Detailed Finite Element Method Research Articles

A novel approach for the lifetime prediction and structural health monitoring of concrete sewer systems exposed to biogenic sulphide corrosion

A novel, combined experimental-modelling approach is presented for the estimation of the corrosion depth, load bearing capacity and lifetime of unreinforced concrete sewer pipes exposed to prolonged biogenic sulphide corrosion. The biogenic sulphide corrosion process is mimicked through two types of representative chemical experiments, namely: (i) long-term experiments performed under moderately controlled pH conditions, where dry-cast concrete cube samples are exposed to monthly refreshed sulphuric acid solutions with initial pH values of 3, 2 and 1 for a period of 12 months, and (ii) short-term experiments carried out under highly controlled pH conditions, in which dry-cast concrete disk samples are subjected to sulphuric acid solutions with almost constant pH values of 2, 1 and 0.5 for a period of two months. By applying X-ray diffraction, optical microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy analyses, insight into the micro-scale morphology and elemental composition of the corrosion profile is obtained. In addition, the corrosion front and depth characteristics are measured by conducting phenolphthalein tests and analyses of surface colour and texture of macro-scale samples. From the experimental results, the time course of the corrosion depth is calibrated with a model for sulphate attack under a constant pH level. The model formulation is combined with detailed finite element method results from the literature to predict the long-term load bearing capacity of a concrete sewer pipe. The model is subsequently generalised for sulphate corrosion under a varying pH level. When combined with the installation of pH measuring devices on the inside of sewer pipes, the current engineering model may serve as an excellent practical tool for continuously monitoring the structural health and predicting the lifetime of in-situ sewer systems subject to sulphate attack under random acidity fluctuations.

Nonlinear compact thermal modeling of self-adaptability for GaN high-electron-mobility-transistors using Gaussian process predictor and ensemble Kalman filter

Thermal issue has been regarded as one of the bottlenecks for GaN high-electron-mobility transistor (HEMT) performance and reliability, which highlights the importance of accurate thermal modeling. In the present work, we propose a GP (Gaussian process)-resistor–capacitor compact thermal model integrated with the ensemble Kalman filter (EnKF) to handle the nonlinear problems attributed to the temperature-dependent properties of GaN HEMTs under large-signal working conditions. The GP predictor is employed for the nonlinear correction term, with strong ability and extendibility to characterize various temperature-dependent relations resulting from different design configurations and materials. The model is identified via the EnKFs by inputting a sequence of channel temperature oscillations induced by imposing a large-signal continuous wave heating source to the device. Furthermore, an adaptation mode is devised for the in situ and timely update of the model parameters to adapt to the thermal variability of GaN devices, avoiding storing a large amount of historical data and repeated offline regressions. The validation of our modeling scheme is conducted through the case study on GaN-on-SiC HEMT’s detailed 3D finite element method simulations.

A computational framework for the lifetime prediction of vertical-axis wind turbines: CFD simulations and high-cycle fatigue modeling

A novel computational framework is presented for the lifetime prediction of vertical-axis wind turbines (VAWTs). The framework uses high-fidelity computational fluid dynamics (CFD) simulations for the accurate determination of the aerodynamic loading on the wind turbine, and includes these loading characteristics in a detailed 3D finite element method (FEM) model to predict fatigue cracking in the structure with a fatigue interface damage model. The fatigue interface damage model allows to simulate high-cycle fatigue cracking processes in the wind turbine in an accurate and robust fashion at manageable computational cost. The FEM analyses show that the blade-strut connection is the most critical structural part for the fatigue life of the VAWT, particularly when it is carried out as an adhesive connection (instead of a welded connection). The sensitivity of the fatigue response of the VAWT to specific static and fatigue modeling parameters and to the presence of a structural flaw is analyzed. Depending on the flaw size and flaw location, the fatigue life of the VAWT can decrease by 25%. Additionally, the decrease of the fatigue resistance of the VAWT appears to be mainly characterized by the monotonic reduction of the tensile strength of the adhesive blade-strut connection, rather than by the reduction of its mode I toughness, such that fatigue cracking develops in a brittle fashion under a relatively small crack opening. It is emphasized that the present computational framework is generic; it can also be applied for analyzing the fatigue performance of other rotating machinery subjected to fluid–structure interaction, such as horizontal-axis wind turbines, steam turbine generators and multistage pumps and compressors.

