High-resolution Finite Element Analysis Research Articles

Highly efficient inverse lumped modeling for the pre-strained circular dielectric elastomer

The equi-biaxially pre-strained circular dielectric elastomer actuator (PCDEA) is a classical actuator configuration to demonstrate the electromechanical deformation mechanism of dielectric elastomer. Till now, there still lacks an effective method to analyze the non-uniform voltage-induced deformation of PCDEA with both high accuracy and low computational cost. Besides, the inverse problem that determines the voltage input for a desired deformation is also a hard problem for the current modeling approaches. In this work, a novel shape function approximation method is proposed to model the non-uniform deformation of PCDEA in a highly accurate lumped-parameter way. The method can solve both the inverse problem and forward problem of the PCDEA, making it possible to position the onset of electromechanical instability (EMI) of this configuration. Detailed comparisons are made between this method and high-resolution finite element analysis to verify its accuracy. This method also features low computational cost, providing opportunities for developing model-based real-time control and proprioception techniques. Further investigation is also carried out on how material types and configurational parameters interact with PCDEAs’ performance, along with checking the findings with experiment observation. Guidelines for DE material development and actuator design are also summarized accordingly.

A combined experimental and numerical method to estimate the elastic modulus of single trabeculae

The elastic modulus at the single trabecular level is an important parameter for the understanding of the mechanical behavior of trabecular bone. Current methods are commonly limited by the irregular trabecular shape and the accuracy of displacement measurement. The aim of this study was to propose a method to estimate the trabecular modulus overcoming some of these limitations. For high-precision displacement measurements, in-situ compression within a synchrotron radiation based X-ray tomograph was used. Trabecular displacements were subsequently estimated by a global digital volume correlation algorithm, followed by high-resolution finite element analyses to account for the irregular geometry. The trabecular elastic moduli were then estimated by comparing the loads from the finite element analyses with those of the experiments. With this strategy, the average elastic modulus was estimated to 3.83 ± 0.54 GPa for three human trabeculae samples. Though limited by the sample size, the demonstrated method shows a potential to estimate the mechanical properties at the single trabecular level.

Analytical investigation of a new direct Column-to-Cased shaft connection

In regions of high seismicity, the large seismic design forces sustained by the superstructure must be transferred to the bridge foundations. In these situations, in particular in situations with weak soils, cased shafts, which are concrete filled steel tube (CSFT) components, are used for piles and drilled shafts. Prior research shows that these components have large flexural, axial and shear strengths. Typically, the forces are transferred from the column to the pile through a pile cap. However, direct connections are becoming more common, in particular because they have efficiency in terms of placement and can facilitate accelerated bridge construction. This research studies two different connections: (1) direct embedment of the column reinforcement into the pile and (2) an enhanced connection using a ring at the top of the tube in the pile to enhance mechancial bond and reduce embedment depth. The study was conducted using a high-resolution finite element analysis modeling approach validated using large-scale test results of reinforced concrete (RC) and CFST components and connections. The model included advanced constitutive modeling of the concrete as well as modeling of the bond between the steel tube and concrete fill and the reinforcing steel and the concrete fill. The transfer of forces from the yielding reinforcing bars in the RC pier to the concrete fill and tube of the pile is critical for this connection. Different approaches were investigated to achieve transfer without loss of strength or ductility. A supplemental rib inside the tube ultimately was used to enhance the load transfer with the minimum embedment length. The model was used to conduct a parametric study with the following study parameters: the rib location (lR), tube diameter (D), reinforcing bar diameter, and embedment depth (ld). The results were used to propose initial design recommendations.

