Geometrical Size Effect Research Articles

Considering size effects in the notch stress concept for fatigue assessment of welded joints

The original idea of the notch stress concept for fatigue assessment of welded joints was to obtain a single ‘master’ Wöhler curve by taking the local geometry into account. Ongoing research has established a number of differing Wöhler curves, specifically focusing on different sheet thicknesses. The differences are due to size effects that were not taken into account while calculating said curves. Therefore, a numerical method was developed which combines the geometrical and statistical size effect to a volumetric approach. To validate the quality of this new developed approach, own results for thin sheet specimens including weld start and end point and results from the literature regarding thick sheet components without weld start and end points were re-evaluated. To be able to compare the results, all specimens were re-modelled as three dimensional (3D) finite element models. To obtain highest accuracy, the actual geometry was either captured using a high resolution 3D scanner or derived from micrographs. The final result is a unified scatter band for fatigue lives of thin and thick sheet specimens with and without weld start and end point.

Electronic states and nonlinear optical properties of a two‐dimensional hexagonal quantum dot: Effects of impurity, geometrical size and confinement potential

AbstractIn the present work, the linear and the third‐order nonlinear optical absorption coefficients as well as refractive‐index changes in a two‐dimensional hexagonal quantum dot with the presence of an impurity are calculated. A matrix diagonalization method is used to calculate energy levels and corresponding wave functions in the effective‐mass approximation. Also, by means of the compact‐density matrix approach the linear and nonlinear optical absorption coefficients and refractive‐index changes are calculated. The effects of the impurity, the size of the dot and the confinement potential on energy eigenvalues, optical absorption coefficients and refractive‐index changes are investigated. It is found that the energy eigenvalues, absorption coefficients and refractive‐index changes are strongly dependent on the dot size, the confinement potential and the effects produced by the impurity.

Damage evaluation of the steel tubular column subjected to explosion and post-explosion fire condition

Fires following explosion are considered one of the most serious secondary disasters in the blast events, which are frequently occurring in the terrorist attacks and other blast accidents. In the recent decades, a significant amount of research work has been carried out to study the structural response under blast loads, and it is found that due to the superior mechanical properties of steel material, steel structures are adequate to withstand a medium-scaled blast loading. However, since the strength of steel is sensitive to temperature, the post-explosion fire action should be involved in the evaluation of structural damage in the blast events. This paper is devoted to introduce a numerical method for predicting the integrative damage of steel tubular column subjected to blast load and the following fire action. The damage caused by blast load would significantly reduce the fire resistance of steel column, which includes two aspects: mechanical damage and geometrical damage. In order to describe the mechanical damage under the blast loading, a damage scalar is defined in the constitutive model, and it is used to represent the reduction of material strength. The geometrical deformation induced by blast load is treated as the initial condition in the fire analysis. Pressure–Impulse diagram is employed to describe the damage of the steel tubular column under blast loading. In the second step, fire analysis for the explosion-survived column is carried out. Obviously, the residual vertical capacity of steel column is related with the fire exposure time. In order to clarify the interaction between the explosion and the post-explosion fire action, a more inclusive function, characterized by three variables: pressure, impulse and fire exposure time, is presented in this paper, and it can be used to predict the residual capacity of the steel column subjected to blast load and exposed to fire for a specific time. In the last section, parametric studies are conducted to observe the effects of geometric size on the failure evolution of steel columns. The main objective of this research work is to provide guidance for assessing damage level of the steel tubular columns that have survived blast loading and expose to the following fire condition.

Effect of cross-sectional geometry on superelasticity of helical nanobelts

The shape memory effect (SME) and the superelasticity make nanoscale chiral structures such as helical ZnO nanobelts promising for a wide range of important applications. In this paper, based on the non-linear rod model with surface effect (Wang et al, 2011), the effects of surfaces and geometrical size of the cross-section on the superelasticity of the helical nanobelts have been investigated quantitatively. Our results demonstrate the superelasticity of helical nanobelts exhibit a distinct size effect, depending on the cross-sectional geometry. We also show the efficiency of the energy storage and retrieval of the helical nanobelts induced by the superelasticity depends strongly on the cross-sectional geometry.

