Time-dependent Finite Element Research Articles

The transient thermal ageing of Eurofer 97 by mitigated plasma disruptions

Plasma disruptions in a commercial-scale tokamak will impose high magnitude, short-duration thermal loads on its plasma-facing first wall. Despite a plethora of mitigation measures, these severe, off-normal events are likely to briefly expose the structural materials of the first wall to significant temperature excursions. Eurofer 97, the reference structural material for the plasma-facing first wall of the EU DEMO tokamak, is a reduced activation 9Cr steel with a normalised and tempered ferritic/martensitic microstructure. Repeated exposure to the thermal effects of disruptions over the operating lifetime of a reactor may promote a cumulative evolution of Eurofer 97′s microstructure, affecting key material properties crucial to first wall performance. This novel transient thermal degradation mechanism has been explored via a laser-based transient heating experiment, supported by time-dependent finite element thermal analysis studies of DEMO’s water-cooled lithium–lead first wall during a mitigated plasma disruption. Transient-affected samples of Eurofer 97 were characterised via scanning and transmission electron microscopy techniques (SEM/TEM), including electron backscatter diffraction (EBSD), energy-dispersive X-ray spectroscopy (EDX), and selective area electron diffraction (SAED). Microhardness and magnetisation testing data are also presented. A single 700 °C thermal transient was found sufficient to coarsen and partially recrystallise Eurofer 97′s tempered martensite sub-grains at the tungsten-Eurofer 97 interface. Further transient exposure at 700 °C resulted in the significant growth of equiaxed grains, the nucleation of intergranular Cr-rich M7C3 and M23C6 carbides, and the coarsening of V-rich and intra-granular Ta-rich MX precipitates. The microstructural effects of 850 °C transients are also reported. Notably, after 1,000 transients at 850 °C the hardness of Eurofer 97 was found to have decreased by 32 %.

Modeling Particle-Doped Materials for Performance Improvement of Contact-Separation Triboelectric Nanogenerators

Triboelectric nanogenerators (TENGs) are an attractive energy harvesting technology due to their high efficiency and vast applications in self-powered sensors. In this work, dielectric–dielectric contact-separation TENGs were modeled with time-dependent finite element simulations with the objective of improving TENG’s performance by enhancing the relative permittivity (εr).To achieve this, the chosen material (PDMS, εr=2.75) was doped with SrTiO3 (εr = 300) particles. The open-circuit voltage (VOC) and short-circuit current (ISC) remained constant as ϵr increased, as predicted by existent models, but in contradiction with available experimental data. Thus, we introduced a charge correction model relating ϵr and surface charge density, allowing us to observe an increase in TENG performance output (VOC and ISC). This work shows that finite element simulations are suitable for better understanding and optimizing TENGs’ performance.

Running-specific prosthesis' performance characterization by dynamic finite element approach.

Composite running-specific prostheses (RSP) are widely used by athletes with lower limb amputations to simulate the spring-like behavior of biological legs. However, the effect of these devices on the biomechanics of athletes with transtibial amputations remains uncertain. To address this issue, this study proposes a time-dependent finite element model that uses angles and dynamic loads during ground contact to evaluate RSP performance parameters such as stiffness and energy efficiency. The study also examines the impact of running speed and RSP geometry on performance. The in-silico characterization approach used in this study takes into account both the intrinsic characteristics of the RSP and the athlete's biomechanics to identify the influence of fundamental geometric variables of the RSP on performance. The model is verified by comparing its results with experimental data. The study found that as running speed increases, RSP stiffness, vertical ground reaction force (vGRF), and contact time decrease. The force-displacement profiles of RSP are nonlinear, but a linear function can be used to accurately represent their behavior at high sprinting speeds. Using higher RSP reduces energy efficiency and vGRF due to their lower stiffness. J-curve RSP result in higher stiffness, vGRF, and strain energy, while C-curve RSP are associated with longer contact times and higher energy efficiency.

Biomechanical analysis of combi-hole locking compression plate during fracture healing: A numerical study of screw configuration.