Low-order model for impact load prediction on walls of navigation structures

As commercial waterway vessels such as groups of interconnected barges (i.e., barge flotillas) traverse navigable US waterways, collision incidents may occur on nearby structures. Navigation structures such as guide walls are therefore designed to resist loads that arise during collision events. Previous studies involving full-scale experimental impact testing and high-resolution finite element (FE) impact simulations revealed that barge impact loads on wall structures are transient (dynamic) and can be influenced by: flotilla impact-speed, mass, stiffness, and configuration, as well as impacted wall mass and stiffness. Empirical load prediction equations intended for use over wide ranging wall stiffness and mass characteristics often introduce excess conservatism into the design process. Accordingly, the present study was undertaken to develop a fast-running analysis methodology capable of accurately computing impact loads for navigation wall structures with varying mass and stiffness characteristics. Typical “run-times” required by the methodology are short enough so as to facilitate probabilistic design, wherein thousands of such analyses may be required. A standalone “low-order” dynamic analysis procedure was formulated to treat flotilla-wall systems in a simplified low-resolution manner, as contrasted to computationally-expensive high-resolution finite element analysis. The low-order procedure produces time-varying impact loads and wall displacements that are sufficiently accurate for design, yet requires drastically reduced computational resources in comparison to high-order approaches. Presented herein is derivation of an equation of motion for (low-order) barge flotilla impacts on rigid and flexible wall structures. Demonstration of suitable accuracy is achieved by comparing results obtained from the low-order procedure to those obtained from high-resolution impact simulations.

Numerical investigation on progressive collapse resistance of steel-concrete composite floor systems

Steel-concrete composite floor systems are commonly used in steel buildings. Stability under gravity and lateral loading depends on these systems. To better understand the design parameters that govern the progressive collapse behaviour of steel-concrete composite floor systems, high-resolution finite-element analyses (FEA) is conducted. Steel fracture and concrete damage are explicitly considered in the finite-element model (FEM). The model is validated using full-scale test data including comparison of the measured response and failure modes. The validated model is used to conduct a parametric study to investigate untested parameters including (i) concrete strength, (ii) horizontal constraint, (iii) steel deck thickness, (iv) reinforcement ratio in the composite slab, and (v) the number of the shear studs. The simulation results are used to determine the optimum design to increase progressive collapse resistance. Specifically, it is found that increasing the continuity and the horizontal constraint of the steel deck, the steel deck thickness, and number of shear studs increase this resistance. Considering the steel consumption and the common practice in the construction, improving the continuity and the horizontal constraint of the steel deck are the best choices to improve the collapse resistance.

High Resolution Modeling and Modeling of Connections in Pony-Truss Bridges

Pony-truss bridges are usually modeled using simplifying assumptions, such as pinned connections and analyzing the trusses separately from the rest of the structure. Pony-truss bridges are also generally designated as possessing many Fracture Critical Members (FCM), a characteristic that requires more rigorous inspections. Eliminating or reducing the number of FCM is desirable as it reduces inspection costs. A high-resolution model of a pony-truss bridge was assembled in ABAQUS CAE that explicitly models the behavior of the gusset plates in the connections and the angles in the framed stringer connections. This model indicated alternative load paths that are potential sources of redundancy useful for retrofit or reclassification of bridges with FCM designation. Higher-resolution models of framed stringer connections were also made in ABAQUS CAE using solid elements explicitly modeling the bolted connections. The connections were loaded in tension; what the stringers must hold if they supply an alternative load path for the truss in the event of fracture in the bottom chord. The primary objective of the work described is to provide high-resolution finite element analysis results that can be used to create general behavior for lower-resolution finite element models used in fracture critical analyses to reclassify FCM as non-fracture critical. The results of high-resolution modeling reveal the behavior that is necessary to include in these lower-resolution models, as well as potential alternative load paths. Lower-resolution models are assembled in SAP2000 and matched to the higher-resolution models.

Peri-implant stress correlates with bone and cement morphology: Micro-FE modeling of implanted cadaveric glenoids.