HTS Tape Mechanical Behavior Sensitivity on Material Properties and Thickness of Material Layers

Multilayer 2G high-temperature superconductor (HTS) tapes undergo various mechanical loading steps as part of a superconducting magnet operation. The mechanical loading steps include cool-down to cryogenic temperatures and Lorentz forces when powering up the magnets. Studying the mechanical behavior of the constituent layers in a HTS tape can reveal the strain or stress level in each layer and help in predicting the probability of mechanical failure or critical current degradation. A detailed finite element method (FEM) -based simulation models enable us to estimate the mechanical behavior in each layer. However, defining the model parameters for material mechanical properties or thickness of each material layer can have consid-erable uncertainty due to variation in manufacturing processes and missing measurements of material properties within these tapes. The material properties in thin layers may not exactly comply with bulk materials of the same kind. In this paper we present a sensitivity analysis for mechanical behavior dependence on material properties variation in the constituent layers of a HTS tape. The results give important insight about the accuracy and reliability of mechanical simulation results and guide the priorities in future material characterization. In addition, we will investigate the effect of the thickness of Hastelloy layer on effective macroscopic mechanical behavior of the tape. A nonlinear elastoplastic FEM model is used for performing the simulations.

Prediction of 2G HTS Tape Quench Behavior by Random Forest Model Trained on 2-D FEM Simulations

Detailed Finite Element Method (FEM) based simulations for 2 G HTS tapes return high quality results, but the computation takes a long time due to the non-linearity of superconducting properties and they needed high mesh density. This work describes a method for prediction of quench behavior in a long 2 G HTS tape based on a series of 2D FEM model simulations for short length of tape in many different conditions. The random forest model is trained by the set of results from the short-pieces FEM calculations. Subsequently the model can be applied to any length of HTS tape with similar thermal characteristics. Comparison of quench simulation in 10 cm long HTS tape between a detailed FEM model and a fully trained random forest model show that the predicted temperatures are within 0.68 %, while the computation time is significantly faster: The random forest model ran in less than 1 s, while the run time of the FEM model was 5:30 min.

Prediction of post-fire seismic performance of reinforced concrete frames

Due to the importance of evaluating the seismic behavior of structures after a fire, a numerical study was conducted on RC frames using the detailed finite element method. The simulation process consists of two separate steps. The first step is heat transfer analysis to estimate heat distribution in the various elements of RC frames, and the second step is evaluating the post-fire seismic performance of the frames using thermal–mechanical analysis. The test specimens used to verify the proposed FE model include two RC frames with different beam-column bending capacity ratios, which are in two different conditions, including the specimens at ambient temperature as control specimens and fired specimens inside of the furnace chamber. In the heat transfer analysis step, the sensitivity of responses to two effective parameters, including the moisture content and the emissivity of the concrete surface has been investigated. In the post-fire analysis, among the various models presented for the post-fire behavior of concrete, three stress–strain models were selected for cyclic analysis of RC frames and the results were compared with the test data. Also, the bond-slip behavior of the concrete-steel interface has been simulated using the equivalent springs technique. It was seen from the comparison of the numerical results with the test data, that the proposed model accurately estimates the resistance and the dissipated energy of frames in each cycle. In addition, parametric studies showed that every 30 min increase in the fire duration based on the ISO 834 standard curve causes a 10% decrease in post-fire seismic resistance compared to the control specimen.

An Open-Source Framework for Modeling RC Shear Walls Using Deep Neural Networks

Reinforced concrete (RC) shear walls macroscopic models are simplified strategies able to simulate the complex nonlinear behavior of RC shear walls to some extent, but their efficacy and robustness are limited. In contrast, microscopic models are sophisticated finite element method (FEM) models that are far more accurate and reliable. However, their elevated computational cost turns them unfeasible for most practical applications. In this study, a data-driven surrogate model for analyzing RC shear walls is developed using deep neural networks (DNNs). The surrogate model is trained with thousands of FEM simulations to predict the characteristic curve obtained when a static nonlinear pushover analysis is performed. The surrogate model is extensively tested and found to exhibit a high degree of accuracy in its predictions while being extremely faster than the detailed FEM analysis. The complete framework that made this study possible is provided as an open-source project. The project is developed in Python and includes a parametric FEM model of an RC shear wall in OpenSeesPy, the training and validation of the DNN model in TensorFlow, and an application with an interactive graphical user interface to test the methodology and visualize the results.

