Three-dimensional Element Research Articles

Investigation of long-wavelength elastic wave propagation through wet bentonite-filled rock joints

The saturation of the compacted bentonite buffer in the deep geological repository can cause bentonite swelling, intrusion into rock fractures, and erosion. Inevitably, erosion and subsequent bentonite mass loss due to groundwater inflow can aggravate the overall integrity of the engineered barrier system. Therefore, the coupled hydro-mechanical interaction between the buffer and rock during groundwater inflow and bentonite intrusion should be evaluated to guarantee the long-term safety of deep geological disposal. This study investigated the effect of bentonite erosion and intrusion on the elastic wave propagation characteristics in jointed rocks using a quasi-static resonant column test. Jointed rock specimens with different joint conditions (i.e. joint surface saturation and bentonite filling) were prepared using granite rock discs sampled from the Korea Underground Research Tunnel (KURT) and Gyeongju bentonite. The long-wavelength longitudinal and shear wave velocities were measured under different normal stress levels. A Hertzian-type power model was used to fit the wave velocities, and the relationship between the two fitted parameters provided the trend of joint conditions. Numerical simulations using three-dimensional distinct element code (3DEC) were conducted to better understand how the long-wavelength wave propagates through wet bentonite-filled rock joints.

A novel computational fracture toughness model for soft tissue in needle insertion

During the process of percutaneous puncture vascular intervention operation in endoscopic liver surgery, high precision needle manipulation requires the accurate needle tissue interaction model where the tissue fracture toughness is an important parameter to describe the tissue crack propagation, as well as to estimate tissue deformation and target displacement. However, the existing studies on fracture toughness estimation did not consider Young’s modulus and the organ capsule structure. In this paper, a novel computational fracture toughness model is proposed considering insertion velocity, needle diameter and Young’s modulus in insertion process, where the fracture toughness is determined by the tissue surface deformation, which was estimated through energy modeling using integrated shell element and three-dimensional solid element. The testbed is built to study the effect of different insertion velocities, needle diameters and Young’s modulus on fracture toughness. The experiment result shows that the estimated result of computational fracture toughness model agrees well with the physical experimental data. In addition, the sensitivity analysis of different factors is conducted. Meanwhile, the model robustness analysis is investigated with different observation noises of Young’s modulus and puncture displacement.

Dynamic thermomechanical analysis on stiffened composite plates with damage

The dynamic thermomechanical analysis on stiffened composite plates with damages are investigated in this article based on the extended layerwise method (XLWM) and three-dimensional finite elements method; a thermomechanical extended-layerwise/Solid-element method (TELW/SE) is established. In TELW/SE, the thermomechanical extended-layerwise method (TELW) and thermomechanical three-dimensional solid elements are used to simulate plate and stiffeners, respectively. The final governing equations are assembled by using the interface conditions to ensure the compatibility of displacements and temperature, together with the internal force equilibrium. Several typical damages in stiffened composite plates can be described accurately in the proposed method, such as delamination, transverse crack and stiffener/plate interface debonding.

Calibration of Holzapfel-Gasser-Ogden collateral ligament properties in a hybrid post-arthroplasty knee joint model for laxity testing

Knee collateral ligaments play a vital role in providing frontal-plane stability in post-total knee arthroplasty (TKA) knees. Finite element models can utilize computationally efficient one-dimensional springs or more physiologically accurate three-dimensional continuum elements like the Holzapfel-Gasser-Ogden (HGO) formulation. However, there is limited literature defining subject-specific mechanical properties, particularly for the HGO model. In this study, we propose a co-simulation framework to obtain subject-specific material parameters for an HGO-based finite element ligament model integrated into a rigid-body model of the post-TKA knee. Our approach achieves comparable accuracy to spring formulations while significantly reducing coefficient calibration time and demonstrating improved correlation with reference knee kinematics and ligament strains throughout the tested loading range.

An Innovative Approach to Fabrication With Photo-Cured Resins by Shell-Printed-Core-Casting

Abstract Modified LCD-based method was used to print three-dimensional (3D) elements. This innovative method combines printing the external shell and filling, thus obtaining mould by casting resin. In order to compare the properties of prints obtained with this method with the ones fabricated in a standard procedure, we conducted bending tests of vertically/horizontally printed and shell-printed cast specimens. The shell-cast samples showed higher flexural strength and larger values of apparent Young’s modulus. The presented results also concern the kinetics of curing samples obtained with different fabrication routes.

