Calculated Strain Distributions Research Articles

Application of Simplified Finite Element Bolted-Joint Model Using Rigid Body Element 3 to Train Attachment Rail

Many bolted joints are used in the field of rolling stock. From the viewpoint of computational load, it is not realistic to employ a 3D detailed model including the contact of thread surfaces and bearing surfaces. In this study, the simplified shell-beam linear finite element model using interpolated rigid body elements, which was originally proposed for flat plate joints, is applied to outfitting attachment rail fasteners. The calculated strain distribution close to the fastener, stiffness against shear force, and natural frequencies are compared with those of experiments and detailed models. Since the detailed model well reproduces the experimental results, the accuracy of the simplified model is verified through the comparison with the detailed model. It is found that the coefficient of determination of strain distribution is larger than 0.73 and the differences in the stiffness against shear force and eigenvalues are less than 10 %. Therefore, the accuracy of our proposed model is well improved, as compared with the previous Naruse model.

Further Evaluation of Limiting Strain Criteria for Perpetual Asphalt Pavement Design

Perpetual asphalt pavements have been designed with single threshold values for the horizontal strains at the bottom of the asphalt concrete layer and for vertical strains on top of the subgrade to prevent the occurrence of bottom-up fatigue cracking and subgrade rutting. Several thresholds have been utilized for design based on laboratory and field test results. Limiting strain distributions were recently proposed for perpetual pavement design instead of single threshold values for controlling the horizontal and vertical strains. These design criteria were developed with data collected from test sections at the National Center for Asphalt Technology pavement test track. The objective of this study was to conduct additional analyses of eight perpetual pavement sections located in different climatic regions, to support the proposed limiting strain criteria. These sections were simulated in the PerRoad perpetual pavement design software to determine the horizontal strains at the bottom of the asphalt concrete layer and the vertical strains on top of the subgrade. The strains were then analyzed to evaluate the proposed limiting strain criteria. Based on the simulations, the limiting horizontal strain distribution above the 60th percentile can effectively differentiate the calculated strain distributions of the eight perpetual pavement sections from those of the test sections that failed due to bottom-up fatigue cracking on the test track. In addition, the calculated vertical strains on top of the subgrade at the 50th percentile were lower than the proposed 200-microstrain limit. These results provide additional support for utilizing the proposed strain criteria in perpetual asphalt pavement design.

Band bending at the heterointerface of GaAs/InAs core/shell nanowires monitored by synchrotron X-ray photoelectron spectroscopy

Unique electronic properties of semiconductor heterostructured nanowires make them useful for future nano-electronic devices. Here, we present a study of the band bending effect at the heterointerface of GaAs/InAs core/shell nanowires by means of synchrotron based X-ray photoelectron spectroscopy. Different Ga, In, and As core-levels of the nanowire constituents have been monitored prior to and after cleaning from native oxides. The cleaning process mainly affected the As-oxides and was accompanied by an energy shift of the core-level spectra towards lower binding energy, suggesting that the As-oxides turn the nanowire surfaces to n-type. After cleaning, both As and Ga core-levels revealed an energy shift of about −0.3 eV for core/shell compared to core reference nanowires. With respect to depth dependence and in agreement with calculated strain distribution and electron quantum confinement, the observed energy shift is interpreted by band bending of core-levels at the heterointerface between the GaAs nanowire core and the InAs shell.

CONSTITUTIVE MODELING AND FINITE ELEMENT ANALYSIS OF MYXOMATOUS MITRAL LEAFLET TISSUE

Although rarely discussed, material modeling of the myxomatous leaflet is considered as the cornerstone of mitral valve finite element analysis. The present study presents an incompressible, hyperelastic constitutive model to characterize myxoid mitral leaflet tissue mechanics. The model incorporates the transversely isotropic response and the layered structure of the tissue. First, an analytical constitutive model for the tissue is developed based on continuum mechanics and layered composites theory. Second, the material constants of the constitutive equation are determined by fitting the model to the experimental data. The analytical material model is then implemented using solid finite element methods by simulating a biaxial tensile test. A numerical simulation of the out-of-plane pressure loading is also conducted. Both the analytical outcomes and the simulated results agree well with experimental data and show good mutual agreement. The calculated strain distribution of the out-of-plane pressure loading simulation indicates myxoid leaflets exhibit enhanced extensibility and decreased stiffness compared to normal valves; the radial direction is more extensible than the circumferential direction. The presented material approximation is able to capture the myxomatous mitral leaflet mechanics. The results of the numerical simulation conform to those of the experimental tests.