Aseptic loosening of cemented joint replacements is a complex biological and mechanical process, and remains a clinical concern especially in patients with poor bone quality. Utilizing high resolution finite element analysis of a series of implanted cadaver glenoids, the objective of this study was to quantify relationships between construct morphology and resulting mechanical stresses in cement and trabeculae. Eight glenoid cadavers were implanted with a cemented central peg implant. Specimens were imaged by micro-CT, and subject-specific finite element models were developed. Bone volume fraction, glenoid width, implant-cortex distance, cement volume, cement-cortex contact, and cement-bone interface area were measured. Axial loading was applied to the implant of each model and stress distributions were characterized. Correlation analysis was completed across all specimens for pairs of morphological and mechanical variables. The amount of trabecular bone with high stress was strongly negatively correlated with both cement volume and contact between the cement and cortex (r = -0.85 and -0.84, p < 0.05). Bone with high stress was also correlated with both glenoid width and implant-cortex distance. Contact between the cement and underlying cortex may dramatically reduce trabecular bone stresses surrounding the cement, and this contact depends on bone shape, cement amount, and implant positioning.

Oldest Pathology in a Tetrapod Bone Illuminates the Origin of Terrestrial Vertebrates

The origin of terrestrial tetrapods was a key event in vertebrate evolution, yet how and when it occurred remains obscure, due to scarce fossil evidence. Here, we show that the study of palaeopathologies, such as broken and healed bones, can help elucidate poorly understood behavioural transitions such as this. Using high-resolution finite element analysis, we demonstrate that the oldest known broken tetrapod bone, a radius of the primitive stem tetrapod Ossinodus pueri from the mid-Viséan (333 million years ago) of Australia, fractured under a high-force, impact-type loading scenario. The nature of the fracture suggests that it most plausibly occurred during a fall on land. Augmenting this are new osteological observations, including a preferred directionality to the trabecular architecture of cancellous bone. Together, these results suggest that Ossinodus, one of the first large (>2m length) tetrapods, spent a significant proportion of its life on land. Our findings have important implications for understanding the temporal, biogeographical and physiological contexts under which terrestriality in vertebrates evolved. They push the date for the origin of terrestrial tetrapods further back into the Carboniferous by at least two million years. Moreover, they raise the possibility that terrestriality in vertebrates first evolved in large tetrapods in Gondwana rather than in small European forms, warranting a re-evaluation of this important evolutionary event.

Lorentz force evaluation: A new approximation method for defect reconstruction

We propose a new method for contactless, nondestructive evaluation of moving laminated conductors, the so-called Lorentz force evaluation (LFE). The Lorentz force (LF) exerting on a permanent magnet moving relative to the specimen is measured. We propose a novel fast forward calculation of the LF based on a three-dimensional finite volume discretization of the specimen and an approximation of defects using local current distributions in the defect region. The approximate solution is compared with solutions from detailed finite element models developed for parallelepipedic subsurface defects. We obtain differences in LF that range between 1.7% and 6.7%, indicating that our approximation method yields sufficient performance. Furthermore, a linear inverse solution based on the novel forward method is presented. We invert the experimental data measured from a subsurface flaw with the dimensions of 2mm×2mm×12mm located within a laminated conductive bar. The reconstruction method yields the correct position of the flaw with an accuracy of 1mm in each direction. The reconstruction results are compared with high-resolution finite element analysis of the same crack configuration. We obtain correct lateral positions of the cracks, although the depth estimation shows a slight bias.

Local Mechanical Stimuli Regulate Bone Formation and Resorption in Mice at the Tissue Level

Bone is able to react to changing mechanical demands by adapting its internal microstructure through bone forming and resorbing cells. This process is called bone modeling and remodeling. It is evident that changes in mechanical demands at the organ level must be interpreted at the tissue level where bone (re)modeling takes place. Although assumed for a long time, the relationship between the locations of bone formation and resorption and the local mechanical environment is still under debate. The lack of suitable imaging modalities for measuring bone formation and resorption in vivo has made it difficult to assess the mechanoregulation of bone three-dimensionally by experiment. Using in vivo micro-computed tomography and high resolution finite element analysis in living mice, we show that bone formation most likely occurs at sites of high local mechanical strain (p<0.0001) and resorption at sites of low local mechanical strain (p<0.0001). Furthermore, the probability of bone resorption decreases exponentially with increasing mechanical stimulus (R2 = 0.99) whereas the probability of bone formation follows an exponential growth function to a maximum value (R2 = 0.99). Moreover, resorption is more strictly controlled than formation in loaded animals, and ovariectomy increases the amount of non-targeted resorption. Our experimental assessment of mechanoregulation at the tissue level does not show any evidence of a lazy zone and suggests that around 80% of all (re)modeling can be linked to the mechanical micro-environment. These findings disclose how mechanical stimuli at the tissue level contribute to the regulation of bone adaptation at the organ level.