Structural vibration control of spar-buoy floating offshore wind turbines

Floating offshore wind turbines (FOWTs) have complex dynamics problems and strong coupling effects within their entire systems, and they are sensitive to winds and sea waves that may cause structural fatigue damages and power production fluctuations. Structural vibration control is a promising way to reduce FOWT responses, but most previous studies focused on linear control strategies with the limitations of requiring large strokes and having narrow effective frequency bandwidth. Nonlinear energy sinks (NESs) have emerged as an alternative to overcome the above limitations of linear control methods. In the present study, an OC3-Hywind spar-buoy FOWT is selected as a prototype structure for developing an in-house model to investigate its associated aerodynamics, hydrodynamics, and structural dynamics. The wind turbine blades and tower are modelled by three-dimensional Euler-Bernoulli beam elements and the floater is assumed as a rigid body. The blades’ pre-twist, pitch, and rotating angles are also considered to develop the system’s time-varying mass, stiffness, and damping matrices. The nonlinear quasi-static model is adopted to calculate the forces generated by the mooring cables. The straightforward and efficient in-house model is validated against FAST. Subsequently, a bistable track NES with a profile combining both negative second-order and positive fourth-order terms is proposed, designed, and incorporated into the model to mitigate FOWT responses under two investigated environmental conditions. Results show that the in-house model is of high accuracy in estimating dynamic characteristics and responses of FOWTs, paving the way for preliminary design and analysis of FOWTs with different platform types. Moreover, the bistable track NES can effectively control the fore-aft displacements at the tower top with practical strokes.

A new variant of the fem for evaluation the strenght of structures of complex geometry with heterogeneous material structure

Structures are known to feature a complex geometry and a complex material structure which varies depending on their functional purpose, technological capabilities and design. The finite difference method, collocation method, variational method, theoretical and experimental method, finite element method, among other methods, are employed to calculate the stress-strain state (SST) of structures of complex geometry. The finite element method (FEM) has become the most common one of them all. The spline version of the finite element method (SV FEM) has proven to be an effective FEM method used to calculate the SST of structures of complex geometry. Unlike the conventional FEM method, a preliminary parametrization of the entire area of complex geometry using the parameters outlined by the canonical domain and a cubic approximation of the desired variables within each element yields consistent finite elements. The unbroken geometric continuity maintained between the elements along with the continuity of the desired quantities and their first derivatives along the line of contact of the elements makes it possible to obtain reliable results with just a small number of partitions into finite elements. One should note the application of the basics of the spline version of the finite element method in two-dimensional SV FEM-2 and three-dimensional SV FEM-3 arrangements. One should also make mention of the idea of producing a synthesized version of SV FEM. Two-dimensional and three-dimensional elements being synthesized into one element would usher in a new phase in the development of the SV FEM approach which would describe the complex structures of construction materials in addition to complex geometry. SV FEM has made it possible to evaluate the SST of structural elements of complex geometry along with the thin layers that penetrate it in all three directions. The versions of SV FEM that are available now serve to assess the SST of a broad range of various thin-walled and non-thin-walled structures of complex geometry, including multilayer structures with thin layers in all three directions, with hard and soft coatings along the edges and with local defects on the surfaces of structures. This paper presents a number of SST calculations of specific structures that have been carried out via the FEM versions described above.

Effects of irregular pores on the shear properties of the plane

Through section analysis and optical microscope observation of thick-section composite materials, it is found that there are significant differences in pore micro-characteristics of samples with different thicknesses. It is pointed out that the difference in pore micro-characteristics in the composite matrix is one of the reasons for the change in the interlaminar shear strength of composite materials. In this paper, irregular pore profiles are obtained through image processing, and the parameters to quantitatively characterize the microscopic characteristics are proposed. A randomly generated three-dimensional representative volume element (RVE) model including fiber, matrix, fiber/matrix interface, and pores. The effect of irregular pores with different sizes and distributions on the interlaminar shear strength of composite materials is given. The RVE numerical analysis results show that the interface failure between the fiber and the matrix near the pore causes damage. At the same porosity, the shear strength decreases with the increase of the maximum pore size. At the same time, the porosity concentration also has a significant effect on the interlaminar shear strength of thick section composite. The numerical results verify that the influence of pores on the interlaminar shear properties of thick-section composites is related to the microscopic characteristic parameters of pores.

