Changkye Lee | University of Miami Miller School of Medicine (original) (raw)

Papers by Changkye Lee

Computers in Biology and Medicine, 2025

Objective: This study aims to investigate the micro-hemodynamic effects of strut malapposition in... more Objective: This study aims to investigate the micro-hemodynamic effects of strut malapposition in patient-specific stented coronary bifurcations.
Methods: Using the mapping-back technique, three-dimensional reconstructions of clinical post-stenting artery bifurcations with strut malapposition were accurately generated from optical coherence tomography scans of 9 patients. Computational fluid dynamics (CFD) simulations were then conducted with these models to examine the impact of strut malapposition on various fluid dynamic parameters, including flow patterns, vorticity, strain rates, viscosity, and wall shear stress (WSS). For statistical analysis, virtually apposed models were created to evaluate WSS metrics. Additionally, follow-up data for 5 out of the 9 patients were reviewed to assess evidence of late thrombosis and restenosis.
Results: Malapposed struts induce significant alterations in flow dynamics, including the formation of recirculation regions and the transition from laminar to disturbed flow. The local curvature of the lumen also affects the development of these recirculation regions. Our study demonstrates, for the first time, that the vorticity on the
abluminal side of malapposed struts exhibits an opposite sign compared to the surrounding region. The strain rate around these struts shows a distinct transition, with high values at the stent surface that rapidly diminish within the strut-lumen gap. This transition is accompanied by an increase in viscosity within these regions.
Furthermore, as the malapposition distance increases, strain rates on the malapposed struts increase while viscosity decreases. Significant differences in WSS metrics were observed between clinically malapposed and virtually apposed scenarios. In clinical follow-up cases, no evidence of thrombosis was found despite the complex
micro-hemodynamics in these patients.
Conclusion: There is complex interplay between stent malapposition and hemodynamics within a patient-specific bifurcation. The significant impact of local lumen curvature on flow dynamics underscores the limitations of idealized artery models. Moreover, the absence of thrombosis in subsequent clinical follow-up cases suggests that additional factors, such as antiplatelet medication, may play a significant role in mitigating these risks.

International Journal of Non-Linear Mechanics, Jan 2025

Volumetric locking may occur in plastic analysis of incompressible materials using low-order fini... more Volumetric locking may occur in plastic analysis of incompressible materials using low-order finite elements due to incompressibility constraints. This study presents a locking-free smoothed five-node quadrilateral element-based approach for plastic analysis in structural engineering. The proposed Q5-element employing four cell-based smoothing domains effectively alleviates the volumetric locking issues, here in the problems under plane strain conditions. The resulting large-scale optimization problem is formulated in a conic programming form, enabling efficient use of the interior-point optimizer. Numerical investigations demonstrate the method’s effectiveness in alleviating volumetric locking, accurately predicting collapse and shakedown limits, and generating interaction diagrams for load-carrying capacity and structural collapse mechanisms.

Finite Elements in Analysis and Design, 2024

Failure mechanism of 3D structures cannot always be produced by the low-order finite elements due... more Failure mechanism of 3D structures cannot always be produced by the low-order finite elements due to the so-called volumetric locking effect. In this paper, dual numerical approaches based on the bubble face-based smoothed finite element method (bFS-FEM) are developed, ensuring that the locking problem is prevented and accurate load factors of elastic-perfectly plastic structures under cyclic actions are achieved. The failure mechanisms, in terms of plastic dissipation, are realized as incremental or alternative plastic failure modes, enabling different treatments in engineering practices. Moreover, the pseudo-static approach is capable of providing three-dimensional stress fields at the failure state, which is crucial for structural design. Interaction diagrams associated with various load-types and-ranges are illustrated in numerical experiments, showing that the bearing capacity envelopes of structures under cyclic loads are evidently smaller than that of proportional loads.

한국전산구조공학회 논문집, 2024

This paper presents two-dimensional boundary value problems of the stress-based gradient elastici... more This paper presents two-dimensional boundary value problems of the stress-based gradient elasticity within the smoothed finite element method (S-FEM) framework. Gradient elasticity is introduced to address the limitations of classical elasticity, particularly its struggle to capture size-dependent mechanical behavior at the micro/nano scale. The Ru-Aifantis theorem is employed to overcome the challenges of high-order differential equations in gradient elasticity. This theorem effectively splits the original equation into two solvable second-order differential equations, enabling its incorporation into the S-FEM framework. The present method utilizes a staggered scheme to solve the boundary value problems. This approach efficiently separates the calculation of the local displacement field (obtained over each smoothing domain) from the non-local stress field (computed element-wise). A series of numerical tests are conducted to investigate the influence of the internal length scale, a key parameter in gradient elasticity. The results demonstrate the effectiveness of the proposed approach in smoothing stress concentrations typically observed at crack tips and dislocation lines.

