences of a shear zone for a copper rod that was surrounded by compacted bentonite as a clay barrier. The challenge problem is to perform the simulation of the mock-up tests under the different shear rates that was carried out by Borgesson (1968). The results might contribute to a better understanding of the real shearing response of the system and validate the constitutive models used for the simulations.For this purpose, the mock-up test was simulated with CODE_BRIGHT in hydro-mechanical couplings in three dimensions. For the model validation, the results of the simulations were compared with the test measurements. The constitutive model used for the copper simulation was a viscoplastic model developed by Bodner and Partom. For the bentonite simulation, a viscoplastic model for unsaturated soils based on the Desai and Perzyna theory, using an elliptical yield surface corresponding to the modified Cam-Clay model, was used.

A parametric numerical study of ASB-assisted chip formation in high-speed machining of Ti-6Al-4V

ABSTRACT The present study aims at studying the influence of some parameters on the numerical results obtained from the simulation of the high-speed machining process using a commercial finite element computation code, namely Abaqus-Exp. Starting from models available by default in the computation code, comparisons are made between full and weak thermo-mechanical coupling, and the influence of the fracture strain and inelastic heat fraction values is studied. The high-strength Ti-6Al-4 V titanium alloy is taken as an example. Continuous, serrated and segmented chips are reproduced at low, moderate and high cutting speeds and the differences observed between experimental and numerical results are discussed.

Soil-Structure Interaction of Flexible Temporary Trench Box: Parametric Studies Using 3D FE Modelling

This paper presents the results of two parametric finite-element studies that were carried out using the PLAXIS-3D finite element (FE) computer code. The following objectives and corresponding parameters were considered: (i) to evaluate the soil pressure on the steel trench box shield; the parameters studied were related to soil type and material, and the study considered till, dry sand, wet sand, and sensitive clay soil; (ii) to assess the effect of trench box material and geometry on earth pressure; the parameters studied were related to trench box material (steel versus aluminum) as well as geometry (plate thickness and strut diameter). These studies included simulation of two steel (or aluminum) trench box shields stacked upon each other to cover the total 6 m (20 ft) deep trench. A Mohr-Coulomb (MC) constitutive material model was chosen for FE analysis (FEA). The FEA results were compared to empirical apparent earth pressure diagrams for a sensitive clay. Comparisons showed that the parameters related to the soil and the trench box have a significant influence on earth pressures.

Dynamic failure of the aluminium plate under air-blast loading in the framework of the fractional viscoplasticity model - theory and validation

This paper deals with the numerical simulation of the dynamic failure of an aluminium plate under air-blast loading. Constitutive modelling based on the fractional viscoplasticity is used. The material model is non-local due to the properties of the applied fractional differential operator and is implemented as user material in the engineering finite element computation code ABAQUS. It is important that the numerical simulations are contrasted with experiments. Numerical outcomes clearly show the applicability of the adopted modelling for the description of salient stages of dynamic structural failure.

Active crustal deformation in the Tian Shan region, central Asia

The Tian Shan mountain ranges in central Asia are one of the largest and most active intracontinental orogenic belts in the world. Its Cenozoic reactivation and deformation manifest the far-field impact of the Indian-Eurasian continental collision. How the collision-induced crustal shortening is accomondated in the Tian Shan region, however, remains debated. Some of the shortening is clearly accommodated by the piedmont fold-and-thrust belts; but the role of deformation in the interior of the orogen, and the overall pattern of crustal deformation in the Tian Shan region, differs in previous studies. We have used NeoKinema, a kinematic finite-element computer code, to analyze the long-term average rates of strain and its partitioning in the Tian Shan region. The model is constrained by updated kinematic data sets, including fault traces, geological fault slip rates, GPS site velocities, principal stress directions, and kinematic boundary conditions. Our results indicate that, in addition to shortening in the piedmont fold-and-thrust belts, significant shortening and strike-slip faulting have occurred in the interior of the Tian Shan orogen. The intra-orogen strain is concentrated north of the Nalati Fault, around the intramontane basins. The overall pattern of crustal deformation in the Tian Shan region is pure shear shortening, facilitated by NEE-trending sinistral and NW-NWW trending dextral strike-slip faults that cut across the mountain ranges. We also calculated the long-term potential of seismicity in the region and compared it with earthquake records.

