Gunjin Yun

Seoul National University | SNU · Department of Mechanical Aerospace Engineering

Acquiring reliable microstructure datasets is a pivotal step toward the systematic design of materials with the aid of integrated computational materials engineering (ICME) approaches. However, obtaining three-dimensional (3D) microstructure datasets is often challenging due to high experimental costs or technical limitations, while acquiring two-d...

Acquiring reliable microstructure datasets is a pivotal step toward the systematic design of materials with the aid of integrated computational materials engineering (ICME) approaches. However, obtaining three-dimensional (3D) microstructure datasets is often challenging due to high experimental costs or technical limitations, while acquiring two-d...

Acquiring reliable microstructure datasets is a pivotal step toward the systematic design of materials with the aid of integrated computational materials engineering (ICME) approaches. However, obtaining three-dimensional (3D) microstructure datasets is often challenging due to high experimental costs or technical limitations, while acquiring two-d...

This paper puts forward a novel integrated microstructure design methodology that replaces the common existing design approaches for multifunctional composites: 1) reconstruction of microstructures, 2) analyzing and quantifying material properties, and 3) inverse design of materials using the diffusion-based generative model (DGM). The problem of m...

The recent trends of vitrimer studies enhance the thermoset material with superior properties, therefore, it is particularly important to address the critical scientific inquiries in this area using their research metrics. The reported vitrimer systems have been highly required for future real-time applications; however, the inquisitiveness of mate...

In this paper, a chemo-mechanically coupled behavior of Nafion 212 is investigated through predictive multiphysics modeling and experimental validation. Fuel cell performance and durability are critically determined by the mechanical and chemical degradation of a perfluorosulfonic acid (PFSA) membrane. However, how the degree of chemical decomposit...

This paper proposes a microstructure reconstruction framework with denoising diffusion models for the first time. The novelty and strength of the proposed model lie in its universality and generality for the microstructure characterization and reconstruction (MCR) that can be applied to various types of composite materials. The applicability of the...

This paper presents the NTS-AR (node-to-segment with area regularization) method to analyze the three-dimensional dynamic frictional contact bodies under large deformation and plastic material behavior. The extended NTS-AR method considers the 3D geometric structure of the slave surface and frictional constraint in a convected coordinate system. De...

This paper presents a novel modeling framework to generate an optimal microstructure having ultimate multifunctionality using a diffusion-based generative model. In computational material science, generating microstructure is a crucial step in understanding the relationship between the microstructure and properties. However, using finite element (F...

This paper presents a multiscale progressive fatigue damage model to predict the fatigue life of composite laminates. The multi-level damage model was employed considering the interfacial debonding effect, and effective material properties were calculated through homogenization. Damage variables and damage slopes were defined at the constituent lev...

N-doped graphene stabilized Cu(I)-catalyzed self-healing nanocomposites are developed. This study found the use of N-doped graphene as both a nanostructured material for enhancing mechanical and conductive properties and a catalyst promoter (a scaffold for catalytic copper(I) particles), helpful to trigger self-healing via "click chemistry". Due to...

In this paper, catalyst-free room-temperature healing epoxy vitrimer-like materials (S-vitrimer) are introduced. The S-vitrimer can be healed at room temperature without any external stimuli such as solvent, pressure, heat, and catalyst through an aromatic disulfide exchange reaction and a hydrogen bond because the glass transition temperature of t...

This paper describes a newly proposed viscoelastic–viscoplastic constitutive model that is used to characterize the effective mechanical behaviors of fiber-reinforced composites based on micromechanical modeling. A homogenization approach is developed in which the effective stress of the fiber-reinforced composite is decomposed into an elastoplasti...

This paper presents a progressive fatigue damage model to predict the damage progress and the fatigue life of composite laminates under cyclic loading. First, the maximum stress criterion was applied in the fiber direction for fiber failure (FF), and Puck’s failure criteria were employed in the matrix direction in which the fracture plane is define...

The design and utilization of polymers with healing capability have drawn increasing attention owing to their enhanced chain mobility and opportunity to heal minor cracks in composites. Rehealable thermoset polymers promise a reduction in maintenance cost and thus prolonged lifetime as well as reshaping and recyclability. Introducing reversible cov...