International Journal of Mechanics and Materials in Design, 2024

This study presents an efficient numerical approach for pseudo-lower bound limit analysis of stru... more This study presents an efficient numerical approach for pseudo-lower bound limit analysis of structures. The total stress field is decomposed into two components: an elastic component associated with the safety factor and a self-equilibrating residual component. Subsequently, equilibrium conditions within the optimization problem are satisfied in a weak manner. The application of the adaptive quadtree edge-based smoothed finite element method (ES-FEM), combined with the transformation into the second-order cone programming (SOCP) form, ensures the resulting optimization problem remains minimal in size. Moreover, employing a yield stress-based adaptive strategy in the proposed procedure either accurately provides limit loads with low computational effort or effectively predicts the collapse mechanism through the concentration of elements after mesh refinement progress. The investigation of a series of numerical tests confirms the effectiveness and reliability of the proposed method.

Computers & Structures, Apr 1, 2024

This study presents a robust numerical method for the computational homogenization analysis of mi... more This study presents a robust numerical method for the computational homogenization analysis of microstructures at limit state. The periodic boundary conditions for asymmetric meshes are effectively handled using piecewise cubic Hermite interpolation. The accuracy of the numerical results is greatly enhanced through the utilization of the edge-based smoothed finite element method (ES-FEM) enriched with a cubic bubble function. The bubble-enhanced ES-FEM (bES-FEM) is also able to avoid volumetric locking induced by the incompressibility constraints in plane strain limit analysis. The optimization problem is formulated as conic programming and is rapidly solved using primal-dual interior point-based software packages. Adaptive mesh refinement offers substantial benefits in reducing computational costs and predicting the failure mechanism of microscopic heterogeneous materials. The interaction surface of the effective macroscopic strengths can also be rapidly and directly obtained. Investigation of various representative volume elements (RVE) with arbitrary mesh designs demonstrates the computational efficacy of the proposed method.

Journal of Medical Imaging, 2024

Purpose Optical coherence tomography has emerged as an important intracoronary imaging technique... more Purpose
Optical coherence tomography has emerged as an important intracoronary imaging technique for coronary artery disease diagnosis as it produces high-resolution cross-sectional images of luminal and plaque morphology. Precise and fast lumen segmentation is essential for efficient OCT morphometric analysis. However, due to the presence of various image artifacts, including side branches, luminal blood artifacts, and complicated lesions, this remains a challenging task.

Approach
Our research study proposes a rapid automatic segmentation method that utilizes nonuniform rational B-spline to connect limited pixel points and identify the edges of the OCT lumen. The proposed method suppresses image noise and accurately extracts the lumen border with a high correlation to ground truth images based on the area, minimal diameter, and maximal diameter.

Results
We evaluated the method using 3300 OCT frames from 10 patients and found that it achieved favorable results. The average time taken for automatic segmentation by the proposed method is 0.17 s per frame. Additionally, the proposed method includes seamless vessel reconstruction following the lumen segmentation.

Conclusions
The developed automated system provides an accurate, efficient, robust, and user-friendly platform for coronary lumen segmentation and reconstruction, which can pave the way for improved assessment of the coronary artery lumen morphology.

Engineering Analysis with Boundary Elements, 2023

This paper discusses an adaptive framework based on the edge-based strain smoothing approach with... more This paper discusses an adaptive framework based on the edge-based strain smoothing approach with polygonal meshes for largely deformable quasi-incompressible hyperelasticity. The proposed approach employs the quadtree decomposition for spatial discretization and the strain smoothing technique to compute the bilinear/linear form. The local refinement is based on the stress distribution and the element that has hanging nodes due to adaptive local refinement are treated as polygonal element within the strain smoothing framework. The accuracy and the robustness of the proposed framework are numerically studied with a few examples. When compared to uniform refinement, it is seen that the proposed framework yields comparable results with fewer degrees of freedom.

한국전산구조공학회 논문집, 2023

This study introduces a smoothed finite-element implementation into the phase-field framework. In... more This study introduces a smoothed finite-element implementation into the phase-field framework. In recent years, the phase-field method has recieved considerable attention in crack initiation and propagation since the method needs no further treatment to express the crack growth path. In the phase-field method, high strain-energy accuracy is needed to capture the complex crack growth path; thus, it is obtained in the framework of the smoothed finite-element method. The salient feature of the smoothed finite-element method is that the finite element cells are divided into sub-cells and each sub-cell is rebuilt as a smoothing domain where smoothed strain energy is calculated. An adaptive quadtree refinement is also employed in the present framework to avoid the computational burden. Numerical experiments are performed to investigate the performance of the proposed approach, compared with that of the finite-element method and the reference solutions.