Elliptic-Spline Solutions for Large Localizations in a Circular Blatz–Ko Cylinder Due to Geometric Softening

It has been known that geometric softening can induce strain localizations in solids. However, it is very difficult to analytically capture the localized deformation states within a three-dimensional framework, especially when the deformation is large. In this paper, we introduce a novel approach, which resembles the use of a spline to approximate a curve, to construct analytical (asymptotic) solutions for large localizations in a circular cylinder composed of a Blatz–Ko material due to geometric softening. The asymptotic normal form equation (in the form of an ODE) valid for the axial stretch in a small neighborhood is first derived and then a set of these equations, each valid in a small neighborhood, can be obtained. The union of these small neighborhoods can cover a large range of the axial stretch, and as a result this set of equations governs the deformation states for the axial stretch in a large interval. Through a phase-plane analysis on this set of ODEs we manage to obtain the analytical solutions (in the form of a spline of elliptic integrals) for the large strain localizations. Both a force-controlled problem and a displacement-controlled problem are solved and the analytical results capture well the nonuniqueness of the stress-displacement relation and the snap-through phenomenon, which are often observed in experiments when strain localizations happen. In addition, some insightful information on the bifurcation points is obtained. The important geometric size effect is also discussed through the analytical solutions.

Casting defects and fatigue behaviour of ductile cast iron for wind turbine components: A comprehensive study

AbstractTwo types of EN‐GJS‐400‐18‐LT ductile cast iron were investigated in this research, clean baseline material in the shape of castings with different thicknesses and also defective material from a rejected wind turbine hub. P‐S‐N curves for baseline EN‐GJS‐400‐18‐LT specimens with different dimensions and from castings with different thicknesses at different load ratios were established. Geometrical size effect, technological size effects and mean stress effect on fatigue strength of baseline EN‐GJS‐400‐18‐LT were evaluated. Fatigue strength of baseline EN‐GJS‐400‐18‐LT was compared with that of defective material from the rejected hub. The effect of defects type, shape, size and position on fatigue strength of this material was evaluated. The hypothesis that the endurance observed in an S‐N test can be predicted based on the analysis of crack growth from casting defects through defect‐free ‘base’ material was tested for the analyzed defective material. 3D X‐ray computed tomography was use to detect defects in defective specimens and find the defect size distribution. The obtained defect size distribution for the defective material was used in random defect analysis to establish the scatter of fatigue life for defective specimens. Finally both safe‐life design and damage tolerant design of wind turbine castings were analyzed and compared.

Effects of geometric structure, orientation and size on structural stability and thermal behavior of zinc oxide nanowires

The structural stability, orientation effect and melting characteristic of zinc oxide (ZnO) nanowires are simulated by using the molecular dynamics with many-body tightbinding potential. The structural stability is affected by the geometric shape of the cross section of a nanowire. The nanowire with a hexagonal cross section is more stable than that with another cross section type, namely, a rectangular, triangular, rhombohedral, octagonal, and circular cross section. The structural stability and melting temperature of a nanowire is sensitive to its diameter because of the surface energy and unfavorable coordination. Remarkably, it is observed that hexagonal ZnO nanowires transform to metastable circular-type structures at temperatures lower than the melting point.

Anisotropic Interface Magnetoresistance inPt/Co/PtSandwiches

We report on an effect of reduced dimensionality on the magnetotransport in cobalt layers sandwiched by platinum. In a current in-plane geometry it is found that the resistivity depends on the magnetization orientation within the plane perpendicular to the current direction. The resistivity shows a symmetry adapted cos(2) dependence on the angle to the surface normal, with the maximum along the surface normal. The Co thickness dependence of the effect in Pt/Co/Pt sandwiches clearly points out that the mechanism behind this effect originates at the Co/Pt interfaces and is disparate to the texture induced geometrical size effect.