Locking compression plates (LCPs) have become a widely used option for treating femur bone fractures. However, the optimal screw configuration with combi-holes remains a subject of debate. The study aims to create a time-dependent finite element (FE) model to assess the impacts of different screw configurations on LCP fixation stiffness and healing efficiency across four healing stages during a complete fracture healing process. To simulate the healing process, we integrated a time-dependent callus formation mechanism into a FE model of the LCP with combi-holes. Three screw configuration parameters, namely working length, screw number, and screw position, were investigated. Increasing the working length negatively affected axial stiffness and healing efficiency (p < 0.001), while screw number or position had no significant impact (p > 0.01). The time-dependent model displayed a moderate correlation with the conventional time-independent model for axial stiffness and healing efficiency (ρ ≥ 0.733, p ≤ 0.025). The highest healing efficiency (95.2%) was observed in screw configuration C125 during the 4-8-week period. The results provide insights into managing fractures using LCPs with combi-holes over an extended duration. Under axial compressive loading conditions, the use of the C125 screw configuration can enhance callus formation during the 4-12-week period for transverse fractures. When employing the C12345 configuration, it becomes crucial to avoid overconstraint during the 4-8-week period.

Prediction of elastic wave propagation in composites using 3D CNN

Performing time-dependent finite element simulations for wave propagation in composites is a particularly complex task that consumes a lot of computational energy as it involves modeling the interactions between waves and various constituents that make up the composite material. In this study, we have developed a surrogate model of elastic wave propagation in composites based on three-dimensional conventional neural networks. The input to the model consists of a three-dimensional matrix representing the architecture of the composites and a vector representing the input waves, while the output is a vector representing the output elastic waves. After training the model using 60 000 randomly generated samples, it has shown high accuracy and efficiency in predicting the output elastic waves. This significantly reduces computational resources required to conduct simulation using commercial software, making it a more practical solution for real-world applications, such as composite optimization, nondestructive testing, and material characterization.

Dynamic behavior of a zero-group velocity guided mode in rail structures.

Important characteristics of a zero-group velocity (ZGV) mode in a standard rail are investigated through numerical simulation and experiment. First, the semi-analytical finite element analysis is implemented to compute dispersion curves for the rail structure and the first ZGV point is identified. Backward waves are identified through opposing senses of group and phase velocities. Next, a time-dependent finite element model is used to understand the dynamic response of the rail. Finally, experimental measurements confirm that ZGV modes in rail structures are formed through interferences between two opposite-traveling waves, which is analogous to the S1-S2b ZGV Lamb mode in plate structures.

Numerical Assessment of Standard Firebrand Accumulation Curve When Transferring Temperature to Contact Surfaces

This work presents research concerning the numerical assessment of two previously measured temperatures due to firebrand accumulation on surfaces, which was determined in former thermal experimental campaigns. A 3D numerical model using thermal transient non-linear analysis is used to validate the thermal outputs of these two previous experimental campaigns, and therefore, corroborating the previous temperature vs. time curves created with a prescribed flux in the firebrand accumulation area. The firebrand thermal heat transfer to the plane surface is simulated using convection and radiation film conditions, in which a 3D non-linear, time-dependent finite element simulation is used. Then, the previous proposed standard firebrand accumulation curve, ISO 834, and external fire curve are numerically compared with the results from previous firebrand accumulation curves in a wood corner wall. Finally, the merit assessment of the proposed standard firebrand accumulation curve shows a visible improvement, which has low values and is in accordance with the experimental results in the temperature field distribution of firebrand accumulation onto a contact surface. It is fair to argue that it constitutes a point to search for an efficient design for structures at elevated temperatures due to firebrand accumulation.