Measurement of the Deformation in Pulsed Magnets by Means of Optical Fiber Sensors

A typical compact pulsed magnet coil wound from copper and Zylon fiber was fabricated with optical fiber sensors embedded at different places in the Zylon layers. Measurements of deformation have been carried out during winding, cooling and with magnetic field pulses up to 10 T at room temperature and 41 T at 77 K. The results are in agreement with the calculated strain distribution by taking into account the anisotropy of fiber/epoxy composites.

Monitoring the Wall Mechanics During Stent Deployment in a Vessel

Clinical trials have reported different restenosis rates for various stent designs1. It is speculated that stent-induced strain concentrations on the arterial wall lead to tissue injury, which initiates restenosis2-7. This hypothesis needs further investigations including better quantifications of non-uniform strain distribution on the artery following stent implantation. A non-contact surface strain measurement method for the stented artery is presented in this work. ARAMIS stereo optical surface strain measurement system uses two optical high speed cameras to capture the motion of each reference point, and resolve three dimensional strains over the deforming surface8,9. As a mesh stent is deployed into a latex vessel with a random contrasting pattern sprayed or drawn on its outer surface, the surface strain is recorded at every instant of the deformation. The calculated strain distributions can then be used to understand the local lesion response, validate the computational models, and formulate hypotheses for further in vivo study.

Monitoring the Wall Mechanics During Stent Deployment in a Vessel

Clinical trials have reported different restenosis rates for various stent designs. It is speculated that stent-induced strain concentrations on the arterial wall lead to tissue injury, which initiates restenosis. This hypothesis needs further investigations including better quantifications of non-uniform strain distribution on the artery following stent implantation. A non-contact surface strain measurement method for the stented artery is presented in this work. ARAMIS stereo optical surface strain measurement system uses two optical high speed cameras to capture the motion of each reference point, and resolve three dimensional strains over the deforming surface. As a mesh stent is deployed into a latex vessel with a random contrasting pattern sprayed or drawn on its outer surface, the surface strain is recorded at every instant of the deformation. The calculated strain distributions can then be used to understand the local lesion response, validate the computational models, and formulate hypotheses for further in vivo study.

Mechanism of GaN quantum dot overgrowth by Al0.5Ga0.5N: Strain evolution and phase separation

The capping of GaN quantum dots (QDs) with an Al0.5Ga0.5N layer is studied using transmission electron microscopy and atomic force microscopy in combination with theoretical calculations. The capping process can be divided into several well-distinguishable stages including a QD shape change and a local change of the Al0.5Ga0.5N capping layer composition. The phase separation phenomenon is investigated in relation with the capping layer thickness. Amount of the chemical composition fluctuations is determined from separate analysis of scanning transmission electron microscopy and high-resolution transmission electron microscopy images. The local distortion of atomic lattice in the QD surroundings is measured by high-resolution electron microscopy and is confronted with theoretically calculated strain distributions. Based on these data, a possible mechanism of alloy demixing in the Al0.5Ga0.5N layer is discussed.

Validated finite element models of the proximal femur using two-dimensional projected geometry and bone density

Two-dimensional finite element models of cadaveric femoral stiffness were developed to study their suitability as surrogates of bone stiffness and strength, using two-dimensional representations of femoral geometry and bone mineral density distributions. If successfully validated, such methods could be clinically applied to estimate patient bone stiffness and strength using simpler and less costly radiographs. Two-dimensional femur images were derived by projection of quantitative computed tomography scans of 22 human cadaveric femurs. The same femurs were fractured in a fall on the hip configuration. Femoral stiffness and fracture load were measured, and high speed video was recorded. Digital image correlation analysis was used to calculate the strain distribution from the high speed video recordings. Two-dimensional projection images were segmented and meshed with second-order triangular elements for finite element analysis. Elastic moduli of the finite elements were calculated based on the projected mineral density values inside the elements. The mapping of projection density values to elastic modulus was obtained using optimal parameter identification in a set of nine of the 22 specimens, and validated on the remaining 13 specimens. Finite element calculated proximal stiffness and strength correlated much better with experimental data than areal bone mineral density alone. In addition, finite element calculated strain distributions compared very well with strains obtained from digital image processing of the high speed video recordings, further validating the two-dimensional projected subject-specific finite element models.

Strain influenced indium composition distribution in GaN/InGaN core-shell nanowires

The optical properties, indium concentration and distribution, defect morphology, and strain distribution of GaN/InGaN coaxial nanowires grown by metal organic chemical vapor deposition were investigated using spatially resolved cathodoluminescence, scanning transmission electron microscopy, and finite element analysis. The results indicate that InGaN layers with 40% or greater indium incorporation and low defect density can be achieved. The indium distribution in the InGaN shell layer was measured and qualitatively correlated with the calculated strain distribution. The three-dimensional compliance of the GaN nanowire leads to facile strain relaxation in the InGaN heteroepitaxial layer, enabling high indium incorporation and high crystalline quality.