Progressive failure of ductile metals: Description via a three-dimensional coupled CZM–XFEM based approach

The present work pertains to the numerical prediction of the current residual strength of large metallic engineering structures when submitted to accidental overloads. In this context, is developed a unified 3D numerical methodology reproducing the successive stages of the progressive failure of structures made of ductile metals, viz. (i) more or less diffuse micro-voiding induced damage, (ii) strain/damage localization in a narrow band, and (iii) macro-crack formation and propagation. This is notably realized via a combination of the GTN model and an XFEM/CZM coupling. Localization is addressed here as a phenomenon driven either by plastic instability or void coalescence. In the latter case an original transition criterion is proposed, accounting for the competition between Mode I/II type localization, utilizing the local triaxiality as a mode indicator. The methodology is implemented as a user element subroutine (UEL) within the commercial finite element computation code ABAQUS and its performance is assessed considering 3D numerical simulations of various loading cases. The proposed methodology is shown to be mesh objective and able to fairly reproduce ductile crack patterns, while it gives promising results regarding global specimen responses.

A constitutive model for a rate and temperature-dependent, plastically anisotropic titanium alloy

Aircraft engine fan blades are notably designed to withstand impact loading involving large deformation, high strain rate, non-proportional loading paths and self-heating. Due to their high strength-to-weight ratio and good toughness, Ti–6Al–4V titanium alloys are promising candidates for the blades leading edge. An extensive experimental campaign on a Ti–6Al–4V titanium alloy provided in the form of cold rolled plates has been carried out. The thermo-mechanical characterization consisted in tension, compression and shear tests performed at various strain rates and temperatures, and under monotonic as well as alternate loading paths. A constitutive model has been accordingly developed accounting for the combined effect of plastic orthotropy and tension/compression asymmetry, nonlinear isotropic and kinematic strain hardening, strain rate hardening, and thermal softening. The constitutive model has been implemented as a user material subroutine into the commercial finite element computation code LS-DYNA. The performances of the model have been estimated by conducting numerical simulations considering a volume element under various loading paths as well as the specimens used for the experimental campaign.

Numerical investigation and experimental verification of thermal frequency of carbon nanotube-reinforced sandwich structure

The modal responses of multi-walled carbon nanotube-reinforced composite sandwich structural plate are computed under the elevated temperature environment using a higher-order polynomial kinematic model and the isoparametric finite element steps. The proposed model accuracy has been verified with experimental modal values under the influence of elevated temperature and ambient conditions. To perform the modal experiment the nanotube-reinforced composite sandwich panel filled with the epoxy core is fabricated. Further, the experimental elastic properties of epoxy, nanotube/epoxy composite and the sandwich are obtained individually for the current computational purpose. A tailor-made finite element computer code (MATLAB environment) is prepared using the multiscale mathematical formulation for the evaluation of thermal frequencies of the nanotube sandwich panel. The impact type vibration analyser has been utilized for the current testing purpose with the help of three components i.e. hardware (compact data acquisition system, cDAQ-9178), software (LAB-VIEW) and a microcontroller controlled thermal chamber (to maintain temperature profile). Finally, wide varieties of numerical examples are solved using the proposed computational model for the different design-related parameters. The intrinsic behaviour of each parameter on the epoxy-filled nanotube sandwich construction including the elevated temperature loading is discussed in details.