Simulating creep induced moment redistribution in prestressed concrete bridges constructed by the balanced cantilever method: ad hoc traditional formulae versus real time-dependent analysis

Assessing available numerical techniques adopted to determine the design time-dependent moment for prestressed concrete segmental bridges constructed by the balanced cantilever method is of utmost importance to the bridge design community. In essence, despite some apparent diversity, there are basically two key conventional approaches to compute the design time-dependent moment accounting for creep effects for this type of bridges. The first is a family of varied simplified methods typically known to practicing designers and with pre-consensus on their reliability and effectiveness. Time-dependent moments retrieved from these classical methods always reside in an intermediate state falling between the results from “two” time-independent analysis cases, namely, (a) sequentially adding all partial permanent loads and prestressing pertaining to various construction steps using the part-bridge structural system corresponding to each step, and (b) assuming all loads and prestressing forces to be applied at-once to the final completed bridge. The second approach is through performing real sophisticated step-by-step time-dependent analysis using a specialized software. The research primary objective is to assess the validity/reliability of commonly used ad-hoc approaches that evolved over the years relying on simplified analyses/formulae to cater for time-dependent creep effects for this type of bridges. Aiming at realistic conclusions, three case-study real-world segmental balanced cantilever bridges over the Nile River in Egypt are elected. Midas Civil commercial package is used to perform time-dependent finite element analyses for the three bridges. Main parameters considered are, inter alia, time-dependent effects of creep and shrinkage of concrete, relaxation of prestressed steel, losses due to friction and anchor setting of prestressing tendons, sequence of construction, and construction-driven temporary change of support conditions (where applicable). The study concludes that creep-induced moment redistribution from simplified traditional formulae typically adopted in the literature may lead to a considerable error in estimating the design time-dependent moment in balanced cantilever bridges.

Wave propagation in a phononic material with periodic bilinear elastic layers

This presentation discusses how bilinear elastic layers influence wave propagation in phononic materials. Bilinear elastic layers have been shown to produce wave propagation phenomenon such as shock waves, non-reciprocity,phase-reversal effects, and harmonic generation. Here, we examine how a propagating wave through a periodic array of linear elastic layers coupled with springs that exhibit bilinear stiffness can be tuned with the stress state of the excited wave. Using time-dependent finite element simulations, we study the interactions of bilinear springs and phononic materials, analyzing how the tensile or compressive stress state influences dispersion, band gaps, and energy transfer between frequencies. We select excitation waveforms to exploit the bilinear stiffness to fundamentally change the shape of the propagating wave. Results demonstrate phase-reversal effects, attenuation of tensile strain, and cumulative displacement offsets. To experimentally probe these results, bilinear stiffness couplings are physically realized by fabricating samples with prescribed delaminations between stiff and soft materials so the samples exhibit higher stiffness in compression than tension. Single layered samples are experimentally studied using both propagating waves and resonant methods. The nonlinear response from delaminations may be used to control frequency energy transfer and wave transformation for applications such as blast mitigation and passive mechanical sensing.

Effect of implant placement depth on bone remodeling on implant-supported single zirconia abutment crown: A 3D finite element study.

This study aimed to evaluate the influence of subcrestal implant placement depth on bone remodeling using time-dependent finite element analysis (FEA) with a bone-remodeling algorithm over 12 months. Seven models of different subcrestal implant placement depths (0, 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 mm) were analyzed using FEA to evaluate the biomechanical responses in the bone and implant, including von Mises equivalent stress, strain energy density (SED), and overloading elements. SED was used as a mechanical stimulus to simulate cortical and cancellous bone remodeling over the first 12 months after final prosthesis delivery. The highest increase in cortical bone density was observed at Depth 1.5, whereas the lowest increase was observed at Depth 3.0. In contrast, the highest increase in bone density was observed at Depth 3.0 in the cancellous bone, whereas the lowest increase was observed at Depth 0. The highest peak von Mises stress in the cortical bone occurred at Depth 2.5 (107.24 MPa), while that in the cancellous bone was at Depth 2.5 (34.55 MPa). Notably, the maximum von Mises stress values in the cancellous bone exceeded the natural limit of the bony material, as indicated by the overloading elements observed at the depths of 2.0, 2.5, and 3.0 mm. Greater bone density apposition is observed with deeper implant placement. An implant depth of more than 1.5 mm exhibited a higher maximum von Mises stress and greater overloading elements.