Microstructure reconstruction has been an essential part of computational material engineering to reveal the relationship between the microstructures and the material properties. However, it is still challenging to find a general solution for microstructure characterization and reconstruction (MCR) tasks although there have been many attempts such...

This paper proposes an integrated unified elasto-viscoplastic fatigue and creep damage model with modified hardening equations to simulate the elasto-viscoplasticity, primary-secondary and tertiary creep, relaxation, cyclic softening, temperature-dependency, and creep-fatigue interaction (CFI) damage. For this purpose, we integrate the creep damage...

This paper proposes a novel time-domain homogenization model combining the viscoelastic constitutive law with Eshelby's inclusion theory-based micromechanics model to predict the mechanical behavior of the particle reinforced composite material. The proposed model is intuitive and straightforward capable of predicting composites' viscoelastic behav...

Although there has been a substantial volume of research for mitigating the thermally-induced residual stress with the heat accumulation in powder bed fusion (PBF) process, it has been challenging to quantify, access, and optimize the heat dissipation at part-scale. Furthermore, a practical method of designing three-dimensional support with a reaso...

This paper presents a multiscale modeling method for sheet molding compound (SMC) composites through a novel bundle packing reconstruction algorithm based on a micro-CT (Computed Tomography) image processing. Due to the complex flow pattern during the compression molding process, the SMC composites show a spatially varying orientation and overlappi...

This paper presents a novel hierarchical micromechanics model to simulate Sheet Molding Compound (SMC) composites with wavy fibers. The three-step homogenizations are integrated to calculate the effective properties of SMC chips, layers, and composites, respectively. During homogenization, it has a unique capability of modeling the wavy fibers with...

Polybenzimidazoles (PBIs) containing two and three ether groups per a repeating unit are prepared to improve the processability of the PBI system for the first time. Poly[2,2′-(m-phenylene)-1,4-bis(5-oxybenzimidazole)benzene] (DOPBI, PBI containing two ether groups per a repeating unit) is synthesized using 1,4-bis(3,4-diaminophenoxy)benzene (DAPB)...

This paper presents a multiscale failure and damage analysis methodology for sheet molding compound (SMC) composites through a new bundle packing reconstruction algorithm and complete failure and damage constitutive modeling. Due to the complex flow pattern during the compression molding process, the SMC composites are featured by a spatially varyi...

This paper presents a molecular structure-informed viscoelastic constitutive equation that adopts the Doi-Edward’s tube model with coarse-grained molecular dynamics (MD) simulation and primitive path analysis. Since this model contains polymer physics-related parameters directly obtained from molecular simulations, it can reflect molecular informat...

This research provides a multiscale framework of failure analysis for carbon-fiber sheet molding compound (CF-SMC) through a bundle packing reconstruction algorithm and material constitutive modeling, respectively. SMC composites have attracted the attention of many industries due to their high strength-to-weight ratio. However, depending on the sp...

Concerns have been ramping up with regard to the propagation of infectious droplets due to the recent COVID-19 pandemic. The effects of filter microstructures and ambient air flows on droplet dispersion by sneezing are investigated by a fully coupled Eulerian–Lagrangian computational modeling with a micro-to-macroscale bridging approach. Materials...

In this paper, we present a novel efficient multi-scale modeling methodology for three-dimensional (3D) needle-punched (NP) carbon/silicon carbide (NP Cf/SiCm) with a complex microstructure. Variations of the material properties induced by the needle-punching process and complex geometrical features could pose challenges when estimating the materia...

A new nano-micro–macro multiscale modeling approach that combines molecular dynamic (MD) simulations with micromechanics and stochastic continuum models is proposed to model carbon-fiber-reinforced graphene/epoxy nanocomposites. Halpin–Tsai and shear lag micromechanics models are employed to relate elastic properties from MD simulations with graphe...

Coarse-grained molecular dynamics simulations are a widely accepted methodology in the field of studying the viscoelasticity of elastomers. In this paper, a thermophysically balanced multiscale coarse-grained potential for glass-forming polymers is presented with the energy renormalization (ER) method by redefining temperature transferable correlat...

