• Progress in Superconductivity and Cryogenics
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• v.23 no.4
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• pp.1-5
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• 2021

Among elemental metals, niobium (Nb) has the highest superconducting transition temperature (T_c) at ambient pressure. Thus, Nb films have been used in superconducting electronics and radio frequency cavity applications. In this study, the depositional factors determining the crystallinity and T_c of Nb films were investigated. An Nb film grown at a sputtering temperature of 240℃ exhibited the maximum crystallinity of Nb and the minimum crystallinity of niobium oxide. X-ray photoelectron spectroscopy confirmed a maximum atomic percent of niobium and a minimum atomic percent of oxygen. A sputtering power of 210 W and a sputtering time of 50 min were the optimal conditions for Nb deposition, and the T_c of the optimized film (9.08 K) was close to that of bulk Nb (9.25 K). Transmission electron microscopy images of the thick film directly confirmed the removal of the typical in-plane compressive strain in the (110) plane caused by residual stress.

• Steel and Composite Structures
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• v.21 no.5
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• pp.1045-1067
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• 2016

Concrete bridges with corrugated steel webs and prestressed by both internal and external tendons have emerged as one of the promising bridge forms. In view of the different behaviour of components and the large shear deformation of webs with negligible flexural stiffness, the assumption that plane sections remain plane may no longer be valid, and therefore the classical Euler-Bernoulli and Timoshenko beam models may not be applicable. In the design of this type of bridges, both the ultimate load and ductility should be examined, which requires the estimation of full-range behaviour. An analytical sandwich beam model and its corresponding beam finite element model for geometric and material nonlinear analysis are developed for this type of bridges considering the diaphragm effects. Different rotations are assigned to the flanges and corrugated steel webs to describe the displacements. The model accounts for the interaction between the axial and flexural deformations of the beam, and uses the actual stress-strain curves of materials considering their stress path-dependence. With a nonlinear kinematical theory, complete description of the nonlinear interaction between the external tendons and the beam is obtained. The numerical model proposed is verified by experiments.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.9
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• pp.1073-1084
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• 1999

The effects of secondary flow on the structure of a turbulent wake generated by a flat plate was investigated experimentally. The secondary flow was induced In a $90^{\circ}$ curved duct in which the flat plate wake generator was installed. The wake generator was installed in such a way that the wake velocity gradient exists in the span wise direction of the curved duct. Measurements were made in the plane containing the mean radius of curvature where pressure gradient and curvature effects were small compared with the secondary flow effect. All six components of the Reynolds stresses were measured in the curved duct. Turbulence intensities in the curved wake are higher than those in the straight wake due to an increase of the turbulent kinetic energy production by the secondary flow. In the inner wake region, shear stress and strain in the plane containing the velocity gradient of the wake show opposite signs with respect to each other, so that eddy viscosity Is negative in this region. This indicates that gradient-diffusion type turbulence models are not appropriate to simulate this type of flow.

• Journal of the korean Society of Automotive Engineers
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• v.12 no.1
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• pp.26-32
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• 1990

This study investigated a method for the determination of the J-integral for a tough glassy polymer such as polycarbonate plates by using the method of caustics. Comparing the values of J-integral determined by a numerical analysis and by the method of caustics, the method of caustics was found to be an effective experimental technique for the determination of the J-integral. The ratio between two J-integrals determined by the method of caustics and by finite element method converged into 1 within the limit of low load. However, it was noticed that the greater the plastic zone at the crack tip was, the lower the J-integral obtained by the reflect method of caustics. This difference may be deduced from the damage at the crack tip such as craze appeared in the polycarbonate plate. It was confirmed that the ratio of longitudinal diameter( $D_{l}$ ) to transverse diameter ( $D_{t}$) of caustics generally converged into 1 at the low load. The transition of the state of stress at the vicinity of a crack tip from plane strain to plane stress was deduced by noticing that the longitudinal diameter( $D_{l}$ ) grew faster than the transverse diameter( $D_{t}$) of caustics within the higher load range.

• Journal of Theoretical and Applied Mechanics
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• v.2 no.1
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• pp.31-50
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• 1996

In this paper, a finite element analysis based on the local approach concept to fracture in the continuum damage mechanics is performed to analyze ductile fracture in two dimensional quasi-static state. First an isotropic damage model based on the generalized concept of effective stress is proposed for structural materials in the context of large deformation. In this model, the stiffness degradation is taken as a measure of damage and so, the fracture phenomenon can be explained as the critical deterioration of stiffness at a material point. The modified Riks' continuation technique is used to solve incremental iterative equations. Crack propagation is achieved by removing critically damaged elements. The mesh size sensitivity analysis and the simulation of the well known shearing mode failure in plane strain state are carried out to verify the present formulation. As numerical examples, an edge cracked plate and the specimen with a circular hole under plane stress are taken. Load-displacement curves and successively fractured shapes are shown. From the results, it can be concluded that the proposed model based on the local approach concept in the continuum damage mechanics may be stated as a reasonable tool to explain ductile fracture initiation and crack propagation.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.8
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• pp.1429-1437
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• 1992

A new finite element formulation based on the relaxation type hereditary integral is presented for a time-domain analysis of isotropic, linear viscoelastic problems. The semi-discrete variational approximation and elastic-viscoelastic correspondence principle are used in the theoretical development of the proposed method. In a time-stepping procedure of final, linear algebraic system equations, only a small additional computation for past history is required since the equivalent stiffness matrix is constant. The viscoelasticity matrices are derived and the stress computation algorithm is given in matrix form. The effect of time increment and Gauss point numbers on the numerical accuracy is examined. Two dimensional numerical examples of plane strain and plane stress are solved and compared with the analytical solutions to demonstrate the versatility and accuracy of the present method.