Numerical investigations on high-speed turbo-compressor rotor systems with air ring bearings: Nonlinear vibration behavior and optimization

Regarding aerodynamic bearings, mainly two basic types can be distinguished: air bearings with a rigid housing and air foil bearings with an elastic supporting structure. Here, a modified bearing concept -- called air ring bearing -- is investigated, which may be considered as a further development of rigid air bearings or as a combination of the two basic bearing types. The idea of air ring bearings is to insert a ring-shaped bearing bushing between the shaft and the foil structure. Alternatively, a visco-elastic supporting structure (e.g., an elastomer) can be applied to connect the bushing ring with the housing. In the first case, external dissipation is mainly generated by dry friction. In the latter case, viscous damping is used to provide external dissipation. Due to the external friction/damping generated by the ring mounting structure, rotor systems with air ring bearings can be operated above the linear threshold speed of instability so that stable self-excited vibrations with moderate amplitudes can be achieved in the complete speed range. Here, a detailed transient co-simulation model for rotor systems with air ring bearings is presented. The rotor is represented by a multibody model. The air films of the two bearings are represented by two nonlinear time-dependent finite element systems. The multibody model and the two finite element subsystems are solved simultaneously by means of an explicit sequential co-simulation technique. Due to the strong nonlinearities, the system shows interesting vibration and bifurcation effects, which are investigated in detail with the help of run-up simulations.

On the nonlinear seismic response of liquid filled thin-walled steel elevated containers isolated by bearings to earthquake ground motions

In the present study two base isolation systems, i.e. lead-rubber and elastomeric bearings are analyzed and their seismic applicability is compared with reference to a liquid-filled conical elevated container undergoing different ground motions. The drilled shaft system is unlinked either from the base or from the top-mounted bowl by means of damping devices and the impact of such amendment on seismic performance of elevated liquid tank is examined by carrying out time-dependent finite element (FE) analyses on several pedestal-container systems. The developed FE procedure is competent in assaying the precise three-dimensional performance of the isolated elevated containers including the hysteretic performance of bi-linear isolation devices. For the input ground motions, the impacts of various parameters on the efficiency of the seismic bilinear hysteretic members are inspected. The results obtained from the FE analysis are validated against available numerical predictions in the time domain, and a good agreement is found between two methods. The sensitivity of induced forces, the internal hydrodynamic pressure, base moments and displacements in regard to some system parameters comprising lateral versus vertical isolation and locations of isolation devices, have been highlighted. Finally the numerical procedure suggests that earthquake response spectrum in various intensity levels has an influential role in the performance of elevated containers and special attention should be given to the dynamic analysis of such structures.

Investigating the relationship of hardness and flow stress in metal forming

Hardness is routinely utilised to link the bulk material properties to surface contact mechanics. Besides possible differences in the material properties between bulk and surface, there is no established relation between hardness and the thermo-viscoplastic flow stress of materials used in the context of metal forming processes. The purpose of this study is to investigate such relationship using 6061 and 6016 aluminium alloys manufactured by hot rolling. Optical microscopy and Vickers hardness tests at different loads and temperatures ranging from 22°C up to 450°C were carried out. A time-dependent Finite Element model of an indentation using thermo-viscoplastic material model based on bulk samples was developed and compared with experiments. Overall, no significant difference between bulk and surface for neither alloy was experimentally identified. The hardness decrease with temperature of the alloys is quantified and ready-to-use constraint factor maps associating the thermo-viscoplastic flow stress with hardness of the materials are presented. The numerical model allowed visualisation of viscoplastic effects, whereas comparisons to the experiments in terms of hardness and topography validated the model and constitutive equations. Constitutive equations derived from compression tests of bulk samples can confidently describe deformation at the surface level and so, be used to develop a contact model in the tribological context of metal forming.

Thermal performance and parameter study of steel fiber-reinforced concrete segment lining in energy subway tunnels