Large scale axial fatigue testing of ductile cast iron for heavy section wind turbine components

The present paper deals with axial fatigue testing of heavy section EN-GJS-400-18-LT ductile cast iron specimens with 120 mm × 140 mm cross section and also smaller cylindrical specimens with 21 mm diameter at room temperature and load ratio R = −1. The load levels for heavy section specimens were adjusted to cover endurances from 10,000 to 14 million cycles. Each heavy section specimen weights 380 kg. The specimens were cut from castings with 150 mm thickness. The heavy section specimens thickness is close to the common thicknesses of heavy section wind turbine ductile cast iron components. Fatigue strength of large 120 mm × 140 mm specimens were compared with fatigue strength of smaller 21 mm diameter specimens and geometrical size effect was evaluated. To evaluate the wall thickness effect on fatigue strength (technological size effect), the obtained fatigue test results for 21 mm specimens from billets with 150 mm thickness were compared with the published fatigue test results of the same specimens but from billets with 95 mm thickness. Metallography analysis of 21 mm specimens from billets with 95 mm and 150 mm thicknesses were performed and important microstructural parameters were measured and compared. Finally the effect of casting thickness and microstructure on fatigue strength of this material was evaluated.

Modeling the Influence of Initial Geometry on the Equilibrium Responses of Glucose-Sensitive Hydrogel

A model entitled the MECglu model is employed to investigate the effect of the initial geometric size on the equilibrium responses of glucose-sensitive hydrogel. With consideration of the chemo-electro-mechanical coupling effect, the model simulates the finite stimulus responsive deformation, which is associated with the electric potential and the fixed charge density in the hydrogel and the surrounding solvent, as the response to the change in the glucose concentration of the external solution. Influence of various material properties and environmental conditions on the performance of the smart material is analyzed by the model, especially focusing on the effect of the initial thickness of the hydrogel strip. The parameters studied include the ionic species, electric potential, fixed charge density, displacement, and swelling ratio of hydrogel. The parameter studies demonstrate that the response of the hydrogel can be optimized by varying the thickness of the hydrogel strip. In other words, while immersed in a glucose buffer solution, the glucose sensitivity of the thin membrane is higher than that of the thick one.

The effect of stacked graphene flakes on the electronic transport of zigzag-edged graphene nanoribbons

The effect of stacked graphene flakes (GFs) on the electronic transport property of zigzag-edged graphene nanoribbon (ZGNR) is investigated. By using the Greens function method, we calculate the conductances of ZGNRs with two different stacked-type GFs. It is found that the coupling effect between ZGNRs and GFs can induce dips at the conductance profiles in two different stacked-types. For both stacked-types, the dips far away from the Fermi level are nearly overlapped. However, the position of conductance dip near the Fermi level depends on the stacked-type. In addition, we discuss the effect of geometric size of GF on the electronic transport property. The results show that with the increase of the size of GF, the dips far away the Fermi level in two stacked-types gradually move toward the Fermi level, while the discrepancy of the dips near the Fermi level is much evident. Our results indicate that the stacked GFs can effectively tune the electronic transport of ZGNR.

QUANTUM SIZE EFFECTS IN SUPERCONDUCTING JUNCTIONS WITH A FERROMAGNETIC INTERLAYER

Motivated by a recent experiment evidencing an unusual double-peak structure in tunneling conductance spectra of a nano-sized ferromagnet — superconductor bilayer, we propose a clean-limit scattering-like model for a quasi-ballistic transport in superconducting heterostructures with conducting ferromagnetic films. The model also includes an effect of charge interactions with an environment which partially degrade quantum correlations in the system. Our numerical simulations show that (i) the observed feature arises from a quantum-mechanical geometrical size effect and (ii) such measurements can serve as a sensitive tool for controlling quantum-mechanical phenomena in the studied junctions.

