Field Finite Element Research Articles

Numerical Calculation and Analysis on the Surface Electric Field Characteristics of Hot-Line Robot

Hot-Line robot (HLR) is an automatic transmission line inspection device developed in recent years. The mounting device should be contacted with the high voltage transmission line thus to realize the voltage transfer at the first operation stage of the HLR, and the HLR device which is regarded as a floating electrode might be damaged by discharge induced by the filed distortion during the voltage transfer process. The surface electric field of HLR device is optimized to improve its operation reliability, and a static-electric field finite element model of the transmission line which includes the HLR device is developed in this paper, and the dynamic surface electric filed characteristics of HLR during elevating operation process are studied by the finite elements model. Results show that influenced by the electric field which induced by high-voltage transmission line, the induced electric field can be generated on the surface of HLR during the process of elevating operation. The induced electric field distorts on the edge chamfer of HLR device which is regarded as a floating electrode. The maximum field strength position is influenced by the arrangement of the overhead wire conductor, and the edge chamfers are the key points of the electric field optimization of the HLR. The results of this study could provide theoretical basis and technical support for the optimization of HLR insulation characteristics, which is of great value to improve its operation reliability.

Field observations and finite element 3-D analysis of soil displacements close to unsupported excavation

Soil displacements due to unsupported deep excavation may cause severe damages to the nearby structures and foundations systems. Such excavations affect the state of stresses and displacements field of the surrounding soil. In this study, the soil displacements at five observation points were continuously monitored for23 days, which was the time period of excavation of about 7 m deep open tunnel. The reference points were installed on the ground surface at horizontal distance ranging from 1.25 to 3.25 m from tunnel excavation edge. The construction work was related to the project of developing the Army Channel/ Zeyouna section in the Eastern part of Baghdad City. The field observations indicate upward vertical displacement and outward lateral displacements during the first 10 days when the excavation depth was within 4 m. After that, the displacement trend was generally reversed. The displacement values were within 12 mm and vary from one observation point to another depending on the lateral distance of each point from excavation edge. The finite element package PLAXIS 3D was used to simulate the problem after obtaining the required soil parameters by an extensive site investigation. The analysis results in general, compare well with the field observations in terms of soil displacements at the reference points, especially during the first 10 days. This finding may reveal the reliability of the analysis results at other locations in the surrounding soil.

Geoacoustic sediment model discrimination using acoustic color comparison

Many competing geoacoustic models exist to represent sandy sediments. These models can be fluid, elastic, or poroelastic, depending on the number and types of waves the sediment is assumed to support. In this talk, a fully scattered field finite element approach is utilized to construct acoustic color templates for a buried spherical aluminum shell and a spherical bubble. The simulated results are then used to assess the ability of target strength measurements to discriminate between the predictions of the competing sediment models. Such discrimination may help determine the physical validity of a given geoacoustic model and aid in model selection. Five geoacoustic sediment models are considered: a simple fluid, constant-Q fluid model, the effective density fluid model of Williams, the viscous grain shearing model of Buckingham, the Biot-Stoll poroelastic model, and the extended Biot model of Chotiros. [Work supported by ONR, Ocean Acoustics.]

Earth pressures on the trenched HDPE pipes in fine-grained soils during construction phase: Full-scale field trial and finite element modeling

Abstract High density polyethylene (HDPE) pipes have been widely used in civil engineering applications. The peaking deflection of the buried HDPE pipes caused by the compaction of the pipe sidefill during the construction phase is not considered in the current pipe design standards. Fine-grained soil may be an alternative backfill when relative high quality coarse-grained soil is absent in-situ or difficult to transport from a long-distance away in urban areas. The effect of construction on the earth pressures around HDPE pipes buried in fine-grained backfills has not been well understood. In this study, a full-scale field trial was undertaken to monitor the earth pressures acting on HDPE pipes buried in fine-grained soils during the construction phase. Three HDPE pipes with norminal diameters of 600, 600, and 300 mm, respectively, were used in the field trial. Numerical modeling which was able to consider the peaking deflection was conducted to investigate the effects of pipe diameter, pipe stiffness, degree of compaction of the backfill, and trench width on the earth pressures acting on the pipes. The measured earth pressures and deflection were utilized to verify the effectiveness of numerical modeling procedures. The finite element analyses indicated that the trench width had a more significant effect on the earth pressures, as compared to other factors. Two empirical equations were proposed, based on the numerical results, for predicting the earth pressures at the top and the springline of the pipes during the construction phase. The validity of the proposed equations was evaluated using the field data obtained from both this study and published studies.

