Formability of AHSS under an Attach-Detach Forming Mode

Majidi, O.,Barlat, F.,Lee, M. G.,Kim, D. J. Wiley 2015 STEEL RESEARCH INTERNATIONAL Vol.86 No.2

Thermomechanical response of a TWIP steel during monotonic and non-monotonic uniaxial loading

Majidi, O.,De Cooman, B.C.,Barlat, F.,Lee, M.G.,Korkolis, Y.P. Elsevier Sequoia 2016 Materials science & engineering. properties, micro Vol.674 No.-

<P>The tensile properties of a Fe-18%Mn-0.6%C-1.5%Al Twinning-Induced Plasticity (TWIP) steel were investigated at different strain rates in three loading modes, i.e. uniaxial monotonic loading, stress relaxation and loading-unloading-reloading. Infrared thermography was used to investigate the effect of the dynamic strain aging, the strain rate and the temperature on the flow stress. In addition to the standard, i.e., non-isothermal tensile tests, isothermal uniaxial tensile tests were performed at 25 degrees C, 45 degrees C and 65 degrees C. While the non-monotonic loading modes resulted in an increase of the total elongation at a low strain rate of 10(-3) s(-1), no increase was observed for strain rates higher than 6 x 10(-3) s(-1). The temperature gradients observed during non-isothermal tests were reduced when non-monotonic loading conditions were used. Temperature changes were found to influence the hardening behavior, and consequently the ductility, of the TWIP steel. Deformation twinning also had a significant influence on the results as its kinetics in TWIP steel are determined by the temperature dependence of the stacking fault energy. (C) 2016 Elsevier B.V. All rights reserved.</P>

Effect of slide motion on springback in 2-D draw bending for AHSS

Majidi, O.,Barlat, F. d.,Lee, M. G. Springer Science + Business Media 2016 International journal of material forming Vol.9 No.3

<P>The springback behavior of two advanced high strength steel (AHSS) grades, DP 780 and DP 980, after different forming conditions, was investigated. 2-D draw bending experiments were performed using a direct-drive digital servo-press in three operation modes, conventional (V-mode), relaxation mode (Stepwise and U-mode) and the attach-detach (A-D mode). Numerical simulations were conducted to in an attempt to reproduce the results and to perform parametric studies. The material behavior was captured using the homogeneous anisotropic hardening (HAH) distortional plasticity approach together with the chord elastic modulus model. In addition, the stress relaxation effects were implemented in the code by using a creep law in order to study the influence of a stepwise slide motion mode as well as holding at the bottom dead center. Both experimental and finite element (FE) simulation results demonstrate that detachment of tools from the work-piece was effective to reduce the springback while the effect of stress relaxation was insignificant. The numerical analysis was validated and successfully explained the importance of a forming path change on the final stress state. Based on the result of this study, a new method to reduce springback was introduced.</P>

HELIUM ADSORPTION ON CARBON NANOCONES WITH DIFFERENT DISCLINATION ANGLE: MOLECULAR DYNAMICS SIMULATION

ROYA MAJIDI 성균관대학교(자연과학캠퍼스) 성균나노과학기술원 2012 NANO Vol.7 No.3

We have used molecular dynamics simulation to study the in°uence of disclination angle on helium adsorption capacity of carbon nanocones (CNCs). Adsorption capacities of 180?, 240? and 300? CNCs have been compared at various temperatures. The results indicate that helium atoms adsorb on internal and external surfaces of the CNCs. At saturation conditions, the endohedral adsorption coverage is decreased by increasing the disclination angle, while the exohedral adsorption coverage is independent of this angle. It is also found that temperature has a considerable e®ect on the adsorbed amount. Comparison of the adsorption on CNCs with carbon nanotubes indicates that the adsorption capacity of cones is greater than those of tubes. Our results con¯rm the fact that the CNCs are good candidate for gas storage.

A ghost fluid method for sharp interface simulations of compressible multiphase flows

Sahand Majidi and Asghar Afshari 대한기계학회 2016 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.30 No.4

A ghost fluid based computational tool is developed to study a wide range of compressible multiphase flows involving strong shocks and contact discontinuities while accounting for surface tension, viscous stresses and gravitational forces. The solver utilizes constrained reinitialization method to predict the interface configuration at each time step. Surface tension effect is handled via an exact interface Riemann problem solver. Interfacial viscous stresses are approximated by considering continuous velocity and viscous stress across the interface. To assess the performance of the solver several benchmark problems are considered: One-dimensional gas-water shock tube problem, shock-bubble interaction, air cavity collapse in water, underwater explosion, Rayleigh-Taylor Instability, and ellipsoidal drop oscillations. Results obtained from the numerical simulations indicate that the numerical methodology performs reasonably well in predicting flow features and exhibit a very good agreement with prior experimental and numerical observations. To further examine the accuracy of the developed ghost fluid solver, the obtained results are compared to those by a conventional diffuse interface solver. The comparison shows the capability of our ghost fluid method in reproducing the experimentally observed flow characteristics while revealing more details regarding topological changes of the interface.