Stochastic structural dynamic analysis using Bayesian emulators

The probabilistic characterisation of a frequency response function can be very valuable for structural design and control. Unfortunately, obtaining a sufficiently large sample for wide ranges of vibration can easily become unaffordable. A variety of modelling assumptions can dramatically add to the computational cost: nonproportional damping, multiscale material properties, and high-resolution finite element analysis are some examples. This paper explores Bayesian emulators as surrogates for expensive finite element models in structural dynamics. We demonstrate the effectiveness of the method by performing uncertainty analysis of the frequency response of a nonproportionally damped plate made of a carbon fibre/epoxy composite material.

Seeing Double

Experimental and computational assessment of thickness, porosity, biomechanical behavior, and adjacent disc glycosaminoglycan content in double- and single-layer bony endplate samples harvested from human cadaver spines. To determine if the second layer of bone in double-layer vertebral endplates allows the superficial layer to achieve a more optimal balance between its biomechanical and nutritional functions. Proper disc health requires the endplate to balance opposing biomechanical and nutritional functions. Previous studies investigating endplate function report seeing double: some endplates have a second layer of bone. However, it remains unclear whether the second layer of bone has any functional advantage. Such information could shed light on the factors that protect against disc degeneration. Six lumbar spines were obtained from human cadavers (32-84 yr) and scanned with magnetic resonance imaging. Cylindrical cores that included the endplate and underlying trabecular bone were harvested from the center of the superior vertebral endplates (6 double- and 12 single-layer endplates) and imaged using micro-computed tomography. The thickness and porosity of the bony endplate layers were measured for each core. High-resolution finite element analysis was performed to assess biomechanical behavior. Glycosaminoglycan content within the adjacent nucleus tissue was quantified using the dimethylmethylene blue technique. The superficial layer of the double-layer endplates was 50% thinner (P = 0.009) and tended also to be more porous than single-layer endplates. Strains were higher in thinner endplates; however, the second layer of bone in the double-layer endplates had a stiffening effect so that despite being thinner than single-layer endplates, the superficial layer of the double-layer endplates had a similar risk of damage. After adjusting for age, glycosaminoglycan content was significantly higher in the nucleus tissue adjacent to the double-layer endplates (P = 0.01). Compared with single-layer endplates, double-layer endplates seem to permit a more optimal balance between endplate biomechanical and nutritional functions, and may therefore offer a significant protective factor against disc degeneration.

Local and regional mechanical characterisation of a collagen-glycosaminoglycan scaffold using high-resolution finite element analysis

Artificial tissue growth requires cells to proliferate and differentiate within the host scaffold. As cell function is governed by mechano-sensitive selection, tissue type is influenced by the microscopic forces exposed to the cells, which is a product of macroscopically straining the scaffold. Accordingly, the microscopic strain environment within a CG scaffold is offered here. Using μ CT to characterise CG scaffold architecture, two high-resolution 3D FE models were used to predict the deformation mechanics. While also providing an analysis of region-specific features, such as relative density, pore diameters and microstructural elastic stability, the deformation patterns afforded strains to be inferred for seeded cells. The results indicate a regional dependence, in terms of architectural and mechanical properties. Specifically, the peripheral regions demonstrated the lowest volume fraction, the highest stress concentrations and the greatest potential for elastic instability. Conversely, the mid-region exhibited the most homogeneous environment. Based on the proviso of mechano-sensitive proliferation and differentiation, the findings suggest cell function will vary between CG scaffold regions. Further work should investigate the possibility of improving the fabrication process in order to deliver a construct in line with the mid-region, or alternatively, isolation of the mid-region may prove beneficial for cell culturing.