ЧИСЛЕННОЕ МОДЕЛИРОВАНИЕ ИСПЫТАНИЯ ПАДАЮЩИМ ГРУЗОМ ВЯЗКОЙ ТРУБНОЙ СТАЛИ ПРИ ТРЕХТОЧЕЧНОМ ИЗГИБЕ

The processes in the metal sample of a supply pipeline realized under drop-weight tests (DWT, or DWTT according to ASTM) are studied. DWT is a proof test of the pipeline metal that should ensure high resistance of the pipeline against extensive destruction. Numerical simulation of DWT with the steel sample of full thickness was performed; the steel had К65 strength grade. Parallel finite-element computer code DANCO developed in RFNC-VNIIEF was used for simulations. A detailed description of the rupture formation process required a fine-enough mesh and a supercomputer. To carry out the numerical simulation of the process, the constants of the deformation diagram were used, obtained on the basis of static and dynamic tensile tests of samples at room temperature. A modified Gurson–Tvergaard–Niedelman (GTNm) model for macro-viscous steel destruction (ductile failure) was used to describe the strain and the destruction of the metal. The modification makes it possible to describe direct and oblique cuts and their combinations in case of ductile failure of small-size objects (rods, plates, shells). The calculated dependences of the movement of the crack tip on the movement of the load and the resistance force of the sample on the movement of the crack tip are presented. We have got a good agreement between the computations and experimental data with regard to the “force–displacement” strength parameter, the deformed profiles and macro-geometry of the ruptured sample after the tests. The computation results reveal the mechanics of the crack origination, start and propagation in the sample, describe the plastic-flow energy distribution in the process of dynamic destruction. The results of the work can be used in the development of the requirements and of the implementation conditions of the “tooled” DWT, and for numerical simulation of the extensive destruction at the main pipeline.

Application of Nonlinear Fracture Mechanics Parameter to Predicting Wire-Liftoff Lifetime of Power Module at Elevated Temperatures

Power modules are utilized for electric power control and play a key role in efficient energy conversion. A structural reliability problem of wire bonding, wire-liftoff, in power modules becomes important at high-temperature operation. Wire-liftoff is a thermal fatigue phenomenon caused by thermal stress due to mismatch of coefficients of thermal expansion between a wire and a chip material. According to experimental studies, a saturation phenomenon of wire-liftoff lifetime was observed in power modules above the maximum junction temperature of 200 °C. In this paper, we examine the failure models for predicting wire-liftoff lifetime proposed in the previous studies and also propose a new failure model based on the nonlinear fracture mechanics parameter $T^\ast $ -integral range $\Delta T^\ast $ . Among various failure models, the proposed model is the best to represent a saturation phenomenon of wire-liftoff lifetime. For practical use of the failure model based on $\Delta T^\ast $ -integral range, we propose a simple calculation method of $\Delta T^\ast $ -integral range that can be calculated by using the commercial finite element computer code such as Marc, Ansys, and so on.

Analytical study for allowable bearing pressures of CLSM bridge abutments

Abstract Controlled low strength materials (CLSMs) are increasingly gaining importance in the construction industry. Among many applications, CLSMs can be used for rapid and sustainable construction of bridge abutments. A study was undertaken to investigate the allowable bearing pressures of bridge sills over an abutment backfilled with CLSMs referred to as a “CLSM bridge abutment.” The study was conducted by the finite element method of analysis. The capability of the finite element computer code for analyzing the performance of a CLSM bridge abutment has been evaluated prior to this study. A series of finite element analyses were carried out to examine the effect of facing type, sill width, CLSM stiffness/strength, and foundation stiffness/strength on the load-carrying capacity of CLSM abutments. The allowable bearing pressures of CLSM abutments were determined based on results of the analytical study and two performance criteria: a limiting displacement criterion and a limiting linear strain criterion. In addition, a recommended procedure for determining the allowable bearing pressure is provided.