Mechanical Characterization of MWCNT-Reinforced Cement Paste: Experimental and Multiscale Computational Investigation.

Computational approaches could provide a viable and cost-effective alternative to expensive experiments for accurately evaluating the nonlinear constitutive behavior of cementitious nanocomposite materials. In the present study, the mechanical properties of cement paste reinforced with multi-wall carbon nanotubes (MWCNTs) are examined experimentally and numerically. A multiscale computational approach is adopted in order to verify the experimental results. For this scope, a random sequential adsorption algorithm was developed to generate non-overlapping matrix-inclusion three-dimensional (3D) representative volume elements (RVEs), considering the inclusions as straight elements. Nonlinear finite element analyses (FEA) were performed, and the homogenized elastic and inelastic mechanical properties were computed. The use of a multiscale computational approach to accurately evaluate the nonlinear constitutive behavior of cementitious materials has rarely been explored before. For this purpose, the RVEs were analyzed both in pure tension and compression. Young's modulus as well compressive and tensile strength results were compared and eventually matched the experimental values. Moreover, the effect of MWCNTs on the nonlinear stress-strain behavior of reinforced cement paste was noted. Subsequently, three-point bending tests were conducted, and the stress-strain behavior was verified with FEA in the macro scale. The numerical modeling reveals a positive correlation between the concentration of MWCNTs and improved mechanical properties, assuming ideal dispersion. However, it also highlights the impact of practical limitations, such as imperfect dispersion and potential defects, which can deteriorate the mechanical properties that are observed in the experimental results. Among the different cases studied, that with a 0.1 wt% MWCNTs/CP composite demonstrated the closest agreement between the numerical model and the experimental measurements. The numerical model achieved the best accuracy in estimating the Young's modulus (underestimation of 13%), compressive strength (overestimation of 1%), and tensile strength (underestimation of 6%) compared to other cases. Overall, these numerical findings contribute significantly to understanding the mechanical behavior of the nanocomposite material and offer valuable guidance for optimizing cement-based composites for engineering applications.

Multi-axial elastic–viscoplastic curves of unidirectional carbon nanotube-polymer nanocomposites considering interphase and interface debonding using finite element modeling

This paper aims to predict the elastic-viscoplastic curves of unidirectional carbon nanotube (CNT)-reinforced polymer nanocomposites under the multi-axial tensions, including bi-axial transverse-transverse, bi-axial longitudinal-transverse and tri-axial longitudinal-transverse-transverse loadings by the use of finite element (FE) modeling. The role of the interphase between the CNT and polymer matrix as well as the debonding at the CNT/interphase interface in the stress-strain curves of nanocomposites is investigated. The polymer matrix and interphase region are characterized as elastic-viscoplastic materials, while the CNT behaves as the elastic material. The present results in uni-axial loading are compared to the available numerical outcomes for indicating the accuracy of the FE modeling. The simulation of multi-axial load conditions is based on the three-dimensional representative volume element (RVE) of the unidirectional CNT-polymer nanocomposite. The effect of interface strength on the elastic-viscoplastic stress-strain curves is examined under bi-axial and tri-axial loadings.

Numerical investigation and calculation method of concrete-filled bimetallic tubes subjected to concentric tension

Concrete-filled stainless steel (outer)-carbon steel (inner) bimetallic tubes (CFBT) are potential to be applied as structural members in the corrosion-prone environments. A detailed numerical investigation on the mechanical performance and calculation method of CFBT members under concentric tension is carried out in this paper. A finite element analysis (FEA) model adopting three-dimensional elements is established, and related experimental results of conventional concrete-filled carbon steel tubes (CFST) and concrete-filled stainless steel tubes (CFSST) are used to validate the FEA model. Then, the calibrated FEA model is used to investigate the performance of CFBT tensile members, especially the composite actions and stress distribution laws between the two tube-layers and the infilled concrete, which play the key role in the load-carrying capacity of the composite member. To quantitatively determine the influence of different parameters on the load-carrying capacity of CFBT tensile members, a wide-range parametric analysis is performed. Finally, a calculation model is proposed to be used to predict the ultimate tensile strength of CFBT members subjected to concentric tension, and the model-predicted values show good agreement with the FEA-computed results.