EFFECT OF DOPING ON THE ELECTRONIC PROPERTIES OF GRAPHYNE

ROYA MAJIDI 성균관대학교(자연과학캠퍼스) 성균나노과학기술원 2013 NANO Vol.8 No.6

We have used density functional theory to study the effect of doping on the electronic properties of graphyne. The graphyne with alpha type has been considered since it is analogous to graphene. The electronic properties of graphynes containing B, N or O impurity have been compared with those of pure graphyne. It is found that the electronic properties of alpha graphyne change from semimetal to semiconductor by doping. The B-doped graphyne becomes a p-type semiconductor, while N-doped and O-doped graphynes are n-type semiconductors. Our results provide possibility of opening an energy gap in graphyne as required for fabricating high-performance nanoelectronic devices based on graphyne.

Thermal Effects on the Enhanced Ductility in Non-Monotonic Uniaxial Tension of DP780 Steel Sheet

Omid Majidi,Frederic Barlat,Yannis P. Korkolis,Jiawei Fu,Myoung-Gyu Lee 대한금속·재료학회 2016 METALS AND MATERIALS International Vol.22 No.6

To understand the material behavior during non-monotonic loading, uniaxial tension tests were conducted inthree modes, namely, the monotonic loading, loading with periodic relaxation and periodic loading-unloadingreloading,at different strain rates (0.001/s to 0.01/s). In this study, the temperature gradient developing duringeach test and its contribution to increasing the apparent ductility of DP780 steel sheets were considered. In order toassess the influence of temperature, isothermal uniaxial tension tests were also performed at three temperatures(298 K, 313 K and 328 K (25 °C, 40 °C and 55 °C)). A digital image correlation system coupled with an infraredthermography was used in the experiments. The results show that the non-monotonic loading modes increasedthe apparent ductility of the specimens. It was observed that compared with the monotonic loading, the temperaturegradient became more uniform when a non-monotonic loading was applied.

The importance of friction coefficient at different forming velocities of AHSS

Omid Majidi(오미드),F. Barlat,M. G. Lee(이명규) 한국자동차공학회 2013 한국자동차공학회 부문종합 학술대회 Vol.2013 No.5

Adsorption Patterns of Helium on Carbon and Cellulose Nanotubes: Molecular Dynamics Simulations

ROYA MAJIDI,Hamid Reza Taghiyari,Mahsa Ekhlasi 성균관대학교(자연과학캠퍼스) 성균나노과학기술원 2017 NANO Vol.12 No.3

Molecular dynamics simulations were performed to study helium adsorption on carbon and cellulose nanotubes. Adsorption isotherms were analyzed at different temperatures and pressures. All adsorption isotherms for carbon and cellulose nanotubes were predicted to be of Langmuir shape type I. Helium adsorption was observed both inside and outside of open-ended tubes. Increasing temperatures caused lower helium adsorption on carbon and cellulose nanotubes. The calculated quantities confirmed that the adsorption capacity of the cellulose nanotube was greater than that of the carbon nanotube. The adsorption capacity, isosteric heat of adsorption and binding energy indicated that cellulose nanotubes as well as carbon nanotubes are proper materials for gas storage.

Maps preserving $m$- isometries on Hilbert space

Alireza Majidi 강원경기수학회 2019 한국수학논문집 Vol.27 No.3

Let $\mathcal{H}$ be a complex Hilbert space and $\mathcal{B}(\mathcal{H})$ the algebra of all bounded linear operators on $\mathcal{H}$. In this paper, we prove that if $\varphi:\mathcal{B}(\mathcal{H})\to \mathcal{B}(\mathcal{H})$ is a unital surjective bounded linear map, which preserves $m$- isometries $m=1, 2$ in both directions, then there are unitary operators $U, V\in \mathcal{B}(\mathcal{H})$ such that \begin{eqnarray*} \varphi(T)=UTV\quad {\rm or}\quad \varphi(T)=UT^{tr}V \end{eqnarray*} for all $T\in \mathcal{B}(\mathcal{H})$, where $T^{tr}$ is the transpose of $T$ with respect to an arbitrary but fixed orthonormal basis of $\mathcal{H}$.