IOP Conference Series: Earth and Environmental Science, Aug 31, 2023

대한건축학회연합논문집, 2023

This paper discusses the performance of linear regression, regression tree, support vector regres... more This paper discusses the performance of linear regression, regression tree, support vector regression, and ensemble learners in modelling airflow between two spaces based on accuracy and training time. To obtain training data, different scenarios from an existing computational fluid dynamics (CFD) model are simulated via transient analysis using Cradle scSTREAM. The raw dataset is transformed to having time step sizes of 2.5s, 5.0s, and 50.0s. Feature scaling is also employed on the each data set using both min-max scaling and z-score methods for a total of 9 datasets. Hyperparameters according to machine learning (ML) algorithms are varied such that 15 ML models across the four algorithms are developed. The results show that the regression trees perform the best over all other algorithms, with all models maintaining R2 values above 0.95 at the different datasets. On the other hand, as expected, all linear models demonstrated poor performance compared to nonlinear models. Data resolution affects model accuracy and training time, with accuracy declining slightly as time step size increased. It is also found that there is no significant effect of feature scaling. Lastly, ML models yield substantially cheaper simulation costs than CFD to simulate airflow.

한국구조물진단유지관리공학회 논문집, 2023

In this paper, the effect of corrosion level and crack width on the cohesive strength-slip behavi... more In this paper, the effect of corrosion level and crack width on the cohesive strength-slip behavior of corroded steel rebar and concrete interface is conducted. The existing studies mainly focus on the decrease in bond strength with respect to the level of corrosion; there are, however, few studies on the decrease in cohesive strength according to the crack width of the concrete surface due to corrosion. Therefore, in this study, a series of tests for the cohesive strength, slip behavior and mass loss of the reinforcing bar is evaluated at the surface of corroded rebar and concrete. It is found that the tendency to decrease the bond strength is closely related to the crack width rather than the corrosion level. Hence, to determine the degradation performance for the bond strength-slip behavior relation, the occurrence of cracks on the concrete surface can be a suitable index.

Engineering With Computers, Nov 16, 2022

In this paper, the cell-based smoothed finite-element method (CS-FEM) is proposed for solving bou... more In this paper, the cell-based smoothed finite-element method (CS-FEM) is proposed for solving boundary value problems of gradient elasticity in two and three dimensions. The salient features of the CS-FEM are: it does not require an explicit form of the shape functions and alleviates the need for iso-parametric mapping. The main idea is to sub-divide the element into simplicial sub-cells and to use a constant smoothing function in each cell to compute the gradients. This new gradient is then used to compute the bilinear/linear form. The robustness of the method is demonstrated with problems involving smooth and singular solutions in both two and three dimensions. Numerical results show that the proposed framework is able to yield accurate results. The influence of the internal length scale on the stress concentration is studied systematically for a case of a plate with a hole and a plate with an edge crack in two and three dimensions.

Remote Sensing, 2022

This paper presents a novel approach for typhoon track prediction that potentially impacts a regi... more This paper presents a novel approach for typhoon track prediction that potentially impacts a region using ensemble k-Nearest Neighbor (k-NN) in a GIS environment. In this work, the past typhoon tracks are zonally split into left and right classes by the current typhoon track and then grouped as an ensemble member containing three (left-center-right) typhoons. The proximity of the current typhoon to the left and/or right class is determined by using a supervised classification k-NN algorithm. The track dataset created from the current and similar class typhoons is trained by using the supervised regression k-NN to predict current typhoon tracks. The ensemble averaging is performed for all typhoon track groups to obtain the final track prediction. It is found that the number of ensemble members does not necessarily affect the accuracy; the determination of similarity at the beginning, however, plays an important key role. A series of tests yields that the present method is able to pro...

Sustainability, 2022

Today, the world is experiencing a tremendous catastrophic disaster that can lead to potential en... more Today, the world is experiencing a tremendous catastrophic disaster that can lead to potential environmental damage. However, awareness of how to deal with this catastrophic situation still remains very low. One of the most critical issues in disaster response is assigning disaster victims to the best emergency shelter location. This article reviews various existing studies to develop a new approach to determining emergency shelter locations. There are four evaluation criteria that are reviewed: optimization objective, decision variable, methodology, and victim identification. From the investigation, there are two major evaluations that can be further developed. In terms of decision variables, most of the previous research applies direct distance (Euclidean Distance) in the analysis process. However, the application of travel distance can represent a real evacuation process. Another interesting point is the victim identification process. Recent research applies grid-based partitioni...