This study proposes an effective approach for improving the energy performance of subway tunnels by using steel fiber-reinforced concrete (SFRC) segments to obtain shallow geothermal energy. Three-dimensional time-dependent thermohydraulic finite element analyses of the subway tunnel lining with reference to the existing Shenzhen subway tunnels were conducted. The width of each pipe segment is 1.5 m and a tunnel lining-ground model with size of 25 m × 25 m × 3 m was studied. The influencing factors including the inlet temperature, flow velocity, operation mode, pipe arrangement, and thermophysical parameters of SFRC were considered in evaluating the heat transfer performance of the tunnel lining. The results confirm that the heat-exchange performance of the SFRC segment is noticeably better than that of the ordinary concrete segment. Also, there is a linear relationship between the thermophysical parameters and the heat exchange per unit length. Heat exchange increased with an increase in flow rate until a certain level and became stabilised. Based on the thermal results and economical considerations, the inlet fluid velocity around 0.6 m/s is recommended. Besides, the heat exchange tube spacing between 0.5 m and 0.7 m can ensure both the heat exchange per unit length of the heat exchange tube and the heat extraction per unit tube sheet inner surface area to have high values at the same time. In terms of heat exchange, the advantages are more obvious for cooling operation than heating operation. The heat exchange per unit segment area of the energy tunnel increased with the inner diameter of the embedded pipes. However, the relationship between the rate of heat exchange increase and the pipe spacing changed with the thermal conductivity of the concrete segment.

Bayesian inference for predicting the long-term deflection of prestressed concrete bridges by on-site measurements

Predicting the long-term deflection of prestressed concrete bridges (PSCB) by the deterministic models is subject to significant epistemic uncertainties, and the Bayesian statistics provide a practical way to make more reliable prediction by use of the long-term measurements. In this paper, a comprehensive Bayesian inference frame was built, in which the model parameters associated with creep, shrinkage, dead load and prestress level were updated by the Bayesian inference of the deflection measurements. A 3-D time-dependent finite element model was constructed, based on which a surrogate model was built to enhance computation efficiency. The Markov Chain Monte Carlo (MCMC) algorithm was adopted to approximate the manifold of the posterior distribution. In the case study, two sets of deflection of the case bridge were utilized to perform the Bayesian inference in a sequential manner. The results revealed that the updated models could produced better fit to the measurements than the deterministic model, while the variance of the prediction was significantly reduced. The updated model revealed changing trends of the time-dependent deflection, which characterizes a later-age acceleration from the initial moderate trend. Based on the updated models, the time-dependent reliability associated with the limit states defined by deflection limits was computed. For such a task, the MCMC-based pre-sampling and the quasi-optimal importance sampling were adopted to reduce the variance of the computation.

Numerical investigation of thermal residual stress distribution for the sustainability of laser welded joints of dissimilar materials

In this work, a three-dimensional (3D) time dependent finite element method is used to simulate the thermo-mechanical behavior of laser welded joints. Laser welding of two dissimilar materials (AISI 316L and Ti6Al4V) of 2 mm thickness has been considered while modeling in COMSOL multiphysics with a coupling of heat transfer and solid mechanics module. A total of six temperature dependent physical properties, i.e. density, specific heat, thermal conductivity, coefficient of thermal expansion, Young’s modulus, and Poisson’s ratio have been considered for making the numerical model more realistic. The experimental results show that AISI 316L experiences more than 2 times more maximum stresses than Ti6Al4V near the weld bead region during the process. High value of Von mishes stress in the welded zone can be observed due to high temperature gradients of welds. The simulated value of residual after one entire thermal cycle is near to 80 MPa on a point over weld bead.

Large Deformation Hermitian Finite Element Coupled Thermoelasticity Analysis of Wave Propagation and Reflection in a Finite Domain

In the present paper, a finite element nonlinear coupled thermoelasticity formulation is presented for analysis of the wave propagation, reflection, and mixing phenomena in the finite length isotropic solids. The governing equations are derived based on the second Piola-Kirchhoff stress and the full form of Green’s strain-displacement tensors to account for the large deformations and finite strains. In contrast to the available researches, the assumption of very small temperature changes compared to the reference temperature is released in the present research. Galerkin’s method, a weak formulation, and cubic elements are employed to obtain the time-dependent non-linear finite element governing equations. The proposed solution procedure to the resulting highly nonlinear and time-dependent governing equations employs an updating algorithm and Newmark’s numerical time integration method. The wave propagation and reflection phenomena are investigated for both the mechanical and thermal shocks and time variations of distributions of the resulting displacements, temperature rises, and stresses are illustrated graphically and discussed comprehensively. Furthermore, the effects of the non-linear terms are discussed comprehensively. Results reveal that in the non-linear analysis, no fixed speed of wave propagation can be defined.