• International Journal of Highway Engineering
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• v.18 no.1
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• pp.73-83
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• 2016

PURPOSES : The purpose of this study is to develop a method for improving the accuracy of analysis results obtained from a two-dimensional (2-D) numerical analysis model of continuously reinforced concrete pavement (CRCP). METHODS : The analysis results from the 2-D numerical model of CRCP are compared with those from more rigorous three-dimensional (3-D) models of CRCP, and the relationships between the results are recognized. In addition, the numerical analysis results are compared with the results obtained from field experiments. By performing these comparisons, the calibration factors used for the 2-D CRCP model are determined. RESULTS : The results from the comparisons between 2-D and 3-D CRCP analyses show that with the 2-D CRCP model, concrete stresses can be overestimated significantly, and crack widths can either be underestimated or overestimated by a slight margin depending on the assumption of plane stress or plane strain. The behaviors of crack width in field measurements are comparable to those obtained from the numerical model of CRCP. CONCLUSIONS : The accuracy of analysis results from the 2-D CRCP model can be improved significantly by applying calibration factors obtained from comparisons with 3-D analyses and field experiments.

• Computers and Concrete
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• v.16 no.4
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• pp.625-642
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• 2015

Prestressed concrete bridges with corrugated steel webs have emerged as one of the promising bridge forms. This structural form provides excellent structural efficiency with the concrete flanges primarily taking bending and the corrugated steel webs primarily taking shear. In the design of this type of bridges, the flexural ductility and deformability as well as strength need to be carefully examined. Evaluation of these safety-related attributes requires the estimation of full-range behaviour. In this study, the full-range behaviour of beam sections with corrugated steel webs is evaluated by means of a nonlinear analytical method which uses the actual stress-strain curves of the materials and considers the path-dependence of materials. In view of the different behaviour of components and the large shear deformation of corrugated steel webs with negligible longitudinal stiffness, the assumption that plane sections remain plane may no longer be valid. The interaction between shear deformation and local bending of flanges may cause additional stress in flanges, which is considered in this study. The numerical results obtained are compared with experimental results for verification. A parametric study is undertaken to clarify the effects of various parameters on ductility, deformability and strength.

• International Journal of Highway Engineering
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• v.8 no.1 s.27
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• pp.139-152
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• 2006

Many experimental and numerical approaches have been developed to evaluate paving materials and to predict pavement response and distress. Micromechanical simulation modeling is a technology that can reduce the number of physical tests required in material formulation and design and that can provide more details, e.g., the internal stress and strain state, and energy evolution and dissipation in simulated specimens with realistic microstructural features. A clustered distinct element modeling (DEM) approach was implemented In the two-dimensional particle flow software package (PFC-2D) to study the complex behavior observed in asphalt mixture fracturing. The relationship between continuous and discontinuous material properties was defined based on the potential energy approach. The theoretical relationship was validated with the uniform axial compression and cantilever beam model using two-dimensional plane strain and plane stress models. A bilinear cohesive displacement-softening model was implemented as an intrinsic interface and applied for both homogeneous and heterogeneous fracture modeling in order to simulate behavior in the fracture process zone and to simulate crack propagation. A disk-shaped compact tension test (DC(T)) with heterogeneous microstructure was simulated and compared with the experimental fracture test results to study Mode I fracture. The realistic arbitrary crack propagation including crack deflection, microcracking, crack face sliding, crack branching, and crack tip blunting could be represented in the fracture models. This micromechanical modeling approach represents the early developmental stages towards a 'virtual asphalt laboratory,' where simulations of laboratory tests and eventually field response and distress predictions can be made to enhance our understanding of pavement distress mechanisms, such its thermal fracture, reflective cracking, and fatigue crack growth.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.27 no.7
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• pp.1159-1169
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• 2003

This paper investigates the effects of T-stress and plastic hardening mismatch on the interfacial crack-tip stress field via finite element analyses. Plane strain elastic-plastic crack-tip fields are modeled with both MBL formulation and a full SEC specimen under pure bending. Modified Prandtl slip line fields illustrate the effects of T-stress on crack-tip constraint in homogeneous material. Compressive T-stress substantially reduces the interfacial crack-tip constraint, but increases the J-contribution by lower hardening material, J$\_$L/. For bimaterials with two elastic-plastic materials, increasing plastic hardening mismatch increases both crack-tip stress constraint in the lower hardening material and J$\_$L/. The fracture toughness for bimaterial joints would consequently be much lower than that of lower hardening homogeneous material. The implication of unbalanced J-integral in bimaterials is also discussed.