Computer Modeling in Engineering & Sciences, 2022

The aim of this work is to employ a modified cell-based smoothed finite element method (S-FEM) fo... more The aim of this work is to employ a modified cell-based smoothed finite element method (S-FEM) for topology optimization with the domain discretized with arbitrary polygons. In the present work, the linear polynomial basis function is used as the weight function instead of the constant weight function used in the standard S-FEM. This improves the accuracy and yields an optimal convergence rate. The gradients are smoothed over each smoothing domain, then used to compute the stiffness matrix. Within the proposed scheme, an optimum topology procedure is conducted over the smoothing domains. Structural materials are distributed over each smoothing domain and the filtering scheme relies on the smoothing domain. Numerical tests are carried out to pursue the performance of the proposed optimization by comparing convergence, efficiency and accuracy.

International Journal for Computational Methods in Engineering Science and Mechanics, 2022

In this study, we present a displacement based polygonal finite element method for compressible a... more In this study, we present a displacement based polygonal finite element method for compressible and nearly-incompressible elastic solids undergoing large deformations in two dimensions. This is achieved by projecting the dilatation strain onto the linear approximation space, within the framework of volume averaged nodal projection method. To reduce the numerical integration burden over polytopes, a linear strain smoothing technique is employed to compute the terms in the bilinear/linear form. The salient features of the proposed framework are: (a) does not require derivatives of shape functions and complex numerical integration scheme to compute the bilinear and linear form and (b) volumetric locking is alleviated by adopting the volume averaged nodal projection technique. The efficacy, convergence properties and accuracy of the proposed framework is demonstrated through four standard benchmark problems.

Journal of Computational Design and Engineering, 2021

In this study, three variants of strain smoothing technique, viz. the cell-based, edge-based, and... more In this study, three variants of strain smoothing technique, viz. the cell-based, edge-based, and node-based smoothed finite element method, are employed for structural topology optimization. The salient features of the strain smoothing technique are: (i) does not require an explicit form of shape functions and (ii) less sensitive to mesh distortion. Within the proposed framework, the structural materials are modelled as the relative material density powered by the power-law approach. An optimum structural topology is estimated from the condition that minimizes the total strain energy of the structures of interest. The efficacy and the robustness of the strain smoothing technique, when applied to topology optimization, are demonstrated with a few standard benchmark problems. A systematic parametric study is done to find suitable and optimal control parameters for the topology optimization, viz. filter size, tuning parameter, and move limit. The relative performance of different strain smoothing techniques for structural topology optimization is also presented.

Applied sciences, Jun 10, 2021

Abstract A series of earthquake events give impetus to research on the ductile fracture behavior... more Abstract
A series of earthquake events give impetus to research on the ductile fracture behavior of steel materials. In the last decades, many fracture models have been developed and utilized in the mechanical or aerospace engineering. Nevertheless, very little application to structural members used in the construction industry has been made due to the lack of a suitable model for the fracture behavior of constructional steel. This paper presents the experimental and finite element (FE) technique to predict ductile fracture in mild carbon structural steel (SS275) sheets, which has been widely used in building structures. The post-necking true stress–strain responses were successfully estimated using the weighted-average method. The Bao and Wierzbicki (BW) model, which requires only two model parameters, was selected for the identification of fracture locus. Each model parameter was calibrated from uniaxial tension and in-plane shear specimens with the aid of digital image correlation (DIC) and finite element analysis. Fracture simulation was then performed and validated based on the experimental results of the specimens under combined tension and shear stress state.

Computers in Biology and Medicine, 2025

Objective: This study aims to investigate the micro-hemodynamic effects of strut malapposition in... more Objective: This study aims to investigate the micro-hemodynamic effects of strut malapposition in patient-specific stented coronary bifurcations.
Methods: Using the mapping-back technique, three-dimensional reconstructions of clinical post-stenting artery bifurcations with strut malapposition were accurately generated from optical coherence tomography scans of 9 patients. Computational fluid dynamics (CFD) simulations were then conducted with these models to examine the impact of strut malapposition on various fluid dynamic parameters, including flow patterns, vorticity, strain rates, viscosity, and wall shear stress (WSS). For statistical analysis, virtually apposed models were created to evaluate WSS metrics. Additionally, follow-up data for 5 out of the 9 patients were reviewed to assess evidence of late thrombosis and restenosis.
Results: Malapposed struts induce significant alterations in flow dynamics, including the formation of recirculation regions and the transition from laminar to disturbed flow. The local curvature of the lumen also affects the development of these recirculation regions. Our study demonstrates, for the first time, that the vorticity on the
abluminal side of malapposed struts exhibits an opposite sign compared to the surrounding region. The strain rate around these struts shows a distinct transition, with high values at the stent surface that rapidly diminish within the strut-lumen gap. This transition is accompanied by an increase in viscosity within these regions.
Furthermore, as the malapposition distance increases, strain rates on the malapposed struts increase while viscosity decreases. Significant differences in WSS metrics were observed between clinically malapposed and virtually apposed scenarios. In clinical follow-up cases, no evidence of thrombosis was found despite the complex
micro-hemodynamics in these patients.
Conclusion: There is complex interplay between stent malapposition and hemodynamics within a patient-specific bifurcation. The significant impact of local lumen curvature on flow dynamics underscores the limitations of idealized artery models. Moreover, the absence of thrombosis in subsequent clinical follow-up cases suggests that additional factors, such as antiplatelet medication, may play a significant role in mitigating these risks.

