Application of Ductile Fracture Criteria to Fracture Prediction of Punch-Stretch Tests

Yanshan Lou,Hoon Huh(허훈),Gihyun Bae(배기현),Younki Ko(고윤기),Seokbong Kim(김석봉) 한국자동차공학회 2011 한국자동차공학회 부문종합 학술대회 Vol.2011 No.5

This paper applies various ductile fracture criteria into finite element analysis (FEA) to predict the fracture initiation of punch-stretch tests. Punch-stretch tests are carried out to construct forming limit diagrams (FLDs) of SPCC (Steel plated cold-rolled commercial quality). Limit dome heights (LDHs) are measured from the fractured specimens in punch-stretch tests. The limit strains in uniaxial tension, plane strain and equibiaxial tension conditions are measured in FLDs and utilized to evaluate the material constants in ductile fracture criteria. To implement ductile fracture criteria into FEA, user subroutines are developed for the commercial software ABAQUS/Explicit with the Hill48 yield function. Simulations are carried out to predict fracture initiation of punch-stretch tests by ductile fracture criteria. Fracture locations predicted by ductile fracture criteria are compared with fractured specimens in experiments. LDHs are measured from simulations and compared with experimental results. The comparison demonstrates that LDHs predicted by various ductile fracture criteria match the experimental results with higher accuracy for arc-shape specimens while the Lou-Huh criterion predicts more accurate LDHs than other ductile fracture criteria for square-shaped specimens.

Fracture-based forming limit criteria for anisotropic materials in sheet metal forming

Park, Namsu,Huh, Hoon,Lim, Sung Jun,Lou, Yanshan,Kang, Yeon Sik,Seo, Min Hong Elsevier 2017 International journal of plasticity Vol.96 No.-

<P><B>Abstract</B></P> <P>This paper is concerned with modeling of fracture-based forming limit criteria for anisotropic materials in sheet metal forming to predict the sudden fracture in complicated forming processes. The Lou–Huh ductile fracture criterion is modified using the Hill's 48 anisotropic yield function instead of the von Mises isotropic yield function to take account of the influence of anisotropy on the equivalent plastic strain at the onset of fracture. For the derivation of an anisotropic ductile fracture criterion, the principal stresses (<I>σ</I> <SUB>1</SUB>,<I>σ</I> <SUB>2</SUB>,<I>σ</I> <SUB>3</SUB>) are expressed in terms of the stress triaxiality, the Lode parameter, and the equivalent stress ( <SUB> η H </SUB> , <SUB> L P </SUB> , <SUB> σ ¯ H </SUB> ) based on the Hill's 48 yield function. Three different kinds of fracture-based forming limit criteria are suggested and investigated with an assumption that the stress state is under the plane stress condition with proportional loading. To determine the parameters of the model proposed, the two-dimensional digital image correlation (2D-DIC) method is utilized to measure the strain histories on the surface of three different types of specimens during deformation and the measurement results are investigated to identify the anisotropy effect on the equivalent plastic strain at the onset of fracture. This paper also discusses about a scaling method for a strain-based fracture forming limit criterion in order to capture the onset of fracture using a single forming limit curve for an anisotropic material. From the comparison of various forming limit criteria suggested, it is concluded that a polar effective plastic strain-based (PEPS) fracture forming limit diagram (FFLD) is suitable for prediction of the sudden fracture in AHSS sheets in complicated sheet metal forming processes on the basis of its path independence and simplicity of measuring strains in real forming processes.</P> <P><B>Highlights</B></P> <P> <UL> <LI> This paper is concerned with fracture-based forming limit criteria of DP980 sheets. </LI> <LI> The anisotropic ductile fracture criterion is suggested on the basis of the Lou–Huh ductile fracture criterion. </LI> <LI> The principal stresses (<I>σ</I> <SUB>1</SUB>,<I>σ</I> <SUB>2</SUB>,<I>σ</I> <SUB>3</SUB>) are expressed in terms of ( <SUB> η H </SUB> , <SUB> L P </SUB> , <SUB> σ ¯ H </SUB> ) . </LI> <LI> Three different kinds of fracture-based forming limit criteria are suggested for anisotropic materials. </LI> <LI> A fracture-based polar effective plastic strain FFLD is suitable for prediction of the sudden fracture of AHSSs. </LI> </UL> </P>