A homogenization model for porous ductile solids under cyclic loads comprising a matrix with isotropic and linear kinematic hardening

In this work, we propose a semi-analytical micromechanical model to study the elasto-plastic response of porous materials subjected to cyclic loading with isotropic and linear kinematic hardening at finite strains. To this end, we use an approximate but numerically efficient decoupled homogenization strategy between the elastic and plastic parts. The resulting effective back stress in the porous solid, similar to the macroscopic stress and plastic strain, has non-zero hydrostatic terms and depends on the porosity, the void shape and orientation as a result of the homogenization process. Subsequently, a complete set of equations is defined to describe the evolution of the microstructure, i.e., void volume fraction (porosity), (ellipsoidal) void shape and orientation both in the elastic and the plastic regimes. The model is then numerically implemented in a general purpose user-material subroutine. Full field finite element simulations of multi-void periodic unit cells are used to assess the predictions of the proposed model. The latter is found to be in good qualitative and quantitative agreement with the finite element results for most of the loading types, hardening parameters and porosities considered in this study, but is less accurate for very small porosities. The combined analytical and numerical study shows that elasticity is an important mechanism for porosity ratcheting in addition to strain hardening. Specifically, in order to recover the main qualitative features of porosity ratcheting for all cyclic loads considered in the present study, it is shown to be critical to take into account the evolution of the microstructure not only during the plastic loading, as is the usual hypothesis, but also during elastic loading. Finally, the effect of isotropic and linear kinematic hardening is found to be highly non-monotonic and non-trivial upon porosity ratcheting for most cases considered here.

Scattering from objects buried in sandy sediments using a fully scattered field finite element approach

The finite element method is utilized to study the scattering response from objects buried in a homogeneous sand half-space. An approach is adopted where the total field is separated into contributions due to the incident, reflected, transmitted fields in the absence of the buried object and the scattered field due to the presence of the buried object. Since the incident, reflected, and transmitted fields can be determined analytically for the case of a flat water-sediment interface, only the scattered field needs to be solved for numerically. This approach results in much faster run times while reducing spurious reflections from the boundaries of the computational domain. The buried object and the sediment half-space can be treated as fluid, viscoelastic, or poroelastic. [Work supported by ONR, Ocean Acoustics.]

J-Integral Calculation by Finite Element Processing of Measured Full-Field Surface Displacements

A novel method has been developed based on the conjoint use of digital image correlation to measure full field displacements and finite element simulations to extract the strain energy release rate of surface cracks. In this approach, a finite element model with imported full-field displacements measured by DIC is solved and the J-integral is calculated, without knowledge of the specimen geometry and applied loads. This can be done even in a specimen that develops crack tip plasticity, if the elastic and yield behaviour of the material are known. The application of the method is demonstrated in an analysis of a fatigue crack, introduced to an aluminium alloy compact tension specimen (Al 2024, T351 heat condition).

Overheated pipe due to scale: Field failure investigation and finite element analysis

Through a field failure investigation and finite element analysis, a study on overheated pipe due to scale was performed. The field failure samples were collected from primary reformer convection tube made of material based on ASTM A 335 grad P91 modified specification. The type of the object was a fin tube coil. The tube shows rupture with the shape of fish mouth accompanied by bulging. The analysis performed using finite element mimics the actual field conditions. It was discovered that long term overheating due to scale both at the outside and in the inside of the tube hinders the smooth heat transfer process. This consequently leads to poor heat transfer and violates the original design philosophy. Metallurgical evaluation proves this verdict. Furthermore, the finite element analysis also supports the conclusion.

Coupled field modeling of E/M impedance of piezoelectric wafer active sensor for cataphoretic coating thickness measurement