International Journal of Non-Linear Mechanics, Jan 2025

Volumetric locking may occur in plastic analysis of incompressible materials using low-order fini... more Volumetric locking may occur in plastic analysis of incompressible materials using low-order finite elements due to incompressibility constraints. This study presents a locking-free smoothed five-node quadrilateral element-based approach for plastic analysis in structural engineering. The proposed Q5-element employing four cell-based smoothing domains effectively alleviates the volumetric locking issues, here in the problems under plane strain conditions. The resulting large-scale optimization problem is formulated in a conic programming form, enabling efficient use of the interior-point optimizer. Numerical investigations demonstrate the method’s effectiveness in alleviating volumetric locking, accurately predicting collapse and shakedown limits, and generating interaction diagrams for load-carrying capacity and structural collapse mechanisms.

Finite Elements in Analysis and Design, 2024

Failure mechanism of 3D structures cannot always be produced by the low-order finite elements due... more Failure mechanism of 3D structures cannot always be produced by the low-order finite elements due to the so-called volumetric locking effect. In this paper, dual numerical approaches based on the bubble face-based smoothed finite element method (bFS-FEM) are developed, ensuring that the locking problem is prevented and accurate load factors of elastic-perfectly plastic structures under cyclic actions are achieved. The failure mechanisms, in terms of plastic dissipation, are realized as incremental or alternative plastic failure modes, enabling different treatments in engineering practices. Moreover, the pseudo-static approach is capable of providing three-dimensional stress fields at the failure state, which is crucial for structural design. Interaction diagrams associated with various load-types and-ranges are illustrated in numerical experiments, showing that the bearing capacity envelopes of structures under cyclic loads are evidently smaller than that of proportional loads.

한국전산구조공학회 논문집, 2024

This paper presents two-dimensional boundary value problems of the stress-based gradient elastici... more This paper presents two-dimensional boundary value problems of the stress-based gradient elasticity within the smoothed finite element method (S-FEM) framework. Gradient elasticity is introduced to address the limitations of classical elasticity, particularly its struggle to capture size-dependent mechanical behavior at the micro/nano scale. The Ru-Aifantis theorem is employed to overcome the challenges of high-order differential equations in gradient elasticity. This theorem effectively splits the original equation into two solvable second-order differential equations, enabling its incorporation into the S-FEM framework. The present method utilizes a staggered scheme to solve the boundary value problems. This approach efficiently separates the calculation of the local displacement field (obtained over each smoothing domain) from the non-local stress field (computed element-wise). A series of numerical tests are conducted to investigate the influence of the internal length scale, a key parameter in gradient elasticity. The results demonstrate the effectiveness of the proposed approach in smoothing stress concentrations typically observed at crack tips and dislocation lines.

International Journal of Mechanics and Materials in Design, 2024

This study presents an efficient numerical approach for pseudo-lower bound limit analysis of stru... more This study presents an efficient numerical approach for pseudo-lower bound limit analysis of structures. The total stress field is decomposed into two components: an elastic component associated with the safety factor and a self-equilibrating residual component. Subsequently, equilibrium conditions within the optimization problem are satisfied in a weak manner. The application of the adaptive quadtree edge-based smoothed finite element method (ES-FEM), combined with the transformation into the second-order cone programming (SOCP) form, ensures the resulting optimization problem remains minimal in size. Moreover, employing a yield stress-based adaptive strategy in the proposed procedure either accurately provides limit loads with low computational effort or effectively predicts the collapse mechanism through the concentration of elements after mesh refinement progress. The investigation of a series of numerical tests confirms the effectiveness and reliability of the proposed method.