A Case Study of EFL College Students' English Learning Style Preferences in South Korea

Yan Lou(Yan Lou ), 김정인(Jungyin Kim) 학습자중심교과교육학회 2023 학습자중심교과교육연구 Vol.23 No.9

목적 이 연구는 두 EFL 영어 전공자의 학습 스타일을 탐색하여 학습 스타일에 영향을 미치는 요인을 식별하는 것을 목표로 했다. 방법 질적 연구를 통해 문화적 배경이 다른 2명의 영어 전공자(한국인 1명, 중국 유학생 1명)를 사례 연구로 선정했다. 연구원들은 2020년 10월부터 2021년 4월까지 한국 국립대학교에서 진행된 고급 영어 회화 수업의 현장 노트를 작성하고 인터뷰를 완료했고 데이터 분석 단계에서 연구자는 인터뷰 내용을 코딩하고 교실 관찰 기록 및 인터뷰 노트와 같은 추가 자료를 조사했다. 결과 한국학생 Alice의 시각 스타일이 동작 스타일을 통해 제고된 반면 국제중국학생 Cindy는 시각 스타일을 통해 청각 스타일을 향상시켰다. 다두 EFL 참가자의 서로 다른 학습 스타일에 영향을 미치는 요인이 가족 환경, 학교 환경 및 문화적 배경임을 발견했다. 결론 본 연구는 학습 선호도가 타고난 능력이 아니라 개인의 가정환경, 학교 환경 및 교사의 스타일, 문화적 배경에 따라 달라질 수 있음을 밝혔다. 이러한 결과를 바탕으로 개인이 자신의 학습 스타일을 알게 되면 이를 학습 과정에 도움이 된다. 또한 학생의 학습 스타일 특성을 파악하면 교사가 자신의 강의 스타일과 수업 자료를 탐색하는 데 도움이 된다. Objectives This study aimed to explore two EFL English majors’ learning styles to identify factors affecting their learning styles. Methods The study conducted qualitative research and selected two English majors with different cultural background (i.e., one Korean and one international Chinese student) as a case study. The researchers took field notes of an advanced English conversation class and completed interviews between October 2020 and April 2021 at a Korean national university. During the data analysis phase, the researchers coded the interview content and examined additional materials, such as classroom observation logs and interview notes. Results The researchers discovered that Korean student Alice had a visual learning style promoted through kinesthetic, whereas International Chinese student Cindy was an auditory learner, promoted through the visual. The factors that influenced the different learning styles of the two participants were: family environment, school environment and their cultural background. Conclusions This study revealed that learning preference is not an innate ability but may vary in accordance with individual's family environment, school environment and the teachers' styles, and culture background. Based on these findings, when the individual knows his/her learning style, s/he will integrate it in the process of learning. Moreover, identifying the characteristics of students' learning styles will assist teachers navigate their teaching style and class materials.

Modeling of ductile fracture from shear to balanced biaxial tension for sheet metals

Lou, Y.,Chen, L.,Clausmeyer, T.,Tekkaya, A.E.,Yoon, J.W. Pergamon Press ; Elsevier Science Ltd 2017 International journal of solids and structures Vol.112 No.-