Understanding the piezoelectric response of ZnO nanotetrapods: Detailed numerical calculations

The complex tetrapod shape of zinc oxide nanostructure, which is constructed from four one-dimensional arms interconnected together via a central core, is a special 3D geometry with multifunctional applications in advanced technologies. The ZnO hexagonal wurtzite crystal lattice with a non-centrosymmetric structure introduces interesting piezoelectric property in nanorods in the bent state, which has been well reported and utilized in piezo- and tribo-electric nanogenerator applications. Considering the broad technological relevance of tetrapods, it is important to understand the piezoelectric response of zinc oxide tetrapods under different conditions. In this study, we explicate the fundamental mechanical and electrical properties of ZnO nanotetrapods (ZnO NTs) through a detailed finite element method analysis. On this basis, the effects of shape factors (including length, height, and aspect ratio) as well as connection strength and packing density on the deformation and piezoelectric potential of ZnO NTs are examined, offering guidance for the fabrication of ZnO NTs. This theoretical model and numerical simulation provide an avenue for further piezoelectric and piezotronic research of ZnO NTs.

Comparison of Modeling Methods for Off-Road Tires

Efficient navigation of off-road vehicles heavily relies on the ability to accurately model the interaction between the vehicle and the terrain. One of the most important parts of this interaction is the deformation of the terrain and the tire. Although high-precision methods like finite element method (FEM) simulation can be used for this purpose, they require significant computational power, which is impractical to install in a vehicle for real-time navigation purposes. Therefore, simplified and less-detailed models are essential for on-board installation in real-time applications. In this study, three two-dimensional static terrain–vehicle models are compared to a detailed FEM reference model, and the results are evaluated both from the perspective of accuracy and computational capacity requirements. The analysis sheds light on the effectiveness of each model in the real-time navigation of off-road vehicles.

Characterisation of a micrometer-scale active plasmonic element by means of complementary computational and experimental methods.

In this article, we investigate an active plasmonic element which will act as the key building block for future photonic devices. This element operates by modulating optical constants in a localised fashion, thereby providing an external control over the strength of the electromagnetic near field above the element as well as its far-field response. A dual experimental approach is employed in tandem with computational methods to characterise the response of this system. First, an enhanced surface plasmon resonance experiment in a classical Kretschmann configuration is used to measure the changes in the reflectivity induced by an alternating electric current. A lock-in amplifier is used to extract the dynamic changes in the far-field reflectivity resulting from Joule heating. A clear modulation of the materials' optical constants can be inferred from the changed reflectivity, which is highly sensitive and dependent on the input current. The changed electrical permittivity of the active element is due to Joule heating. Second, the resulting expansion of the metallic element is measured using scanning Joule expansion microscopy. The localised temperature distribution, and hence information about the localisation of the modulation of the optical constants of the system, can be extracted using this technique. Both optical and thermal data are used to inform detailed finite element method simulations for verification and to predict system responses allowing for enhanced design choices to maximise modulation depth and localisation.

Assessing the impact of rockfall on a bridge by using hybrid DEM/FEM analysis: A case study in Central Taiwan

Two destructive rockfalls occurred in central Taiwan, wherein falling blocks severely damaged an essential bridge of a mountain expressway. A series of field investigations were conducted to recognize the trajectories of the rockfalls and gather information on the falling blocks and bridge damage. According to the obtained field data, a hybrid model of the discrete element method (DEM) and finite element method (FEM) was developed to assess the rockfall impact on the bridge. A DEM analysis was first performed to simulate the rockfall process, which agreed with the actual condition. A detailed FEM was developed with strain-rate-dependent material models to investigate the process, which caused severe damage to the bridge and protective structures under the rockfall impact based on the calculated velocity and kinetic energy of rock blocks obtained from the DEM analysis. The simulated damage mechanisms agreed well with the field observations. The simulated maximum impact force was 4.5 times higher than the original design load and caused severe damage to the bridge structure. The thorough and objective investigation results of this rockfall event and the rigorous hybrid DEM-FEM analysis framework can be essential guidelines for authorities to formulate future hazard mitigation strategies for similar infrastructures under the threats of rockfall impacts.