Computers & Structures, Apr 1, 2024

This study presents a robust numerical method for the computational homogenization analysis of mi... more This study presents a robust numerical method for the computational homogenization analysis of microstructures at limit state. The periodic boundary conditions for asymmetric meshes are effectively handled using piecewise cubic Hermite interpolation. The accuracy of the numerical results is greatly enhanced through the utilization of the edge-based smoothed finite element method (ES-FEM) enriched with a cubic bubble function. The bubble-enhanced ES-FEM (bES-FEM) is also able to avoid volumetric locking induced by the incompressibility constraints in plane strain limit analysis. The optimization problem is formulated as conic programming and is rapidly solved using primal-dual interior point-based software packages. Adaptive mesh refinement offers substantial benefits in reducing computational costs and predicting the failure mechanism of microscopic heterogeneous materials. The interaction surface of the effective macroscopic strengths can also be rapidly and directly obtained. Investigation of various representative volume elements (RVE) with arbitrary mesh designs demonstrates the computational efficacy of the proposed method.

Journal of Medical Imaging, 2024

Purpose Optical coherence tomography has emerged as an important intracoronary imaging technique... more Purpose
Optical coherence tomography has emerged as an important intracoronary imaging technique for coronary artery disease diagnosis as it produces high-resolution cross-sectional images of luminal and plaque morphology. Precise and fast lumen segmentation is essential for efficient OCT morphometric analysis. However, due to the presence of various image artifacts, including side branches, luminal blood artifacts, and complicated lesions, this remains a challenging task.

Approach
Our research study proposes a rapid automatic segmentation method that utilizes nonuniform rational B-spline to connect limited pixel points and identify the edges of the OCT lumen. The proposed method suppresses image noise and accurately extracts the lumen border with a high correlation to ground truth images based on the area, minimal diameter, and maximal diameter.

Results
We evaluated the method using 3300 OCT frames from 10 patients and found that it achieved favorable results. The average time taken for automatic segmentation by the proposed method is 0.17 s per frame. Additionally, the proposed method includes seamless vessel reconstruction following the lumen segmentation.

Conclusions
The developed automated system provides an accurate, efficient, robust, and user-friendly platform for coronary lumen segmentation and reconstruction, which can pave the way for improved assessment of the coronary artery lumen morphology.

Engineering Analysis with Boundary Elements, 2023

This paper discusses an adaptive framework based on the edge-based strain smoothing approach with... more This paper discusses an adaptive framework based on the edge-based strain smoothing approach with polygonal meshes for largely deformable quasi-incompressible hyperelasticity. The proposed approach employs the quadtree decomposition for spatial discretization and the strain smoothing technique to compute the bilinear/linear form. The local refinement is based on the stress distribution and the element that has hanging nodes due to adaptive local refinement are treated as polygonal element within the strain smoothing framework. The accuracy and the robustness of the proposed framework are numerically studied with a few examples. When compared to uniform refinement, it is seen that the proposed framework yields comparable results with fewer degrees of freedom.

한국전산구조공학회 논문집, 2023

This study introduces a smoothed finite-element implementation into the phase-field framework. In... more This study introduces a smoothed finite-element implementation into the phase-field framework. In recent years, the phase-field method has recieved considerable attention in crack initiation and propagation since the method needs no further treatment to express the crack growth path. In the phase-field method, high strain-energy accuracy is needed to capture the complex crack growth path; thus, it is obtained in the framework of the smoothed finite-element method. The salient feature of the smoothed finite-element method is that the finite element cells are divided into sub-cells and each sub-cell is rebuilt as a smoothing domain where smoothed strain energy is calculated. An adaptive quadtree refinement is also employed in the present framework to avoid the computational burden. Numerical experiments are performed to investigate the performance of the proposed approach, compared with that of the finite-element method and the reference solutions.

IOP Conference Series: Earth and Environmental Science, Aug 31, 2023

대한건축학회연합논문집, 2023

This paper discusses the performance of linear regression, regression tree, support vector regres... more This paper discusses the performance of linear regression, regression tree, support vector regression, and ensemble learners in modelling airflow between two spaces based on accuracy and training time. To obtain training data, different scenarios from an existing computational fluid dynamics (CFD) model are simulated via transient analysis using Cradle scSTREAM. The raw dataset is transformed to having time step sizes of 2.5s, 5.0s, and 50.0s. Feature scaling is also employed on the each data set using both min-max scaling and z-score methods for a total of 9 datasets. Hyperparameters according to machine learning (ML) algorithms are varied such that 15 ML models across the four algorithms are developed. The results show that the regression trees perform the best over all other algorithms, with all models maintaining R2 values above 0.95 at the different datasets. On the other hand, as expected, all linear models demonstrated poor performance compared to nonlinear models. Data resolution affects model accuracy and training time, with accuracy declining slightly as time step size increased. It is also found that there is no significant effect of feature scaling. Lastly, ML models yield substantially cheaper simulation costs than CFD to simulate airflow.