The cataphoretic electro-coating is one of the most common methods that are used against corrosion as a primary coating layer. The cataphoretic electro-coating is commonly utilized technique especially in protecting of automobile components in automotive industry. This coating method has many advantages such as high corrosion resistance, ability of homogeneous and complete coating of components in any geometry, less pollution, and less risk of ignition. In this study, some specimens in the form of steel sheets coated by the cataphoretic electro-coating method are examined using electro-mechanical impedance spectroscopy (EMIS) method. One of the extensively employed sensor technologies has been permanently installed piezoelectric wafer active sensor (PWAS) for in situ continuous structural health monitoring (SHM). Using the transduction of ultrasonic elastic waves into voltage and vice versa, PWAS has been emerged as one of the major SHM sensing technologies. EMIS method has been utilized as a dynamic descriptor of PWAS and the structure on which it is bonded. EMIS of PWAS-structure couple is a high frequency local modal sensing technique by applying standing waves to indicate the response of the PWAS resonator by determining the resonance and anti-resonance frequencies. To simulate the actual EMIS measurements in the present work, two-dimensional and three-dimensional coupled field finite element models are created for both uncoated and coated steel plates in a commercial FEA software, ANSYS®. The EMIS values of the specimens in certain sizes and coated in different thickness are going to be simulated in broad-band of frequency spectra. The thickness of the coating layer and coating time are of paramount importance for the corrosion resistance. The coating layer thickness and the corresponding coating period will be optimized by analyses of the values obtained from the 2D and 3D EMIS simulations.

On characterizing spatially variable soil Young’s modulus using spatial average

The purpose of this study is to investigate whether the effective Young’s modulus (Eeff) for a spatial variable soil mass can be strongly correlated to any spatial average. The spatially variable Young’s modulus of the soil mass is modeled as a stationary lognormal random field, and the Eeff of the soil mass is simulated by random field finite element analysis. Spatial averages are calculated from the input random field. If a strong correlation exists, it is possible to replace a random field analysis by a simpler random variable analysis. Two classes of problems are considered: a soil cube subjected to displacement-controlled compression and a footing problem. For the soil cube problem, Eeff is found to be strongly correlated to a suitable spatial average. However, for the footing problem, only the statistics (mean and standard deviation) of Eeff can be well approximated by a suitable spatial average, but the correlation is not strong. It is possible that the two classes of problems behave differently because the finite elements in the soil cube are mobilized uniformly, whereas those in the footing problem are mobilized non-uniformly. This leads to a weighted spatial average model that applies a different weight on the log modulus of each finite element over the domain being averaged.

Evaluation of top-down cracking potential for asphalt pavements with 4.75 mm nominal maximum aggregate size mixture layer using full-scale field tests and finite element analysis

This study primarily focused on evaluating top-down cracking resistance for pavement structures with 4.75 mm nominal maximum aggregate size asphalt mixtures (i.e. SP-4.75) that will help determine more effective application of these mixes for layered flexible pavement system. Crack evaluation and pavement surface response measurements were conducted using full-scale field test sections. In addition, a three-dimensional (3-D) viscoelastic finite-element (FE) pavement model was developed to further validate the results of field tests using analytical solution. The results indicated that the pavement structure with SP-4.75 mixture at the top becomes more prone to top-down cracking when a thin layer with unmodified asphalt binder is applied. It was found that both layer thickness and binder type are key parameters to control top-down cracking potential. Also, the comparative observations between analytical and experimental results confirm that the FE model developed was able to provide accurate, reliable, and realistic prediction for change in pavement surface strain responses due to different conditions of SP-4.75 layer application. It was concluded that adequate thickness and binder type should be considered for proper application of SP-4.75 mixture as a surface course of layered flexible pavement system to effectively resist top-down cracking.

Analysis of the effect of repair welding/grinding on the performance of railway crossings using field measurements and finite element modeling

In this paper, the effects of repair welding and grinding, which are currently the main components used in the maintenance of crossings, on the performance of crossings are analyzed. It has been observed that sometimes the welding and grinding activities that directly affect the geometry of the crossings and/or material properties can have negative effects on the performance and ultimately on the service life of the crossings. In this paper, the effect of the changes of geometry has been studied experimentally, while the effect of the changes in the material properties has been analyzed using a numerical model. When grinding the shape of the crossing nose, the resulting profile can deviate from the original one. To analyze the geometry-related effects of welding and grinding, the geometry of crossings (cross-sectional profiles) as well as the corresponding dynamic accelerations due to passing trains are measured before and after the welding and grinding activities. Based on the comparison of the measured accelerations, the performance of the measured crossings has been assessed. Also, a welding repair that is not properly performed can lead to undesirable changes in the material properties of the rails, resulting in defects in rails. The material-related effects of the welding and grinding are studied using the three-dimensional explicit finite element model wherein a wheelset moves over a railway crossing. To understand the microstructure of the welding defect and provide an input for the numerical model, the results of ultrasonic and microscopic analyses of some welded crossings are presented first. Then, a number of the numerical simulations of the crossing with the welding defect are performed to investigate the failure mechanism of the crossing. Furthermore, assessment using the fatigue model (coupled with the finite element model) that accounts for the ratcheting behavior of material by calculating a number of the load cycles to the crack initiation is performed. Finally, conclusions on the effects of changes in geometry and material of the crossings due to repair welding and grinding are given.