Improvements for building-performance simulations by a comparative finite-element method analysis

This paper presents a method to improve building-performance simulations (BPS) by the comparison and analysis of different approaches based on finite-element method (FEM) models. The lumped parameter method (LPM) is used in several BPS programs and tools. It has the advantage of fast computing times and comparably good accuracy for thermal and energy loads. With the help of detailed FEM simulations, it is possible to further improve the degree of detail and accuracy while maintaining the high simulation speed.In this work, we compare time-dependent results for local temperatures in a generic reference room within a given periode of time. In a second step, the differences between the models with respect to various physical effects are analyzed and used to introduce additional equations into the LPM model in order to improve its accuracy. Thus, we discuss potentials for improvement for BPS and demonstrate a method of a practical implementation.The results show minor differences of less than 0.1 K for radiation and heat transfer, so their level of detail in BPS is appropriate. In these terms, no improvements were pursued within the work. However, the FEM simulation is capable of calculating the internal convective heat transfer and thermal bridges more accurately due to the use of computational fluid dynamics (CFD) and the geometrically precise representation of the FEM model. Here, deviations of up to 1 K in room temperature (convective heat transfer) and up to 0.5 K in wall temperatures (thermal bridges) were pointed out.By improving the LPM with equations obtained from the FEM, these deviations can be reduced to less than 0.2 K, which is a considerable improvement in accuracy.

Effect of biochemical attack on the mechanical performance of used concrete sewer pipes

The structural response and failure mechanisms of 35 used concrete sewer pipes are investigated by means of a combined experimental–numerical approach, whereby specific attention focuses on determining the effect by biochemical attack. Accordingly, the degree of degradation of the inner and outer surfaces of the pipes is carefully analysed by visual inspection, and subsequently categorized in 6 different surface condition classes. The type and degree of biochemical attack are determined by respectively performing X-ray diffraction analyses and phenolphthalein tests. The sewer pipes, which vary in age, size and geometry, are subjected to biaxial loading conditions in a full-scale test set-up. The experiments show that the material degradation of relatively old sewer pipes can be considerable, and for a large part may be attributed to the process of biogenic sulphide corrosion. This process typically induces a weak, corroded layer at the inside of the pipe, which is characterized by the appearance of exposed granulates and porous mortar between the granulates. The experimental failure responses of the used sewer pipes are compared against numerical results obtained from detailed Finite Element Method (FEM) simulations. The failure response of each sewer pipe type is well predicted by the FEM model if the negligible mechanical properties of the biogenic sulphide corrosion layer at the inside of the pipe are accounted for in the simulations. The relatively low concrete tensile strength of old sewer pipes and the reduced effective wall thickness due to biogenic sulphide corrosion cause that the ultimate bearing strength and structural stiffness of these pipes may be more than a factor of two lower than those of completely new sewer pipes. The results of the present work provide a scientific basis for the decision-making process on sewer rehabilitation and replacement, in particular by revealing and quantifying the influence on the structural bearing strength by biogenic sulphide corrosion. • Structural response and failure mechanisms of 35 used concrete sewer pipes are investigated via a combined experimental–numerical approach. • Effect of biochemical attack on bearing strength is quantified. • Mechanical collapse of sewer pipes is characterized by the development of 4 failure cracks. • Results provide a scientific basis for decision-making process on sewer rehabilitation and replacement.