한국구조물진단유지관리공학회 논문집, 2023

In this paper, the effect of corrosion level and crack width on the cohesive strength-slip behavi... more In this paper, the effect of corrosion level and crack width on the cohesive strength-slip behavior of corroded steel rebar and concrete interface is conducted. The existing studies mainly focus on the decrease in bond strength with respect to the level of corrosion; there are, however, few studies on the decrease in cohesive strength according to the crack width of the concrete surface due to corrosion. Therefore, in this study, a series of tests for the cohesive strength, slip behavior and mass loss of the reinforcing bar is evaluated at the surface of corroded rebar and concrete. It is found that the tendency to decrease the bond strength is closely related to the crack width rather than the corrosion level. Hence, to determine the degradation performance for the bond strength-slip behavior relation, the occurrence of cracks on the concrete surface can be a suitable index.

Engineering With Computers, Nov 16, 2022

In this paper, the cell-based smoothed finite-element method (CS-FEM) is proposed for solving bou... more In this paper, the cell-based smoothed finite-element method (CS-FEM) is proposed for solving boundary value problems of gradient elasticity in two and three dimensions. The salient features of the CS-FEM are: it does not require an explicit form of the shape functions and alleviates the need for iso-parametric mapping. The main idea is to sub-divide the element into simplicial sub-cells and to use a constant smoothing function in each cell to compute the gradients. This new gradient is then used to compute the bilinear/linear form. The robustness of the method is demonstrated with problems involving smooth and singular solutions in both two and three dimensions. Numerical results show that the proposed framework is able to yield accurate results. The influence of the internal length scale on the stress concentration is studied systematically for a case of a plate with a hole and a plate with an edge crack in two and three dimensions.

Remote Sensing, 2022

This paper presents a novel approach for typhoon track prediction that potentially impacts a regi... more This paper presents a novel approach for typhoon track prediction that potentially impacts a region using ensemble k-Nearest Neighbor (k-NN) in a GIS environment. In this work, the past typhoon tracks are zonally split into left and right classes by the current typhoon track and then grouped as an ensemble member containing three (left-center-right) typhoons. The proximity of the current typhoon to the left and/or right class is determined by using a supervised classification k-NN algorithm. The track dataset created from the current and similar class typhoons is trained by using the supervised regression k-NN to predict current typhoon tracks. The ensemble averaging is performed for all typhoon track groups to obtain the final track prediction. It is found that the number of ensemble members does not necessarily affect the accuracy; the determination of similarity at the beginning, however, plays an important key role. A series of tests yields that the present method is able to pro...

Sustainability, 2022

Today, the world is experiencing a tremendous catastrophic disaster that can lead to potential en... more Today, the world is experiencing a tremendous catastrophic disaster that can lead to potential environmental damage. However, awareness of how to deal with this catastrophic situation still remains very low. One of the most critical issues in disaster response is assigning disaster victims to the best emergency shelter location. This article reviews various existing studies to develop a new approach to determining emergency shelter locations. There are four evaluation criteria that are reviewed: optimization objective, decision variable, methodology, and victim identification. From the investigation, there are two major evaluations that can be further developed. In terms of decision variables, most of the previous research applies direct distance (Euclidean Distance) in the analysis process. However, the application of travel distance can represent a real evacuation process. Another interesting point is the victim identification process. Recent research applies grid-based partitioni...

Computer Modeling in Engineering & Sciences, 2022

The aim of this work is to employ a modified cell-based smoothed finite element method (S-FEM) fo... more The aim of this work is to employ a modified cell-based smoothed finite element method (S-FEM) for topology optimization with the domain discretized with arbitrary polygons. In the present work, the linear polynomial basis function is used as the weight function instead of the constant weight function used in the standard S-FEM. This improves the accuracy and yields an optimal convergence rate. The gradients are smoothed over each smoothing domain, then used to compute the stiffness matrix. Within the proposed scheme, an optimum topology procedure is conducted over the smoothing domains. Structural materials are distributed over each smoothing domain and the filtering scheme relies on the smoothing domain. Numerical tests are carried out to pursue the performance of the proposed optimization by comparing convergence, efficiency and accuracy.