<P>A ductile fracture model is proposed to describe shear fracture of sheet metals from shear to balanced biaxial tension via uniaxial and plane strain tension. The fracture criterion models plastic damage as strain-induced void nucleation, triaxiality-governed void enlargement, Lode-controlled void torsion, and shear-restrained coalescence of voids. Its flexibility is investigated by a parameter study of the ductile fracture model proposed. The fracture model is employed to describe ductile fracture behavior of an aluminum alloy AA6082 T6 (thickness: 1.0 mm). Dogbone specimens are strained to characterize the strain hardening properties, while another four different specimens are tested to characterize fracture behavior in shear, uniaxial tension, plane strain tension and balanced biaxial tension. The loading processes are analyzed numerically with the stress invariant-based Drucker yield function which is for the first time specified for body-centered cubic and face-centered cubic metals. Fracture strains in various loading conditions are measured with a hybrid experimental-numerical approach. The measured fracture strains are then used to calibrate the ductile fracture model proposed. The ductile fracture model calibrated above is employed to predict the onset of ductile fracture for these four specimens. For the purpose of comparison, the predicted fracture strokes of these four loading conditions are compared with those predicted by the modified Mohr-Coulomb model (Bai and Wierzbicki, 2008), and two micromechanism-inspired criteria proposed recently (Lou et al., 2012, 2014). The comparison reveals that the proposed model predicts the fracture behavior in much better agreement compared with experimental results from shear to the balanced biaxial tension. Accordingly, the proposed ductile fracture criterion is recommended for the prediction of ductile fracture in sheet metal forming processes, optimization of forming parameters and design of tools for both solid elements and shell elements. Besides, the ductile fracture model proposed can also be applied in various bulk metal forming processes in case that the model is calibrated by proper sets of experiments. (C) 2017 Elsevier Ltd. All rights reserved.</P>

Prediction of Fracture Forming Limit for DP780 Steel Sheet

Yanshan Lou,Sung Jun Lim,허훈 대한금속·재료학회 2013 METALS AND MATERIALS International Vol.19 No.4

This paper is concerned with modeling of fracture strains of DP780 using a newly proposed micro-mechanism-motivated ductile fracture criterion (Lou et al., 2012) and its application to predict limit dome heights (LDH) for nine hemispherical punch-stretch tests. Dog-bone specimens are tested to characterize strain hardening behavior. Five arc-shaped specimens and four square-shaped specimens are drawn until fracture to construct a fracture forming limit diagram (FFLD) using circle grid analysis. Fracture strains are approximated from constructed FFLD in uniaxial, plane strain and balanced biaxial tension. The approximated fracture strains are employed to calculate material constants of the proposed criterion as well as six conventional criteria. FFLDs predicted by these criteria are compared with experimental results. The comparison demonstrates that only the proposed criterion describes FFLD perfectly from uniaxial tension to balanced biaxial tension. All criteria are implemented into ABAQUS/Explicit to predict LDHs of punch-stretch tests. Numerical results indicate that LDHs are severely underestimated for the square-shaped specimens by conventional criteria while the proposed criterion predicts LDHs with good agreement for nine tests with strain paths between uniaxial tension and balanced biaxial tension. Thus, the proposed criterion is recommended to access formability from uniaxial tension to balanced biaxial tension.

FDU-12 Mesoporous Materials Detection Hg (Ⅱ) Ions by QCM

Huihui Lou,Chong Shen,Qun Xiang,Jiaqiang Xu,Tianjun Lou 성균관대학교(자연과학캠퍼스) 성균나노과학기술원 2016 NANO Vol.11 No.8

Thiol-functionalized three-dimensional (3D) mesoporous silica FDU-12 and SBA-15 with ordered pore were prepared. All the obtained materials were characterized by small angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Mesoporous silica FDU-12 (Fm3m) materials with various unit cell sizes, multifaceted pore were convenient for the interaction of subject and object; the 2D SBA-15 mesoporous silica materials with short and order pore channel were better than the traditional SBA-15 mesoporous silica, in which the pore channel can be used fully. 3D mesoporous silica FDU-12 is used as a sensing material to reconstruct QCM sensors to enhance the stability and this material is an ideal material to deal with heavy metal ions in water.

Post-earthquake Assessment of Mission-Gothic Undercrossing

Lou, K.Y.,Ger, J.F.,Yang, R.J.,Cheng, F.Y. Computational Structural Engineering Institute of 2001 Computational structural engineering Vol.1 No.1

Collapse behavior of Mission-Gothic Undercrossing under Northridge earthquake is studied by performing nonlinear time-history analysis and three-dimensional nonlinear finite element method for flared columns. Bridge structural model is characterized as three-dimensional with consideration of columns, superstructures, and abutment conditions. Three components of ground motion, corresponding to bridge's longitudinal, transverse, and vertical direction and their combinations are used to investigate bridge collapse. Studies indicate that bridge collapse is dominantly caused by transverse ground motion and the consideration of three-dimensional ground motion leads to a more accurate assessment. Failure mechanism of flared columns is analyzed applying nonlinear finite element method. Reduction of column capacity is observed due to orientation of flare. Further investigation demonstrates that the effects of flare play an important role in predicting of bridge failure mechanism. Suggestions are offered to improve the performance of bridges during severe earthquake.