This paper proposes a stochastically coupled thermo-chemical and viscoelastic model and provides new insights from comparisons of the cure-induced residual stresses and distortions between various ply configurations. Six different laminate configurations were modeled to compare the material response during four manufacturing steps: in-mold curing,...

In this paper, we present a procedure that predicts the dynamic mechanical properties of viscoelastic composites. The time-domain homogenization model of the viscoelastic composites is introduced with Eshelby’s inclusion theory. The constitutive modeling is presented, and the pseudocode is provided to show the numerical computation procedure precis...

Selective laser melting (SLM), one of the most widely used powder bed fusion (PBF) additive manufacturing (AM) technology, enables the fabrication of customized metallic parts with complex geometry by layer-by-layer fashion. However, SLM inherently poses several problems such as the discontinuities in the molten track and the steep temperature grad...

This paper proposes a micromorphic simulation model with novel strain rate relationships between MD simulations and macroscale experiments at the ductile-brittle (DB) transition of glassy polymer and dynamic heterogeneity of deformation. Novelties of this paper are on the strain rate relationship between two different scale regions and investigatio...

Re-melting process has been utilized to mitigate the residual stress level in the selective laser melting (SLM) process in recent years. However, the complex consolidation mechanism of powder and the different material behavior after the first laser melting hinder the direct implementation of the re-melting process. In this work, the effects of re-...

In this paper, a novel nonlinear micromechanics constitutive model is proposed to simulate elastoplastic and ductile damage behavior for wavy carbon nanotube (CNT) nanocomposites. A modified Eshelby tensor corresponding to wavy CNT is derived by integrating a rotational transformation of the effective composite stiffness tensor along the sinusoidal...

In this paper, a novel hybrid heat source model is developed considering the different absorption mechanisms for porous and dense state materials, and an effective absorptivity is adapted to the proposed model to analyze the melting mode transition. The proposed model can predict the melt pool characteristics including the melt pool dimensions and...

In this paper, we elucidate the filler effect of the vitrimer nanocomposites in the atomistic aspect for the first time with the molecular dynamics (MD) simulations. The self-healing properties are compared between GO/vitrimer nanocomposites and pristine vitrimers by the self-healing simulation containing the bond exchange reaction algorithm. The r...

In this paper, we propose a multiscale damage plasticity model for particulate composites within an incremental Mori-Tanaka (MT) micromechanics framework and present numerical and experimental verification. J2 plasticity and Lemaitre-Chaboche ductile damage models account for damage in the matrix and a linear spring model accounts for interface dam...

Mechanoluminescence (ML), which emits light upon external mechanical stress, was applied to fibrous composites. Herein, ML particles were incorporated into poly(vinylidene fluoride) (PVDF) and polyacrylonitrile (PAN) electrospun webs to prepare ML/PVDF and ML/PAN composite fabrics. The produced fabrics were treated with O2 and C4F8 plasma to modify...

This article proposes a three-dimensional (3D) stochastic progressive damage simulation model for the fiber-reinforced polymer matrix composite laminates. Constituents’ strength and fracture energy of each lamina were accounted as spatially varying random fields by 3D Karhunen–Loeve expansion method. Damage mechanisms modeled by continuum damage me...

The surfaces of the flight vehicles passing the low Earth orbit at high speeds of 7–8 km/s are placed in high temperature conditions, and the thermal protection system (TPS) is used to withstand that high temperature. Silicon carbide (SiC) is frequently used in TPSs because of its various material properties suitable for use in high temperature env...

Vitrimers- a class of polymeric networks, those change topology above a threshold temperature have been investigated in recent years. In order to further extent their properties, in this research, we demonstrate disulfide exchange assisted poly dimethyl siloxane (PDMS) and graphene oxide (GO) involved epoxy vitrimers, exhibit a reduction in glass t...

In this study, the two-scale micromorphic theory incorporating tensile experiments and molecular dynamics (MD) simulations, which are compatible in both length scale and time scale is investigated. To the authors’ knowledge, this is the first attempt to combine fully-compatible experiments and MD simulations into a multi-scale continuum framework w...

In this paper, we present a material constitutive model for three-dimensional (3D) needle-punched carbon/silicon carbide (Cf/SiCm) with a complex meso-scale structure and characterize the material properties through an experimental-numerical hybrid inverse identification analysis. Variations of the material properties induced by the needle-punching...