A biomechanical study of osteoporotic vertebral trabecular bone: The use of micro-CT and high-resolution finite element analysis

Osteoporosis is one of the most dangerous skeletal diseases in relation to the highest fracture risk in vertebral bones. A considerable amount of work has been done to investigate the biomechanical characteristics of osteoporotic vertebral trabecular bone. Previous researchers studied the elastic characteristics using a micro-finite element (micro-FE) model, used to analyze realistic trabecular architectures in full detail, based on micro-computed tomography (μCT). Since osteoporotic compression fracture is closely associated with the mechanical characteristics of the vertebral trabecular bone and there were few micro-FE models to account for all of the elastic and plastic characteristics in vertebral trabecular bone, this study analyzed the effect of voxel resolution on the plastic characteristics as well as the elastic characteristics of three-dimensional (3D) osteoporotic lumbar trabecular bone models. Also, we evaluated the effect of specimen geometry on this problem. It has been reported that a cubic specimen with side length 6.5mm was suggested as standard specimens for the experimental test of trabecular bone. Current study examined whether or not the effect of the specimen geometry on the experimental test may be also applied to the simulated compression test of trabecular bone specimens. The experimental test employing the rapid prototyping (RP) technique and INSTRON test machine is performed to indirectly validate the results of the simulated compression test by micro-FE analysis. The review finished with the verification about the effects of the simulated compression test.

Damage in trabecular bone at small strains

Evidence that damage decreases bone quality, increases fracture susceptibility, and serves as a remodeling stimulus motivates further study of what loading magnitudes induce damage in trabecular bone. In particular, whether damage occurs at the smaller strains characteristic of habitual, as opposed to traumatic, loading is not known. The overall goal of this study was to characterize damage accumulation in trabecular bone at small strains (0.20 - 0.45% strain). A continuum damage mechanics approach was taken whereby damage was quantified by changes in modulus and residual strain. Human vertebral specimens (n = 7) were tested in compression using a multi-cycle load - unload protocol in which the maximum applied strain for each cycle, epsilonmax, was increased incrementally from epsilonmax = 0.20% on the first loading cycle to epsilonmax = 0.45% on the last cycle. Modulus and residual strain were measured for each cycle. Both changes in modulus and residual strains commenced at small strains, beginning as early as 0.24 and 0.20% strain, respectively. Strong correlations between changes in modulus and residual strains were observed (r = 0.51 - 0.98). Fully nonlinear, high-resolution finite element analyses indicated that even at small apparent strains, tissue-level strains were sufficiently high to cause local yielding. These results demonstrate that damage in trabecular bone occurs at apparent strains less than half the apparent compressive yield strain reported previously for human vertebral trabecular bone. Further, these findings imply that, as a consequence of the highly porous trabecular structure, tissue yielding can initiate at very low apparent strains and that this local failure has detectable and negative consequences on the apparent mechanical properties of trabecular bone.

Prediction of Pier Response to Barge Impacts with Design-Oriented Dynamic Finite Element Analysis

The loads imparted to bridge piers during barge impact events can be quantified and the structural responses that result from the application of such loads can be determined by using analytical procedures of varying degrees of complexity. At one extreme, sophisticated, nonlinear, dynamic finite element impact simulation techniques may be used to quantify the time variation of the dynamic loads generated during such collision events. At the other end of the spectrum are bridge design specifications with simple computational formulas for computing equivalent static loads for barge impact loading. An intermediate level—design-oriented dynamic finite element analysis technique—that is suitable for use in design practice with moderate computational resources is presented. The proposed method uses barge crush data—predetermined either experimentally or by using high-resolution finite element analysis—to link a design-oriented dynamic pier analysis program to a low-order, nonlinear barge model. For demonstration purposes, the method is implemented in a widely used commercial pier analysis program (FB-Pier). Results obtained from the proposed method, including both dynamic load predictions and structural response predictions, are compared with corresponding results obtained using high-resolution, finite element analyses for validation purposes.