Behaviour of the lockbolt demountable shear connector under combined shear and tension loading

A demountable ‘lockbolt’ shear connector (LB-DSC), consisting of a short partially threaded bolt that is locked on a steel flange to eliminate tolerance and initial slip issues and a compatible machined tube that can be fastened/unfastened over the bolt to facilitate the easy and fast disassembly process for reuse of all components, was proposed for use in composite floors or decks of buildings or bridges. This paper focuses on the numerical investigation of the structural behaviour of the LB-DSC under combined tension and shear loading. A detailed finite element method (FEM) model considering material and contact nonlinearity was built and calibrated based on previous standard pushout tests on the LB-DSC. The calibrated FEM model was used to investigate the effects of various parameters on tensile and shear behaviour of the LB-DSC, including the concrete strength of slab, the bolt diameter and strength, the steel tube thickness, and the presence and strength of infill grout. The shear resistance of the connector is significantly reduced when a tensile force more than 20% of the tensile resistance of the connector is applied. Design equations for predicting the tension resistance and tension-shear interaction are proposed to expand the application of the LB-DSCs in sustainable steel–concrete composite construction.

CMOS-MEMS Thermal-Piezoresistive Resonators and Oscillators for Sensors

Microelectromechanical systems (MEMS) are in widespread commercial use due to their compact size, high performance, and low cost. MEMS resonators have emerged as front runners for sensing (accelerometers, gyroscopes, and particulate matter) and frequency (RF front-end, filters, timing, and frequency source) applications. The excellent stability, resolution, and accuracy of resonators lead them an ideal candidate for sensor implementation. The CMOS-MEMS technology allows for rapid, large-scale, and low-cost manufacturing. Thermal–piezoresistive resonators (TPRs) are promising candidates due to their favorable potential with scaling and robust performance in the ambient environment. A detailed finite element method (FEM) simulation flow is presented along with a mathematical model for device optimization. The devices were fabricated with the commercial CMOS technology utilizing the front-end-of-line (FEOL) polysilicon and back-end-of-line (BEOL) materials like silicon dioxide and interconnect metal. The flexibility of selective material placement in layout and complex routing using multi-metal interconnect is employed to develop a balanced TPR design at 2 MHz. A 5-MHz bulk mode TPR was designed for mass sensing application. The fabricated devices were characterized, and their performance is compared with other state-of-the-art works. Finally, the developed devices were used in real-world applications for mass sensing and pressure sensing. The device achieved 20 kHz/ng. The TPR devices combine principles of Pirani gauge and resonant sensors for improving the sensing range from 2 to 760 Torr (1 atm).

Analysis of condensation and ventilation phenomena for double skin façade units

This paper presents a study of the thermo-hygrometric behaviour of a Double Skin Façade (DSF) unit. The study aims (i) at comparing currently used calculation procedures according to European and American standards (UNI EN ISO 10077, UNI EN ISO 12631:2018, ISO 15099:2003, ANSI/NFRC 100 for the thermal performance and ISO 13788:2012 (2012) for the condensation risk), and (ii) at assessing the 2D hygrothermal performance of a double skin module through a Finite Element Method (FEM)-based model. According to the current standards, a detailed characterization of thermal and fluid dynamic phenomena in closed and ventilated cavities is neglected and a simplified approach is proposed, which tends to overestimate the overall U-value of the curtain wall (UCW) due to an incremental thermal resistance that depends on the thickness of the air gap layer and the level of ventilation. The potential risk of this simplification is that the DSF estimated design performance, whilst complying with regulatory requirements, present inconsistencies respect to the real behaviour, impacting energy, comfort, material degradation, etc. Accurate assessments could be done already during design through detailed FEM multi-physic analyses. Nevertheless, those require a specific knowledge, are cost and time-consuming. As a first step, this study focuses on comparing the normed calculation approach for the design, against a detailed FEM-based multi-physics methodology. Specifically, this couples CFD, hygrothermal and Ray Tracing physics in a tool for the calculation of thermal transmittance, g-value and relative humidity of a DSF with a customizable geometry. As a second step, given a real DSF unit that showed unforeseen phenomena of surface condensation inside the cavity during several hours in spring and autumn, the multi-physic tool has been used to evaluate the condensation risk with the current and modified DSF design, under static and time-dependent boundary conditions.