Unified modelling of adiabatic shear banding and subsequent micro-voiding driven dynamic failure of viscoplastic solids

Adiabatic shear bands (ASBs) are intense shear localisation zones resulting from thermomechanical instability in dynamically loaded, high strength structural materials. In the wake of the ASB, Micro-voids (MVs) have been observed which coalesce and ultimately rupture the material. ASBs thus favour premature MV induced damage and further material failure. In the present work, the coupled effects of ASB+MV on the representative volume element (RVE) are reproduced in a unified model within irreversible thermodynamics framework considering a large scale postulate. The influence of the softening mechanisms such as thermal softening and/or DRX induced softening on the ASB onset is accounted for. The model is numerically implemented as a user material subroutine into the engineering finite element (FE) computation code LS-DYNA and its feasibility and performances are assessed on both RVE and structural scales. The dynamic shear compression loading of a hat shaped structure is simulated for that purpose. The respective contributions of ASB and MV on the progressive material deterioration can be clearly identified.

Nonlinear flutter and limit-cycle oscillations of damaged highly flexible composite wings

A study on nonlinear flutter of damaged high- wings is presented. Because an undamaged, high- wing is long and slender, it may be properly modeled as a 1D beam, using a geometrically exact beam formulation. However, structural damages in the form of cracks and delaminations are in fact 3D phenomena. Therefore, the area near a crack cannot be accurately modeled as a reduced-dimensional beam. However, finite element analysis of a wing structure modeled entirely as a 3D continuum is computationally expensive. To address this issue, a joined 3D/1D finite element approach has been proposed in which a small area surrounding the crack is regarded as a 3D continuum, and the rest of the wing structure is modeled as a reduced-dimensional 1D beam undergoing large displacements and rotations. The continuity conditions at the intersection between the 3D and 1D models are derived using the variational asymptotic method, by which the two parts are rigorously connected. Hence, the joined 3D/1D method provides a computationally inexpensive, yet accurate, analysis for slender wings. The structural model is then coupled with a reduced-order unsteady aerodynamic model to compute the aerodynamic forces and moments acting on the wing. The resulting nonlinear aeroelastic element is integrated into Abaqus, a commercial finite element computer code as a nonlinear user-defined element. Through nonlinear time-marching, using a generalized $$\alpha $$ method, limit-cycle oscillations associated with the free response of both undamaged and damaged wings undergoing large deformations are studied, and the differences are highlighted.

Investigation of Rail Deformation and Stress Wave Propagation in the ISL-NGL60 Railgun

Railgun performance requires a low-resistance electric gliding contact between the rails and the armature. The electromagnetic forces, which accelerate the launch package (0.7 to 3 kg mass) up to more than 2000 m/s, are acting on the rails as repulsive forces. Therefore, the rails of the NGL60 electromagnetic gun, with caliber 60 $\times$ 60 mm2, are held in their position by an arrangement of discrete steel bolts, further by beams and plates made of glass fiber reinforced plastic, as well as by stiffening metal plates. The highly dynamic electromagnetic forces generate stress waves and elastic rail deformation that might contribute to contact transition between the rails and the armature. If the rail deformation enters the plastic domain, the rail geometry may be altered in an unfavorable way for subsequent shots. For our numerical investigation, we used the multiphysics finite-element computation code LS-DYNA to simulate a railgun projectile launch with a moving armature by injecting an electric current into the rails. It is therefore possible to study the dynamic deformations during the launch and get insight in the dynamic stress states. The material models for the rails and for the armature take into account elastic-plastic stress-strain behavior as well as strain rate sensitivity. We expect to simulate launch dynamics with higher accuracy than previously presented models with a nonmoving armature. The principal objective of the study is to show that this procedure is capable of effectively optimizing the structural behavior of the rails inside the barrel.