Diffusion of water molecules confined in slits of rutile TiO 2(1 1 0) and graphite(0 0 0 1)

Titanium dioxide (TiO 2) is well applied in implant materials because of its biocompatibility while carbon is usually regarded as toxicant to cells. Actually, water-surface interaction has significant influence on the biocompatibility of implant materials. To understand the difference between TiO 2 and graphite on the biocompatibility, molecular dynamics simulations were performed to study the structure and diffusion of water in nano-slits formed by rutile TiO 2(1 1 0) and graphite(0 0 0 1) with the separation distance ranging from 0.8 to 2.0 nm. Simulation results show that the residence time of water at TiO 2 surface is considerably longer than that at graphite surface, indicating the stronger interaction between water and TiO 2. Moreover, the microstructure of water molecules absorbed at the TiO 2 and graphite surfaces were analyzed; the bound water molecules at the TiO 2 surface and the hydrogen bond network reduce the diffusivity of water through the TiO 2 slits. Simulation results show that the surface chemistry is crucial to the diffusion of water in nanoscale pores, while geometric size effect may just enhance the chemical effect.

Density functional theory study of BN-doped graphene superlattice: Role of geometrical shape and size

Effects of geometric shape and size of embedded boron nitride (BN) nanodot on the electronic and magnetic properties of the BN-doped graphene superlattices are systematically studied by spin-polarized first-principles calculations. The band gap of graphene superlattice is found to increase with the size of the BN nanodot, regardless of the shape of BN nanodot. Midgap states are found for graphene superlattices with triangular BN nanodots, and the number of such midgap states is determined by the imbalance between the number of carbon atoms occupying A- and B-sublattices, which is closely related to the geometric shape and size of the BN nanodot. When B and N atoms in the superlattices are exchanged, the valance bands and conduction bands are inverted with respect to the Fermi level due to electron-hole symmetry. Furthermore, partial occupation of the midgap states induces spin-polarization, and results in a magnetic ground state for the BN-doped graphene superlattices with triangular BN nanodots. Through electron or hole injection, the magnetic moment of the BN-doped graphene superlattice can be tuned, which may find applications in spintronic devices.

Multiscale Modelling Strategies for Heterogeneous Materials

This paper considers modelling strategies for the multiscale analysis of heterogeneous materials. Initial focus is on computational homogenization (e.g. [1-4]) which represents a powerful, albeit computationally intensive, technique for capturing the influence of an evolving microstrucuture on the overall macroscopic behaviour. First-order homogenization assumes that the representative volume element (RVE) is infinitely small compared to the macroscopic characteristic length scale and that there is clear separation of scales. Second-order schemes [2,4,5] are subsequently discussed as a possible solution. This approach introduces a material length scale into the macroscopic constitutive model and potentially enables geometric size effects to be captured. However, the size of the RVE now becomes an important parameter for the macroscopic response and there is no clear way of determining this length scale. Second-order computational homogenization is demonstrated in the context of an indentation problem. In situations where the RVE cannot be defined and there is no clear separation of scales, computational homogenization is not applicable and full resolution of the microstructure may be necessary. This paper summarises an efficient solution strategy for such situations following the work of Miehe and Bayreuther [6], with particular focus on fracturing heterogeneous materials, which utilises a geometric multi-grid preconditioner and the scale transition methodology from computational homogenization. This solution strategy is coupled with the use of the hybrid-Trefftz stress elements for modelling cohesive cracking [7]. The resulting modelling approach is demonstrated with the analysis of concrete dog-bone specimens where the different phases of matrix, aggregate and interfacial transition zone are explicitly modelled. Such an approach is still computationally intensive and so this paper also presents a novel hybrid multiscale approach that avoids the need for the finite element mesh to fully resolve all heterogeneities [8]. Classical micromechanics is utilised to enhance the approximation space in order to capture the effect of microstructural inclusions. In comparison to where the finite element mesh fully resolves the microstructure, this novel approach significantly reduces the number of degrees of freedom required while still accurately capturing the influence of the inclusions.