Study of notched MEMS specimen: elasto-plastic modeling and experimental testing

This paper investigates the effect of stress and strains concentration, due to the notch presence, on the elasto-plastic behavior of gold microstructures subjected to tensile loading under electrostatic actuation. A kinematic model for the test microstructure which relates the experimentally measured deflection to the induced stress in the central specimen with applied electrostatic load is developed. The local maximum stress and strains at the notch root are analytically estimated using the Neuber’s rule and verified through a detailed non-linear coupled-field electric-structural finite element method (FEM)-based analysis. Several experimental tests are carried out to analyze the accumulation of plastic strain and the consequent development of plastic hinges induced in the central notched specimen due to repeated cyclic tensile loading by measuring the corresponding deflection with each loading cycle. The comparison between the failure condition observed experimentally in the test notched specimens and the FEM-based simulation results shows that the notch acts as stress and strains raiser fostering the initiation and expansion of plastic hinges in the thin film gold specimen which can lead to the specimen breakdown.

Pool evaporation under low Grashof number downward convection

We investigate the free evaporation of water into air from a small-scale (≈0.1 m) circular pool for low Grashof (Gr) number values under dominant downward flow motion. Repeatable experiments performed at atmospheric pressure for air temperatures at 290 K, 300 K and 310 K and relative humidity values ranging from 30% to 60%, are described and compared to detailed Finite Element Method (FEM) numerical simulations. For the free evaporation regimes considered here, a downward thermally-induced flow originated at the rim of the pool overcomes the concentration-induced buoyancy, consequently forcing the far-stream warm dry air to descend into the lower temperature air-water interface. Experimental results show that a steady-state recirculation zone near the air-water interface develops for Gr numbers greater than or equal to 50. The Sherwood number (Sh) for this geometry appropriately scales with Gr1/4 for drier free-stream boundary conditions, while it approaches a constant value for smaller mass transfer potentials (i.e., more humid environments). A stable Sherwood number (Sh correlation as a function of Gr values is proposed, covering Gr values from 10 to 105.

Development of a two-degree-of-freedom vibration generator for fabricating optical microstructure arrays.

Optical microstructure arrays on metallic surfaces are drawing ever-increasing attention due to the increasing requirements in optical systems. Although vibration generators are developed for generating optical microarrays with the ultra-precision diamond cutting process, the systematic research works on its mechanical design, working performance simulation, and numerical simulation of microstructure arrays has received less attention. In this study, a novel two-degree-of-freedom vibration generator (2DOF-VG) is designed based on the triangular amplification mechanism. To precisely simulate the working performance of this designed 2DOF-VG, the detailed multi-physics finite element method is proposed. Considering the three-dimensional geometric shape of the cutting tool, the cutting motion trajectory, and the elastic recovery of the workpiece material, the numerical simulation algorithm of the microstructure arrays generation is then established and used to precisely predict the surface topography of microstructure arrays. Finally, two types of unique microstructure arrays are fabricated, which demonstrates the feasibility and flexibility of the 2DOF-VG.

Modeling the cyclic behavior of composite plate shear walls/concrete filled (C-PSW/ CF)

This paper summarizes experimental results and discusses the development and benchmarking of detailed 3D finite element method (FEM) models for large-scale planar composite plate shear wall/concrete filled (C-PSW/CF) specimens subjected to constant axial force and cyclic lateral loading. The results from the benchmarked 3D FEM models are used to develop phenomenological effective stress-strain relationships for the steel plates and concrete infill of the tested C-PSW/CF specimens. The proposed effective stress-strain relationships implicitly account for various aspects governing behavior such as: (i) steel yielding and hardening in tension, (ii) steel yielding and local buckling in compression; (iii) concrete cracking in tension, (iv) concrete inelasticity in compression, and (v) composite interaction resulting in biaxial stresses in the tension steel and confinement of the concrete in compression. These proposed effective stress-strain relationships are used in simpler 2D FEM models and fiber-based FEM models along with calibrated cyclic hysteresis rules and damage models to simulate the cyclic behavior of C-PSW/CF specimens and compare with experimental results. These simpler 2D FEM models and fiber-based FEM models are recommended for simulating the seismic response of planar C-PSW/CF walls. The detailed 3D FEM models are recommended for conducting additional parametric studies investigating the structural behavior and design of planar C-PSW/CF walls.