Experimental and numerical study of mode I interlaminar behavior of carbon nanotube reinforced glass–epoxy composite: A multiscale approach

In this paper, the effect of adding carbon nanotubes (CNTs) between glass–epoxy composite laminate in the mode I interlaminar behavior was investigated by experimental and numerical methods. Mode I interlaminar fracture toughness of epoxy matrix modified by 0.5 wt%, and 1 wt%, CNTs were compared with an unmodified epoxy matrix using the double cantilever beam (DCB) specimen. By adding a 1% weight ratio of CNT, the mode I interlayer fracture toughness increases by about 50%. A multiscale approach was used for modeling the DCB test. At the micro-scale, toughness mechanisms, including CNT pull-out, its breaking and bridging, and the separation of the nanotube from the surrounding resin were modeled. The separation of the nanotube from the surrounding resin was modeled with a cohesive element. The cohesive element around the nanotube was embedded in the three-dimensional elements of the resin and according to the weight percentage of the nanotube to the resin, a representative volume element (RVE) was created. The orientation, length, and diameter of CNT were considered randomly in the RVE. In the mesoscale, according to the percentage of glass fibers to resin, an RVE was created randomly. By using the damage model whose parameters were extracted from the micro-scale, crack growth was simulated. According to the Hill-Mendel conditions, the relationship between the meso and macro scale was established and the amount of effective quantities of traction and separation from the micro-scale was used as the parameter of the cohesive element in the macro-scale. To consider the bridging effect of glass fibers and its effect on interlayer fracture toughness, random distribution of interlayer beam elements was created. At the macro-scale, the DCB test simulated and cohesive elements were considered for modeling lower-scale mechanisms and beam elements for fiber bridging simultaneously. A favorable agreement between the experimental and simulation results was obtained.

Multiscale progressive damage model for plain woven composites

In this paper, a novel multiscale progressive damage model is proposed to study the complex mechanical behavior and damage mechanism of plain woven composites. In the macroscale, composites are treated as full-scale model with three-dimensional solid elements. The mechanical response of element integration points depends on the mesoscale constitutive relationship, including orthotropic undulating yarn and isotropic matrix. Based on the microscopic representative volume element (RVE), the stress amplification factor (SAF) database is introduced to characterize the microscale stress state of the fiber and matrix. SAF is used to realize the stress correlation calculation from the yarn level to the micro level, and a progressive damage model based on the micro-mechanics of failure (MMF) theory is developed. Quasi-static tension experiments are carried out to validate the accuracy of the model, and the influence of meso‑scale waviness and non-waviness constitutive models on the simulation results is compared. The results indicate the waviness constitutive model predicts are more consistent with the experimental results, and the prediction deviations about modulus, failure strength and failure strain are around 5%. The damage initiation of matrix in the yarn and pure matrix occurs successively, and finally the fiber failure is the main cause of catastrophic damage. Furthermore, the proposed multiscale method can be further used to predict the strength and damage of full-size woven composites under complicated loading.

Investigation of mechanical properties of NbC particle-reinforced iron matrix composites under tensile loading- A numerical simulation study

To study the effect of NbCp on the mechanical properties of iron matrix composites, a reinforced phase model was established that closely matched the microscopic morphology. The interface failure behavior was exhibited with a bilinear cohesive zone element. A three-dimensional periodic element was employed to analyze the matrix strain gradient, plastic deformation, and ductile cracking. The influence of interface damage on the uniaxial tensile elastic-plastic behavior of iron matrix composites reinforced with spherical or cubic particles of the same volume fraction was investigated. The stress concentrations of the NbCp-reinforced iron matrix composites were produced at the sharp corners of the particles. The results showed that the cubic NbCp-reinforced iron matrix composites were prone to crack initiation and propagated faster than the spherical NbCp-reinforced iron matrix composites. By increasing the proportion of cubic NbCp, the crack initiation, advancement of fracture, elongation, and elastic modulus of the composite showed an upward trend, the tensile strength decreased from 446.56 Mpa to 347.78 Mpa, and the elastic modulus increased accordingly. The results revealed that the fracture mechanism of the model was matrix ductile fracture with interface debonding, and the crack propagation was consistent with the stress-strain concentration region.