The correspondence between coronary arterial wall strain and histology in a porcine model of atherosclerosis

Atherosclerotic plaque rupture is the leading cause of mortality in cardiovascular disease. Intravascular ultrasound (IVUS) imaging is a powerful clinical technique that provides real-time cross-sectional images of the arterial wall and atherosclerotic plaques. However, it does not provide sufficient information about the histological composition of plaques to characterize their vulnerability. Arterial wall strain measurements may provide insights into plaque composition and vulnerability, complementing the information directly available in the IVUS echogram. We have developed a method to measure the transverse arterial wall strain tensor in response to luminal pressure change, by registering IVUS images acquired at different pressures. This method has been validated by using IVUS images with simulated motion and IVUS images of a vessel phantom. In this study, we further evaluate the method by assessing the correspondence of the calculated strain distribution and the histological composition of atherosclerotic coronary arteries from Sinclair miniature pigs following 12 months of a high fat diet. The images were acquired in situ using a clinical IVUS system and under computer-controlled pressurization. After image acquisition, the artery segments were fixed for histology to identify plaque components. The strain distributions were aligned with the corresponding histological sections. The stiffness of various components of the lesion, inferred from the wall strain distribution, was consistent with the tissue composition seen in the histological cross-sections. These findings suggest that strain measurements from IVUS are promising for assessing plaque vulnerability.

Self-organized GaN/A1N hexagonal quantum-dots: strain distribution and electronic structure

This paper presents a finite element method of calculating strain distributions in and around the self-organized GaN/AIN hexagonal quantum dots. The model is based on the continuum elastic theory, which is capable of treating a quantum dot with an arbitrary shape. A truncated hexagonal pyramid shaped quantum dot is adopted in this paper. The electronic energy levels of the GaN/AIN system are calculated by solving a three-dimension effective mass Shrödinger equation including a strain modified confinement potential and polarization effects. The calculations support the previous results published in the literature.

A perturbation theory for calculating strain distributions in heterogeneous and anisotropic quantum dot structures

By introducing a homogenous comparison material, a perturbation theory based on Green’s function is proposed to calculate the strain distribution inside and outside an arbitrarily shaped and anisotropic quantum dot (QD) embedded in an alien infinite medium. This theory removes the limitations of the previous analytical methods which are based upon the assumption that the QD is isotropic and has the same elastic properties as the surrounding medium. The numerical results for a truncated pyramidal Ge∕Si QD structure demonstrate that the anisotropy of the materials and the difference between the stiffness tensors of the QD and the matrix have a significant influence on the strain field. It is found that the first-order approximate solution obtained by the proposed method can reduce the relative difference of the strain fields induced by the isotropic approximation from 30% to 6%. Moreover, it is shown that the strain fields obtained by the proposed method with the second-order approximate solution are very accurate for the Ge∕Si QD structure.

Electronic and optical properties of InAs/InP self-assembled quantum dots on patterned substrates

We present atomistic theory of electronic and optical properties of a single InAs quantum dot grown on a pyramidal InP nanotemplate. The shape and size of the dot is assumed to follow the nanotemplate shape and size. The electron and valence hole single particle states are calculated using atomistic effective–bond–orbital model with second nearest-neighbor interactions. The electronic calculations are coupled to separately calculated strain distribution via Bir–Pikus Hamiltonian. The optical properties of InAs dots embedded in InP pyramids are calculated by solving the many-exciton Hamiltonian for interacting electron and hole complexes using the configuration–interaction method. The effect of quantum-dot geometry on the optical spectra is investigated by a comparison between dots of different shapes.

An Approach for Calculating Strain Distributions inArbitrarily Shaped Quantum Dots

We study strain distribution inside and outside an arbitrarily-shapedquantum dot (QD) buried in an infinite isotropic medium. By defining avery simple vector, we derive a compact formula for stress fieldsexpressed by an integral over the interface between the QD and itssurrounding material. Using this method, the analytical solution for acuboidal QD is obtained, which is different from the previous result.It is shown that our solution satisfies the tractioncontinuity condition on the interface. Based on this solution,it is found that thestrain field in the cuboidal QD in the semiconductor heterostructure issensitive to its height. In addition, the strain distribution around ahemispherical QD is also calculated and demonstrated.