International Journal for Computational Methods in Engineering Science and Mechanics, 2022

In this study, we present a displacement based polygonal finite element method for compressible a... more In this study, we present a displacement based polygonal finite element method for compressible and nearly-incompressible elastic solids undergoing large deformations in two dimensions. This is achieved by projecting the dilatation strain onto the linear approximation space, within the framework of volume averaged nodal projection method. To reduce the numerical integration burden over polytopes, a linear strain smoothing technique is employed to compute the terms in the bilinear/linear form. The salient features of the proposed framework are: (a) does not require derivatives of shape functions and complex numerical integration scheme to compute the bilinear and linear form and (b) volumetric locking is alleviated by adopting the volume averaged nodal projection technique. The efficacy, convergence properties and accuracy of the proposed framework is demonstrated through four standard benchmark problems.

Journal of Computational Design and Engineering, 2021

In this study, three variants of strain smoothing technique, viz. the cell-based, edge-based, and... more In this study, three variants of strain smoothing technique, viz. the cell-based, edge-based, and node-based smoothed finite element method, are employed for structural topology optimization. The salient features of the strain smoothing technique are: (i) does not require an explicit form of shape functions and (ii) less sensitive to mesh distortion. Within the proposed framework, the structural materials are modelled as the relative material density powered by the power-law approach. An optimum structural topology is estimated from the condition that minimizes the total strain energy of the structures of interest. The efficacy and the robustness of the strain smoothing technique, when applied to topology optimization, are demonstrated with a few standard benchmark problems. A systematic parametric study is done to find suitable and optimal control parameters for the topology optimization, viz. filter size, tuning parameter, and move limit. The relative performance of different strain smoothing techniques for structural topology optimization is also presented.

Applied sciences, Jun 10, 2021

Abstract A series of earthquake events give impetus to research on the ductile fracture behavior... more Abstract
A series of earthquake events give impetus to research on the ductile fracture behavior of steel materials. In the last decades, many fracture models have been developed and utilized in the mechanical or aerospace engineering. Nevertheless, very little application to structural members used in the construction industry has been made due to the lack of a suitable model for the fracture behavior of constructional steel. This paper presents the experimental and finite element (FE) technique to predict ductile fracture in mild carbon structural steel (SS275) sheets, which has been widely used in building structures. The post-necking true stress–strain responses were successfully estimated using the weighted-average method. The Bao and Wierzbicki (BW) model, which requires only two model parameters, was selected for the identification of fracture locus. Each model parameter was calibrated from uniaxial tension and in-plane shear specimens with the aid of digital image correlation (DIC) and finite element analysis. Fracture simulation was then performed and validated based on the experimental results of the specimens under combined tension and shear stress state.

Conference: Symposium of the International Association for Shell and Spatial Structures, 2009

This paper describes a new topology optimization (TO) technique based on meshless method to evolv... more This paper describes a new topology optimization (TO) technique based on meshless method to evolve two-dimensional truss structures. The meshless method has been considered as a very attractive computational technique since it does not need any mesh generation process during the analysis. It has been gradually and widely used in many engineering disciplines and a particular weak point such as the re-generation of mesh information inherit in other numerical analysis techniques have been naturally solved. However, there have been a few applications of meshless method into structural design optimization so that we here try to apply the meshless method into structural topology optimization problem. We adopt the radial point interpolation method (RPIM) which uses radial basis function (RBF) since it is stable and robust for arbitrary nodal distributions. Then, the hard kill method based on fully stressed design scheme is consistently combined with the adopted meshless method. In order to demonstrate the accuracy of the proposed topology optimization technique, several benchmark tests are tackled to investigate the accuracy and capability of the present TO technique. From numerical results, it is found to be that the proposed TO technique is very simple and easy to produce the optimum topologies of plane structures.

공간구조공학회 춘계학술발표대회 눈문집, 2009

This study proposes a structual topology optimization technique using meshless method. We adopt t... more This study proposes a structual topology optimization technique using meshless method. We adopt the radial point interpolation method (RPIM) which uses the radial basis function (RBF). So far, there is a few application of new meshless method into the structural design optimization. Therefore, we here apply the meshless method into structural topology optimization problem. The hard kill method based on fully stressed design technique is consistently combined with RPIM meshless method. In order to demonstrate the accuracy of the proposed topology optimization technique, a benchmark test is tackled to investigate the performance of new topology optimization process.

대한건축학회지회연합회 학술발표대회논문집, 2008

A meshless method based on the radial point interpolation method(RPIM) is used to analyze cantile... more A meshless method based on the radial point interpolation method(RPIM) is used to analyze cantilever beam. Meshless methods have been considered is a very attractive as new computational method since it does not need mesh generation in analysis procedure. Especially, RPIM, using radial basis function(RBF), has an advantage that do not consider the essential boundary condition. In order to demonstrate the accuracy of the adopted meshless method, numerical examples are tackled with multiple elastic moduli and the different number of background cell. Numerical results are compared with the reference solution produced by finite element analysis program FEAP.