Dynamic assessment of a FRP suspension footbridge through field testing and finite element modelling

The use of advanced fibre composite materials in bridge engineering offers alternative solutions to structural problems compared to traditional construction materials. Advanced composite or fibre reinforced polymer (FRP) materials have high strength to weight ratios, which can be especially beneficial where dead load or material handling considerations govern a design. However, the reduced weight and stiffness of FRP footbridges results in generally poorer dynamic performance, and vibration serviceability is likely to govern their design to avoid the footbridge being too lively. This study investigates the dynamic behaviour of the 51.3 m span Wilcott FRP suspension footbridge. The assessment is performed through a combination of field testing and finite element analysis, and the measured performance of the bridge is being used to calibrate the model through an updating procedure. The resulting updated model allowed detailed interpretation of the results. It showed that non-structural members such as the parapets can influence the dynamic behaviour of slender, lightweight footbridges, and consequently their contribution must be included during the dynamic assessment of a structure. The test data showed that the FRP footbridge is prone to pedestrian induced vibrations, although the measured response levels were lower than limits specified in relevant standards.

Effects of cross-correlated multiple spatially random soil properties on wave-induced oscillatory seabed response

The evaluation of seabed response under wave loading is important for prediction of stability of foundations of offshore structures. In this study, a stochastic finite element model which integrates the Karhunen-Loève expansion random field simulation and finite element modeling of wave-induced seabed response is established. The wave-induced oscillatory response in a spatially random heterogeneous porous seabed considering cross-correlated multiple soil properties is investigated. The effects of multiple spatial random soil properties, correlation length and the trend function (the relation of the mean value versus depth) on oscillatory pore water pressure and momentary liquefaction are discussed. The stochastic analyses show that the uncertainty bounds of oscillatory pore water pressure are wider for the case with multiple spatially random soil properties compared with those with the single random soil property. The mean pore water pressure of the stochastic analysis is greater than the one obtained by the deterministic analysis. Therefore, the average momentary liquefaction zone in the stochastic analysis is shallower than the deterministic one. The median of momentary liquefaction depth generally decreases with the increase of vertical correlation length. When the slope of the trend function increases, the uncertainty of pore water pressure is greatly reduced at deeper depth of the seabed. Without considering the trend of soil properties, the wave-induced momentary liquefaction potential may be underestimated.

Investigation of Distortion-Induced Web-Gap Cracking in a Seismically Retrofitted Steel Bridge: Repair Measures

AbstractThis paper studies distortion-induced fatigue cracks in a 1960s-era welded plate girder bridge that developed cracks in the web-gap region shortly after completion of a comprehensive seismic retrofit. Finite-element analysis (FEA) and field testing were conducted to investigate the cause of cracking and then to recommend appropriate repair measures for the bridge. The field test results validated the high-stress concentrations in the web-gap region of the bridge that led to fatigue cracking after only a limited number of cycles. FEA results indicated that the high-stress concentrations were principally a result of replacing K-type diaphragms with stiffer cross diaphragms as part of the seismic retrofit. Two conventional repair measures in addition to two innovative repair measures were evaluated. The proposed innovative repair measures focused on ease of installation compared with more conventional options and were found to reduce the distortion-induced stresses well below the constant amplitude f...

Application of a self-adaptive Kalman filter approach in alignment control for an extra long span rail transit cable-stayed bridge

Extra long span rail transit bridges are generally slender and sensitive to excitations, and alignment control of the bridges should be conducted during construction. In this paper, the self-adaptive Kalman Filter (SKF) method was applied to the alignment control of the second longest rail transit bridge in the world. Each influential factor of the bridge alignment was firstly introduced. Then, the finite-element analysis and field tests of the bridge were carried out to obtain the influential factors that induced bridge cambers quantitatively. Combining with the cambers, the SKF method was applied to the alignment control of the bridge. Comparisons of the SFK-predicted alignment with the target alignment or the field measured alignment were carried out. Results show the SKF for alignment control of bridges is feasible and reliable.