In this paper, we present a novel approach to enhance the mechanoluminescence (ML) sensitivity of composite made with epoxy resin and strontium aluminate co-doped with europium ions and dysprosium ions (SrAl2O4: Eu²⁺, Dy³⁺) (SAOED) by subjecting it to ultrasonic vibration throughout the curing process. Three different SAOED particle size models wer...

Internal pores in composite materials generated during fabrication processing can induce negative influences on the physical properties. Among them, the fatigue life under repetitive loadings is one of the most detrimentally influenced properties by the undesirable pores. In this paper, we investigated the effect of pore geometries – 1) sizes and 2...

In this paper, we proposed a new material design method by microstructure topology optimization. Novelty of the proposed method is to target the whole nonlinear volume-averaged effective stress-strain curve of microstructure representative volume element (RVE) rather than aiming specific values such as strength, stiffness or Poisson ratio. J2 plast...

Catalyst free graphene oxide (GO) promoted self-healing vitrimer nanocomposites are designed, where the synthesized vitrimer nanocomposites displays self-healing properties via disulfide exchange based covalent adaptive network behavior. This study found that GO based nanofiller enhance the self-healing properties, including the shape memory and fl...

In this paper, design criteria are proposed for an aircraft morphing wing skin and conventional lattice structures are evaluated against the design criteria. Although the structural deformation and aerodynamic loadings during the morphing process directly affect the skin structure, the design of the morphing skin has been overlooked. Five design cr...

This paper demonstrates a set of two-scale asymptotic computational homogenization formalisms of Cf/SiCm thermomechanical properties and proposes a 3D micro-to-mesoscale stochastic bridging methodology considering the uncertainties of the constituents’ properties and geometries. For multiscale thermomechanical analyses, the thermal conductivity (TC...

In this paper, a new concept of a gripper pin structure is proposed for an aircraft morphing wing that can decouple in-plane stiffness from out-of-plane stiffness. There has been a pressing need for morphing wing skin structures that hold both low in-plane and high out-of-plane stiffness. A design methodology for the gripper pin morphing wing skin...

In this paper, we investigate thermomechanical characteristics of silica-mineralized nitrogen-doped carbon nanotube (SC-NCNT)-reinforced poly (methyl methacrylate) (PMMA) nanocomposites for the first time by molecular dynamics simulations. An in-situ mineralization algorithm is employed for the mineralization of the silica layer where the thickness...

In this paper, we proposed a novel molecular dynamics (MD) free radical polymerization algorithm that can dynamically simulate chain growth of the thermoplastic network and produce unit cells having experimentally observed molecular weight distributions. In the proposed algorithm, a coarse-grained force field was implemented by combining bonded ter...

In this paper, an extended Mori-Tanaka (MT) model was proposed to evaluate effective stiffnesses of wavy carbon nanotube (CNT) nanocomposites with interface damage. The proposed model combined MT theory with linear spring model and wavy CNT model. We validated the proposed model by comparing analytically and numerically derived dilute strain concen...

In this paper, we investigated the process variable effects on the damage and deformational behavior of fused deposition modeling (FDM) three-dimensional (3D)-printed specimens by performing tensile tests and inverse identification analyses. A characterization of the effects of different parametric variations of 3D-printed specimens on fracture pro...

In this paper, we report on a new photonic behavior based on a series of photoluminescence and mechanoluminescence experiments conducted on Strontium aluminate (SrAl2O4) (SAO) co-doped with lanthanide ions such as Eu²⁺ (europium ion) and Dy³⁺ (dysprosium ion) (SAOED). It was observed that the increase of the dislocation density in SAOED decreased i...

In this paper, a mathematical proof of the existence of a global minimum of Self-Optim (Self-Optimizing Inverse Analysis Method) cost functional is presented based upon weak-solution theory of partial differential equations. The Self-Optim provides single global minimum rather than having multiple global minima corresponding to unrealistic solution...

In this paper, we present an approach for characterizing the interfacial region using the molecular dynamics (MD) simulations and the shear deformation model (SDM). The bulk-level mechanical properties of graphene-reinforced nanocomposites strongly depend on the interfacial region between the graphene and epoxy matrix, whose thickness is about 6.8-...