Nanoengineering of lateral strain modulation in quantum well heterostructures

We have developed a method to design a lateral band-gap modulation in a quantum well heterostructure. The lateral strain variation is induced by patterning of a stressor layer grown on top of a single quantum well which itself is not patterned. The three-dimensional (3D) strain distribution within the lateral nanostructure is calculated using linear elasticity theory applying a finite element technique. Based on the deformation potential approach the calculated strain distribution is translated into a local variation of the band-gap energy. Using a given vertical layer structure we are able to optimize the geometrical parameters to provide a nanostructure with maximum lateral band-gap variation. Experimentally such a structure was realized by etching a surface grating into a tensile-strained InGaP stressor layer grown on top of a compressively strained InGaAs-single quantum well. The achieved 3D strain distribution and the induced band-gap variation are successfully probed by x-ray grazing incidence diffraction and low-temperature photoluminescence measurements, respectively.

Strain and stress distributions in flexible pavements under moving loads

ABSTRACT Calculated strain distributions in flexible pavements were studied by comparison with measurements made on full-scale structures. The calculated results were obtained by elastic and viscoelastic modelling of these structures. As no stress measurements were collected, the stress distribution was studied by comparing the results of elastic and viscoelastic calculations. For different loading and temperature conditions, and assuming the hypothesis of the French design method for flexible pavements, modelling provided strain signals similar to those measured on real scale structures for strains at two different depths in the studied structures. This first conclusion was used to validate the models, which were then used in the stress study. Some unexpected strains variations were observed near to the surface of the structures. Modelling based on linear elastic behaviour of the materials made it possible to explain these variations by analysing the stress variations, particularly in the vertical direction. Viscoelastic modelling have shown that some additional stresses appear in the bituminous layer when the loads are moving away from the measurement point. These stresses are induced mainly by the reaction of the foundation layers (sub-base and sub-grade) and they are mainly caused by the interaction between the layers of the structures. Finally, the analysis of the strain and stress distributions near to the surface showed that bituminous materials are submitted to higher horizontal than vertical stresses just under the truck wheel. A particular and unexpected result of this situation is that the vertical strains at just under the wheel are tensile ones.

Theory to estimate the stress due to moisture in wood using a transmission-type microwave moisture meter

In recent years a microwave transmission-type moisture meter has been developed in Japan. Its purpose is to measure the average moisture content of thick woods. Since its development I have realized that there is a negative correlation between the moisture content of wood and the power voltage of the meter. This realization suggests that an invisible stress has an effect on the attenuation constant of the wood. The presence of such a stress in the wood could easily be proven by the slicing technique. In this article a theory is presented to explain further the effect of this stress on the attenuation constant. The theory was applied to softwood specimens in various states of moisture. It was concluded that the calculated strain distributions of the various specimens approximated those of the experimental results. Thus, the proposed theory presented herein has validity or adaptability with regard to qualitatively understanding the stress. Future research efforts would also be expected to detect the stress in wood due to moisture.

Study of the relation between evaluation of strain distribution on superconducting coil and mechanical heat generation

In the superconducting Maglev system, on-board superconducting magnets (SCMs) are vibrated at various frequencies according to the train speed by the electromagnetic disturbance which is caused when the train passes over ground coils. Then a mechanical loss is generated inside the inner vessel in the SCM. This phenomenon increases the heat load on the cryogenic equipment in the SCM. It has been surmised that the mechanical heat inside the inner vessel is generated by the frictional heat caused by the relative microscopic slips between fasteners and superconducting coil (SC coil). Nevertheless, heat generation mechanisms inside the inner vessel have not been studied sufficiently. In this study, we suggest a hypothesis that the frictional heat generated by the relative microscopic slips between fasteners and a SC coil will be indicated if the calculated strain distribution on the SC coil is evaluated. The results of this study supported this hypothesis.

Development of a metal test box configuration to test a range of skin panels of a composite horizontal stabilizer

Within the framework of a national technology programme under a contract awarded by the Netherlands Agency for Aerospace Programmes (NIVR), a composite stabilizer was developed by Fokker Aircraft B.V. in close collaboration with NLR. One of the activities in this technology program for NLR was to develop a test box for testing large composite skin panels and to test the composite panels on this box. The main goals of this part of the program were: To develop a single test box on which a number of test panels with different configurations could be loaded to relatively high strain levels without incurring damage to the test box; To load the test panels in such a way that an optimum correspondence between strain distributions in the test panels and the horizontal stabilizer skins was attained; To validate the design concepts of the skin panels by comparison of calculated and measured strain distributions. All objectives have been met successfully. Six different panels were tested on a single metal test box. The panels were loaded up to a maximum of 3.0 times Limit Load with a total of 21 different loading conditions without incurring any damage to the metal test box. The loading of the test panels was such that an acceptable correspondence of the measured strain distributions and the calculated strain distributions in the horizontal stabilizer skins was attained. The measured strain distributions compared quite well with the calculated strain distributions for the test panels.