Numerical simulations of interactions between solitary waves and elastic seawalls on rubble mound breakwaters

Lou, Yun-Feng,Luo, Chuan,Jin, Xian-Long Techno-Press 2015 Structural Engineering and Mechanics, An Int'l Jou Vol.53 No.3

Two dimensional numerical models and physical models have been developed to study the highly nonlinear interactions between waves and breakwaters, but several of these models consider the effects of the structural dynamic responses and the shape of the breakwater axis on the wave pressures. In this study, a multi-material Arbitrary Lagrangian Eulerian (ALE) method is developed to simulate the nonlinear interactions between nonlinear waves and elastic seawalls on a coastal rubble mound breakwater, and is validated experimentally. In the experiment, a solitary wave is generated and used with a physical breakwater model. The wave impact is validated computationally using a breakwater - flume coupling model that replicates the physical model. The computational results, including those for the wave pressure and the water-on-deck, are in good agreement with the experimental results. A local breakwater model is used to discuss the effects of the structural dynamic response and different design parameters of the breakwater on wave loads, together with pressure distribution up the seawall. A large-scale breakwater model is used to numerically study the large-scale wave impact problem and the horizontal distribution of the wave pressures on the seawalls.

Numerical analysis of second-order effects of externally prestressed concrete beams

Lou, Tiejiong,Xiang, Yiqiang Techno-Press 2010 Structural Engineering and Mechanics, An Int'l Jou Vol.35 No.5

A numerical procedure for the geometrical and material nonlinear analysis of concrete beams prestressed with external tendons is described, where the effects of external prestressing are treated as the equivalent loads applied on the concrete beams. The geometrical nonlinearity is considered not only the eccentricity variations of external tendons (second-order effects) but also the large displacement effects of the structure. The numerical method can predict the nonlinear response of externally prestressed concrete beams throughout the entire loading history with considerable accuracy. An evaluation of second-order effects of externally prestressed concrete beams is carried out using the proposed analysis. The analysis shows that the second-order effects have significant influence on the response characteristics of externally prestressed concrete beams. They lead to inferior ultimate load and strength capacities and a lower ultimate stress increase in tendons. Based on the current analysis, it is recommended that, for simply-supported externally prestressed beams with straight horizontal tendons, one deviator at midspan instead of two deviators at one-third span be furnished to minimize these effects.

Effect of motion path of downburst on wind-induced conductor swing in transmission line

Lou, Wenjuan,Wang, Jiawei,Chen, Yong,Lv, Zhongbin,Lu, Ming Techno-Press 2016 Wind and Structures, An International Journal (WAS Vol.23 No.3

In recent years, the frequency and duration of supply interruption in electric power transmission system due to flashover increase yearly in China. Flashover is usually associated with inadequate electric clearance and often takes place in extreme weathers, such as downbursts, typhoons and hurricanes. The present study focuses on the wind-induced oscillation of conductor during the process when a downburst is passing by or across a specified transmission line. Based on a revised analytical model recently developed for stationary downburst, transient three-dimensional wind fields of moving downbursts are successfully simulated. In the simulations, the downbursts travel along various motion paths according to the certain initial locations and directions of motion assumed in advance. Then, an eight-span section, extracted from a practical 500 kV ultra-high-voltage transmission line, is chosen. After performing a non-linear transient analysis, the transient displacements of the conductors could be obtained. Also, an extensive study on suspension insulator strings' rotation angles is conducted, and the electric clearances at different strings could be compared directly. The results show that both the variation trends of the transient responses and the corresponding peak values vary seriously with the motion paths of downburst. Accordingly, the location of the specified string, which is in the most disadvantageous situation along the studied line section, is picked out. And a representative motion path is concluded for reference in the calculation of each string's oscillation for the precaution of wind-induced flashover under downburst.