Tensile behaviors of stainless steel anchor channel systems embedded in concrete: Experimental and numerical analyses

Stainless steel anchor channels with channel bolts (SSAC-CBs) are extensively used in reinforced concrete structures subjected to corrosive environments owing to the excellent heat and corrosion resistance, appreciable inelastic deformation capacity, and high ductility of stainless steel. However, limited tests and numerical simulations have been conducted describing the tensile behaviors of SSAC-CB assemblies. We therefore conducted static monotonic tension tests to investigate the structural behaviors of SSAC-CBs embedded in concrete in terms of failure mode, tensile load-bearing capacity, ductility, and plastic development response. The test variables included the anchor/channel profile type, stainless steel grade, and tensile load position. The SSAC-CB specimens exhibited three dominant failure modes in tension: combined yielding of the channel lips and webs, flexural deformation of the channel, and concrete splitting. The ductility ratios of the test specimens ranged from 4.56 to 8.45, indicating a significant inelastic deformation capacity. Based on the maximum tensile loads obtained in the experiments, the ultimate limit state principle was employed to recommend tensile design resistances according to anchor channel type. In addition, nonlinear finite element analysis models were constructed in ABAQUS using three-dimensional solid elements to facilitate detailed investigations of the tensile behaviors of SSAC-CBs embedded in concrete; the numerically obtained failure modes and loads agreed well with the experimental responses. Finally, the experimental data were applied to propose and validate analytical expressions for the tensile load-bearing capacities of SSAC-CBs embedded in concrete corresponding to their different failure modes under tensile load.

Experimental and DEM numerical analyses of the flexural and diagonal compression behavior of masonry reinforced with polyurea coating

The brick masonry specimens in this paper were strengthened with polyurea reinforcement before flexural and diagonal compression tests. The influence of one-sided and double-sided polyurea coatings on the load capacity and displacement were studied. According to the findings, the polyurea coating used for strengthening provided a considerable increase in load capacity as well as deformation. Two modeling approaches, macro-modeling and micro-modeling based on the Discrete Element Method (DEM), were applied. The Three-Dimensional Distinct Element Code (3DEC) program was used to simulate the behavior of the brick masonry. Several parameters, including joint-normal stiffness, joint-shear stiffness, and friction angle, were investigated, and their influence on the brick masonry structure was evaluated. The research determined that the prediction of ultimate load and behavior failure in the models could be predicted with accurate results. Furthermore, computational models were developed to compare against experimental tests. The results obtained from the simulation showed that the proposed modeling may be effectively used to verify the experimental results.

Effect of graphene reinforcement distribution on energy release rate and vibration of thermally pre/post-buckled delaminated composite plates

The nonlinear thermal stability responses of functionally graded graphene-reinforced composite (FG-GRC) laminated plates with single or multiple through-the-width delaminations, are investigated in the article. The possibility of delamination growth is evaluated using the three-dimensional crack tip element (3D-CTE) method, which determines the energy release rate (ERR) at the delamination edge. Furthermore, the influence of delamination configurations and the types of graphene reinforcement distribution patterns, on the free vibration of FG-GRC delaminated plates in thermally pre/post-buckled regimes is evaluated. The von Karman geometrical nonlinearity is adopted in a solution based on the layerwise third-order shear deformation theory (TSDT). The nonlinear equilibrium equations derived by the minimum total potential energy principle are solved using the Ritz method in conjunction with the Newton–Raphson iterative procedure. A three-dimensional finite element model is also developed using ABAQUS to corroborate the accuracy of the theoretical results. Parametric studies reveal that while the FGX graphene distribution pattern improves the bending stiffness of delaminated composite plates more than FGA, the ERR at the near surface delamination edge in FGX pattern is twice as high as that in FGA. This implies that the possibility of delamination propagation in FGX plates can be higher than FGA. Additionally, while the fundamental frequency of the FGX plate with and without delamination exceeds the frequency of other graphene distribution patterns at the reference temperature, its natural frequency is the lowest among all patterns in the post-buckled regime.