Programmable Self-Regulation with Wrinkled Hydrogels and Plasmonic Nanoparticle Lattices.

This paper describes a self-regulating system that combines wrinkle-patterned hydrogels with plasmonic nanoparticle (NP) lattices. In the feedback loop, the wrinkle patterns flatten in response to moisture, which then allows light to reach the NP lattice on the bottom layer. Upon light absorption, the NP lattice produces a photothermal effect that dries the hydrogel, and the system then returns to the initial wrinkled configuration. The timescale of this regulatory cycle can be programmed by tuning the degree of photothermal heating by NP size and substrate material. Time-dependent finite element analysis reveals the thermal and mechanical mechanisms of wrinkle formation. This self-regulating system couples morphological, optical, and thermo-mechanical properties of different materials components and offers promising design principles for future smart systems.

Determination of individual concentration-dependent diffusivity of binary gas-mixtures in reservoir-fluid systems

Diffusion is one of the main mass transfer principles associated with a gas injection process, e.g., vapor extraction (VAPEX) and cyclic solvent injection (CSI), to reduce the viscosity of heavy oil by dissolution of light gas(es). A few efforts have been made to estimate the diffusivity of each gas component of a binary gas-mixture in heavy oil as a constant, but no attempts have been made to determine individual diffusivity of each gas component in the binary gas-mixture as concentration-dependent. In this study, the concentration-dependent diffusivities of CO2 and C3H8 in a CO2/C3H8 mixture, which preferentially diffuses in a Lloydminster heavy oil, are evaluated implicitly as a power function of gas concentration, pressure, and temperature through the oil viscosity. The coefficient and exponent of such a power function for each gas are determined once the deviations between the measured and calculated values for oil swelling factors and composition of each gas dissolved in heavy oil at the completion of a dynamic volume analysis (DVA) test are minimized by applying the 1D time-dependent finite element method (FEM). Compared with the constant diffusivities for the same test, considering concentration-dependency of the dissolved gases reproduces the experimentally measured oil swelling factors and compositions at the end of the test with a significantly enhanced accuracy. Then, such obtained concentration-dependent diffusivities are extended to reproduce the experimentally measured swelling factors of a pressure-decay test performed with another CO2/C3H8 gas-mixture and the same heavy oil. From such an evaluation, it can be inferred that the coefficient and exponent of the diffusivity correlation associated with a binary gas-mixture are functions of pressure, temperature, feed gas composition, and test duration, respectively. This is because the coefficient and exponent for one gas component may be affected by the presence of the other as the latter alters the composition of heavy oil in which the former diffuses.

Lifetime seismic performance assessment of high-rise steel-concrete composite frame with buckling-restrained braces under wind-induced fatigue

Under a severe environment of multiple hazards such as earthquakes and winds, the life-cycle performance of engineering structures may inevitably be deteriorated due to the fatigue effect caused by long-term exposure to wind loads, which would further increase the structural vulnerability to earthquakes. This paper presents a framework for evaluating the lifetime structural seismic performance under the effect of wind-induced fatigue considering different sources of uncertainties. The seismic behavior of a high-rise steel-concrete composite frame with buckling-restrained braces (FBRB) during its service life is systematically investigated using the proposed approach. Recorded field data for the wind hazard of Fuzhou, Fujian Province of China from Jan. 1, 1980 to Mar. 31, 2019 is collected, based on which the distribution of wind velocity is constructed by the Gumbel model after comparisons. The OpenSees platform is employed to establish the numerical model of the FBRB and conduct subsequent numerical computations. Allowed for the uncertainties caused by the wind generation and structural modeling, the final annual fatigue damage takes the average of 50 groups of simulations. The lifetime structural performance assessments, including static pushover analyses, nonlinear dynamic time history analyses and fragility analyses, are conducted on the time-dependent finite element (FE) models which are modified in lines with the material deterioration models. The results indicate that the structural performance tends to degrade over time under the effect of fatigue, while the influencing degree of fatigue varies with the duration time of fatigue process and seismic intensity. The impact of wind-induced fatigue on structural responses and fragilities are explicitly quantified and discussed in details.