In this paper, a 3D stochastic computational homogenization model for carbon fiber-reinforced (CFR) CNT/epoxy matrix composites was presented. Stochastic waviness, agglomeration and orientation of CNT fillers cause random spatial variations of the elasticity tensor of the CNT/epoxy matrix within a microscale RVE, resulting in probabilistic variatio...

In this paper, a novel investigation on the effect of microstructure morphology on the stress within mechanoluminescent (ML) particles is presented. It is known that the light intensity of ML materials is proportional to deviatoric stresses in the particles. A set of digital image processing techniques is introduced to compute quantitative morpholo...

This paper presented underlying methylene blue (MB) color-fading mechanisms by photoluminescence-photocatalysis (PL-PC) reaction with PL materials coated with titanium dioxide (TiO2). The color fading mechanism means the photocatalytic degradation of organic dye by the coated TiO2photocatalyst. The photocatalyst is activated by photoluminescent mat...

This paper presents a probabilistic multiscale approach to model the random spatial distribution of local elastic properties arising from the heterogeneous waviness and orientation of CNT fillers within a 3D microscale continuum representative volume element (RVE) of a CNT-reinforced polymer matrix. The proposed direction-sensitive 3D Karhunen-Loèv...

In this paper, a generalized discretization scheme is proposed that can derive general-order finite difference equations representing the joint probability density function of dynamic response of stochastic systems. The various order of finite difference equations are applied to solutions of the Fokker-Planck-Kolmogorov (FPK) equation. The finite d...

This paper proposed a probabilistic service life prediction method for reinforced concrete (RC) structures with randomly distributed chloride corrosion-induced cracking. In the proposed method, spatial randomness of environmental, geometric and physical factors was considered that influence corrosion process and crack propagation of RC structures a...

In this paper, a method for extracting stress-strain databases from material test measurements is introduced as one of the potential Integrated Computational Materials Engineering (ICME) tools. Measuring spatially heterogeneous stress and strain evolutionary data during material tests is a challenging and costly task. The proposed method can extrac...

In this paper, a novel fatigue life prediction model for electronic components under non-Gaussian random vibration excitations is proposed based on random vibration and fatigue theory. This mathematical model comprehensively associates the vibration fatigue life of electronic components, the characteristics of vibration excitations (such as the roo...

In this paper, a new calibration method for mechano-luminescence (ML) thin film sensors was proposed to enable quantitative full-field strain measurements in pixel-level resolution for the first time along with two standard reference test methods. The proposed method has a distinct advantage of its facet-free full-field strain sensing capability wi...

This paper introduces a new NDT technique that uses mechanoluminescence sensing materials. The mechanoluminescent sensor technology detects cracks and material discontinuities visually through the application of luminescent materials embedded in a coating. The technology does not require expensive analytical equipment or highly skilled operators. T...

In this paper, probabilistic failure response and damage patterns in polymer matrix composite laminates was investigated by considering spatially varying strength properties. For this purpose, an efficient random field modeling framework for multiple cross-correlated random fields is proposed whereby different a set of uncorrelated random variables...

In this paper, a new methodology for reconstructing spatially varying random material properties is presented by combining stochastic finite element (SFE) models with Self-Optimizing Inverse Method (Self-OPTIM). The Self-OPTIM can identify model parameters based on partial boundary force and displacement data from experimental tests. Statistical in...

In this paper, a new inverse uncertainty quantification method was proposed to identify statistical parameters associated with spatially varying material properties and to reconstruct their heterogeneous distributions from limited experimental measurements. The proposed method parameterizes statistical models of random fields with analytic covarian...

Novel accelerated random vibration fatigue test methodology and strategy are proposed, which can generate a design of the experimental test plan significantly reducing the test time and the sample size. Based on theoretical analysis and fatigue damage model, several groups of random vibration fatigue tests were designed and conducted with the aim o...

A vibration-based testing methodology has been developed that will assess fatigue behavior of the metallic material of construction for the Advanced Stirling Convertor displacer (planar) spring component. To minimize the testing duration, the test setup is designed for base-excitation of a multiplespecimen arrangement, driven in a high-frequency re...