Comprehensive inter-fibre failure analysis and failure criteria comparison for composite materials using micromechanical modelling under biaxial loading

Inter-fibre failure analysis of carbon fibre-reinforced polymer (CFRP) composites, under biaxial loading conditions, has been a longstanding challenge and is addressed in this study. Biaxial failure analysis of IM7/8552 CFRP unidirectional (UD) composites is conducted under various stress states. Two widely accepted failure criteria, the interactive Tsai-Wu and non-interactive Hashin failure criteria, are comprehensively assessed with finite element-based micromechanical analysis. High-fidelity three-dimensional representative volume elements (RVEs) are subjected to biaxial loadings with imposed periodic boundary conditions. Carbon fibres are assumed to be transversely isotropic and linearly elastic. The Drucker-Prager plastic damage constitutive model and cohesive zone model are utilised to simulate the mechanical response of the matrix and fibre-matrix interface, respectively. Coulomb friction is assumed between the fibres and matrix after interface failure. Two sets of biaxial loading scenarios (i.e. transverse stress dominated and shear stress dominated) with the associated failure modes are selected for the failure analysis and assessment of these failure criteria. A data-driven failure envelope for the composites under biaxial loadings is developed using a univariate cubic spline function. Failure mode transition points are determined under biaxial loadings. It is found that the micromechanics-based numerical model is effective in assessing these two existing criteria.

Residual performance of compression stiffened welded hollow spherical joints after exposure to elevated temperatures

The post-fire residual behavior of the joints and their accurate identification are critical issues for the assessment of the fire damage in the commonly used grid structures. Accordingly, it will be determined, whether the structure is directly used, disassembled, or used after repair. In this paper, the residual performance of compression stiffened welded hollow spherical joints after a fire is studied using the methods of air cooling and water cooling. The failure mode, load-displacement curves, and residual mechanical properties of the joints are analyzed. The results show that the residual performance of the joints after a fire is mainly related to the exposure temperature and the cooling mode. In air-cooled joints, the initial axial stiffness and bearing capacity decrease with the increase of exposure temperature, while the ductility increases significantly. In water-cooled joints, the initial axial stiffness and ductility decrease with the increase of exposure temperature, and the bearing capacity shows a decreasing trend before 500 °C and gradually increases after 500 °C. When conducting the FEM simulation of the experimental specimens, three-dimensional hexahedral elements were adopted with the relationship of the steel material after elevated temperature consisting of a combination of straight lines. The validity of the FEM model was verified by comparison with the experimental results. Based on the results of the parametric analyses, a calculation formula is proposed for predicting the residual bearing capacity and initial axial stiffness of compression stiffened welded hollow spherical joints after a fire.

Three-dimensional high aspect ratio interface elements for simulating 3D hydraulic fracturing in deformable porous media

Hydraulic fracturing (HF) is a technique widely used in the oil industry to stimulate low permeability reservoirs and enable their production. However, to model the HF is necessary to consider at least three main coupling process: (i) the fluid pressure deforming the fracture walls, (ii) the fluid flow inside the fracture, and (iii) the fracture propagation through the porous medium. In this context, this work aims to expand the applicability of the high aspect ratio interface elements (HAR-IE) to simulate the formation and propagation of induced hydraulic fracture in three-dimensional scenarios. The proposed methodology considers a deformable porous media in which the fluid flow is governed via Darcy’s law. A proper tension continuum damage model is employed to deal with the fracture formation process and the classical cubic law defines the fluid flow in the fracture. A fully-coupled method solves the system of equations and the numerical results focus on the validation of the method via comparisons against analytical solutions. All the results showed that 3D HAR-IEs satisfactorily reproduce the main characteristics associated with HF, as fracture aperture, length and injection pressure.