Deflagration characteristics of hydrogen-air mixture beyond the original premixed fuel region

Explosions are a major type of accident resulting in casualties in the processing industry. Fires frequently occur after a gas explosion. The attenuation rule of the deflagration state beyond the original premixed gas fuel area is the basis for determining the hazard risk once a gas explosion occurs. In this study, a numerical simulation was conducted using a finite element computational code for the fluid dynamics to reveal the attenuation law of the deflagration process of hydrogen–air mixtures in a tube 10 m long with a 0.1 m × 0.1 m sized section with one end closed and the other end open. The combustion characteristics beyond the original premixed hydrogen–air mixture region were examined. The numerical method was verified and its results showed good agreement with the experimental results. It was found that the fuel region can extend and enlarge the area of the flame, and thus a flame beyond the original fuel occupied zone can clearly be seen, which can expand the area of the damage. The explosion state inside the premixed region has a significant effect on the flame area, and a higher explosion strength will shorten the explosion time and reduce the disturbance of the unburned fuel. It was also found that a fuel-rich mixture has a larger flame area beyond the original premixed hydrogen–air mixture. Under all conditions considered in this study, the flame in the hydrogen–air mixture is 1–2 times as long as the one in the hydrogen–air mixture in the initially occupied area.

Validation of a Physical and Numerical Model to Solve Problems of Seepage Flow

The coefficient of permeability (k value) is an important parameter in civil engineering practice, in hydrology and hydrogeology. It can be determined by field test or by means of laboratory testing. The goal of this paper is to assess this parameter by creating a laboratory model and by validating its results using finite element computer code. The model tests provide that can be applied for estimating permeability of different soils. In a physical model medium-grained sand was tested in the laboratory, for understanding the effects of different flow rates on the validation of the measurement result, the numerical simulation of the physical model was constructed using FEFLOW. Two model variants were developed and both variants were calibrated and validated. Subsequently, the results were converted to real variables based on the model laws. The physical model provides the flow rate of the well in medium-grained sand with sufficient accuracy if the real size of the drawdown is between 0,5 and 1,7 m.

Damage and fracture of fuel rods of nuclear reactors

The paper contains the description of method and calculation algorithms for two-dimensional problems of creep-damage and fracture caused by development the macroscopic defect. The method of problem solution is based on combination of Finite Element and time integration third order predictor-corrector methods. The finite element software FEM creep and computer code FEM Creep damage fracture for modification of grids and boundary conditions, which are gone with fracture process behavior, are used in calculations. The resulting system of differential equations, which includes right part caused by traction and additional forces caused by creep strains, is presented. The plane triangular finite element is used in numerical analysis. The model of fuel rod cross section was regarded. It includes uranium fuel and metal cladding. The constitutive equations for fuel were built by use of Norton and Kachanov-Rabotnov laws for cases of varying temperatures. The constants which are included in creep-damage constitutive equations for fuel were processed by use of experimental curves at different temperatures. The comparison between experimental and calculated data is presented. The fracture algorithm was built by use the idea of grid modification due to exception of ‘failed’ elements. The calculations with modified grids and, if necessary, the boundary conditions, are continued until the moment of achieving the critical dimensions of macroscopic defect. The numerical simulation of creep-damage process and macroscopic defect’s growth was carried out and damage distribution in fuel rod’s cross section as well its current geometry were determined by way of finite elements exclusion.

Experimental and numerical simulation study of perforation effect of steel pipes subject to the impact loadings of ASC and LSC jets

The perforation effect of steel pipes subjected to the circular-shaped charge (ASC) and linear-shaped charge (LSC) jet were studied by experimental research, and the explicit nonlinear dynamic finite element computer code LS-DYNA was adapted to study the nonlinear responses of the steel pipes, which subjected to the impact of the two different jets, using Lagrangian-Eulerian coupling method. The deformation process and the stress of the steel pipes were described and analyzed, and the simulation results are in good agreement with the experiment data. The studies indicated that under the impact of ASC jet, the steel pipe got a circular incision and a deformation process of local perforation, flocculent shear lip forming and axial shock. Under the impact of LSC jet, the steel pipe got a ship-type incision and a deformation process of coupling of local perforation and dent, whole bending and radial shock. The formation of flocculent shear lip attributes to the radial stress concentration. Under the impact of LSC jet, the whole bending leads to the axial stretch and tearing of the cut tip, and there is a bigger radial plastic deformation area than the damage effect for the impact of ASC jet.