A micromechanical constitutive modeling of WC hardmetals using finite-element and uniform field models

For constitutive modeling of WC hardmetals we used a full field finite element simulations of accurately reproduced scanning electron microscope images and a simple uniform field two-phase model. Both models combine isotropic Drucker–Prager elastic–plastic behavior of WC grains and isotropic von Mises elastic–plastic behavior of the binder, and include non-linear hardening. We performed simulations for representative volume elements using the generalized 2.5D formulation and demonstrated a good agreement between the two models in terms of the effective mechanical behavior. Effective elastic properties and coefficients of thermal expansion were obtained. Effective yield stresses evaluated at 0.1% of effective plastic strain were also computed for six different loading paths. Probability and joint probability histograms obtained in FE simulations are presented. We also studied the effect of residual thermal stresses, which appear in WC hardmetals due to cooling from sintering temperatures. Finally, we obtained a realistic yield surface for a three-dimensional microstructure using a uniform field model with spherical inclusions. This surface combines a Drucker–Prager region for moderate pressures and a von Mises region for high pressures, with a sharp transition between these two regions. Four different WC hardmetal grades were considered. In total, nine microstructures were reproduced in finite element models with the binder content ranging from 10% to 19%. A sensitivity study on the binder plastic properties was carried out, thus the obtained results for the yield surface are applicable to real hardmetals with different binder materials and various binder content.

Numerical modeling of mullions in the Taili high deformation zone, North China: Implications for the rheology of granitic rocks

This paper presents a combined field measurement and finite element modeling analysis of the mullions occurring on the contact of two granitic rocks with different grain size in the Taili High-Strain Deformation Zone (THDZ), West Liaoning of North China. All of the field data are located in the plot zone of the modeling results. Numerical modeling results indicate that: (1) The inter-angle between the tangent lines cross the cusp point and the ratio R of amplitude and width of mullions are the most effective parameters to describe the geometric shape and evolution of mullions, as well as useful indicators of the rheology of rocks. (2) The competence contrast controls the growth rate of mullions under shortening. It determines the possible ratio R of final mullions. Moreover, decreasing of the cusp angle in high competence contrast materials is faster than that in low competence contrast model. (3) The initial disturbance is an essential factor for the generation of mullions. Those contacts with higher initial disturbance will develop into mullions more easily and have a high growth rate during the same shortening deformation regime. (4) The rheology and deformation behavior of the granitic rocks in the study area are primarily controlled by the grain sizes of quartz and feldspar. The effective viscosity ratio of biotite adamellite and granitic gneisses is about 0.01–0.5. The deformation mechanisms of these granitic rocks should be dominated by a grain-size-sensitive diffusion creep.

Structural Health Monitoring Using Piezoelectric Sensors and Actuators

Structural health monitoring (SHM) is an inevitable component of modern civil and aerospace structures. It essentially detects the damages in the system by evaluating the performance parameters by the integration of sensing and possibly also actuation devices into the structure. In this paper damage detection process in an aluminium cantilever plate using piezoelectric sensors and actuators is simulated. Possible root damage such as transverse crack and longitudinal crack are studied. The results are compared with undamaged case. The plate is actuated using PZT actuators and performance is evaluated using PVDF sensors. Modelling for PZT/PVDF and structural parts are carried out using coupled field finite element PLANE223 ofAnsys(TM) finite element package. Strain response at the root of the cantilever plate is captured as the voltage output of the PVDF sensor. Strain response is directly related to the voltage generated in the PVDF sensor. The percentage variation of the fundamental frequency is found to be less than 3% in the present study and hence it is not taken as an index of damage. The transient voltage response captured with the transient coupled field analysis shows variation up to 24% as a signature between damaged and undamaged systems. Therefore, the present study suggests transient response evaluation using PVDF sensor is a suitable evaluation technique for the cases under consideration.

Uncertainty analysis of the effect of grout injection on the deformation of multi-wythe stone masonry walls

The eighteenth century Prince of Wales Fort in Manitoba Canada has experienced extensive freeze-thaw damage and mortar washout within the escarp walls resulting in distress and failures at multiple locations. Injection of grout could counteract this degradation of structural stability. However existing literature provides little guidance as to the improvement level that could be expected, especially with respect to out-of-plane performance. As such, the proposed treatment was modelled to include a high level of uncertainty in the system through the application of a Random Field Finite Element Micro-modelling technique. A Latin Hyper cube simulation method was used in conjunction with a parametric finite element model to randomize the material properties of each stone and relevant grouting layers. The numerical results predicted that the stone-grout bond was the most critical parameter in the proposed intervention, and that in the grouted wall sections, collapse would be avoided and lateral displacements stabilized with the proposed treatment.