Zur Ermüdungsfestigkeit hochfester großer Schrauben

StahlbauVolume 79, Issue 9 p. 703-703 Dissertation Zur Ermüdungsfestigkeit hochfester großer Schrauben Frithjof Marten Dr.-Ing., Frithjof Marten Dr.-Ing.Search for more papers by this author Frithjof Marten Dr.-Ing., Frithjof Marten Dr.-Ing.Search for more papers by this author First published: 06 September 2010 https://doi.org/10.1002/stab.201090101Citations: 4AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article.Citing Literature Volume79, Issue9September 2010Pages 703-703 RelatedInformation

SU‐GG‐J‐107: Monte Carlo Modelling the Effect of Translational Shifts and Rotations on Prostate Dose‐Population Histograms ‐ What Margins Are Required for IGRT?

Purpose: To investigate the effect of geometric uncertainties on prostate dose coverage based on population statistics and to determine the most appropriate margins for prostate IGRT. Method and Materials: Monte Carlo based software has been written using Matlab in order to implement Dose Volume Population Histograms (DVPH) for modeling geometric uncertainties. Whereas previously published work utilizing the DVPH concept has only accounted for translational uncertainties, this study has expanded this to include rotational uncertainties also. In this modeling, estimated values for systematic and random geometric uncertainties in 3‐dimensions for both translational and rotational shifts are entered into the software. Presented in this study are the results of modeling prostate motion using translational uncertainties of Σ = σ =1.5mm and Σ = σ= 3mm (representing IGRT and non‐IGRT treatment), these are combined with rotational uncertainties of Σ = σ = 1.5° and Σ = σ = 3° for IGRT and non‐IGRT. This modeling has been applied to six plans for a prostate treatment, based on margins around the prostate body of 0, 2, 4, 6, 8 and 10mm. Results: For the modeled non‐IGRT treatment, results show that a full 10mm margin is required in order to achieve 95% prescribed dose coverage of the CTV to 90% of the population. For the IGRT treatment modeling, 95% prescribed dose coverage to >90% of the population was achieved by all plans — including that with 0mm margin. Conclusion: This work demonstrates the effect of geometric uncertainties (including rotations) and margin size on tumor coverage probability. Currently, these Monte Carlo calculations take several hours to complete on a desktop PC, work is in progress to use a distributed computing server in order to minimize the time taken for calculation.

Spin polarization in a double bend quantum wire with Rashba spin-orbit coupling

Based on Green’s function method, we investigate spin-dependent transport properties of a double bend quantum wire. It is found that a notable spin polarization can be achieved in a broad energy range when magnetic field and Rashba spin-orbit coupling (RSOC) only exist in the intersection of two quantum wires. The strength of RSOC and magnetic field has drastic influence on the spin polarization. The amplitude of spin polarization can be tuned from 0% to 100% by adjusting RSOC or magnetic field. In addition, the effect of geometric size on the spin polarization is discussed. The results show that the double bend quantum wire may be employed for spin filter.

Fatigue life prediction of components made of spheroidal graphite cast iron

In many cast metals fatigue crack initiates from a defect. The lower bound of the fatigue life in cast components can be estimated based on the maximum defect size which is present in a component. In this research the distribution of defects in a ferritic spheroidal graphite cast iron component was determined. Then, Gumbel distribution was used to estimate the maximum sizes of defects in cylindrical fatigue specimens with different dimensions. Based on estimated maximum defect size, the fracture mechanics approach was used to determine the lower bound of the fatigue life for fatigue specimens with different dimensions and the geometrical size effect was modeled.

Effects of magnetic field and geometrical size on the interband light absorption in a quantum pseudodot system

We study in this paper the direct interband transitions in quantum pseudodot system under the influence of an external magnetic field. We obtain the analytical expressions for the light interband absorption coefficient and threshold frequency of absorption as the functions of applied magnetic field and geometrical size of quantum pseudodot system. We study the absorption threshold frequency (ATF) at small and high applied magnetic field and also as a function of size of quantum pseudodot. According to the results obtained from the present work, we find that (i) the ATF is linear at large magnetic field. (ii) It is nonlinear at small magnetic field. (iii) The ATF depends on the geometrical size of quantum pseudodot and decreases when the size of quantum pseudodot increases. Therefore, the magnetic field and quantum pseudodot size play important roles in the ATF.