• Coupled systems mechanics
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• v.8 no.2
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• pp.99-109
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• 2019

This work investigates the accuracy and performance of a $FE^2$ multi-scale implementation used to predict the behavior of composite materials. The equations are formulated assuming the small deformations solid mechanics approach in non-linear material models with hardening plasticity. The uniform strain boundary conditions are applied for the macro-to-micro transitions. A parallel algorithm was implemented in order to solve large engineering problems. The scheme proposed takes advantage of the domain decomposition method at the macro-scale and the coupling between each subdomain with a micro-scale model. The precision of the method is validated with a composite material problem and scalability tests are performed for showing the efficiency.

• Geomechanics and Engineering
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• v.24 no.4
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• pp.337-348
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• 2021

Brillouin Optical Frequency Domain Analysis (BOFDA) is a distributed fiber optic sensing (DFOS) technique that has unique advantages for performance monitoring of piles. However, the complicated production process and harsh operating environment of offshore PHC pipe piles make it difficult to apply this method to pile load testing. In this study, sensing cables were successfully pre-installed into an offshore PHC pipe pile directly for the first time and the BOFDA technique was used for in-situ monitoring of the pile under axial load. High-resolution strain and internal force distributions along the pile were obtained by the BOFDA sensing system. A finite element analysis incorporating the Degradation and Hardening Hyperbolic Model (DHHM) was carried out to evaluate and predict the performance of the pile, which provides an improved insight into the offshore pile-soil interaction mechanism.

• Geomechanics and Engineering
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• v.29 no.3
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• pp.269-279
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• 2022

In this research, a series of dynamic numerical analysis were carried out for deep underground building structures under the various earthquake conditions. Dynamic numerical analysis model was developed based on the PLAXIS2D and calibrated with centrifuge test data from Kim et al. (2016). The hardening soil model with small strain stiffness (HSSMALL) was adopted for soil constitutive model, and interface elements was employed at the interface between plate and soil elements to simulate dynamic interaction effect. In addition, parametric study was performed for fixed condition and embedded depth. Finally, the dynamic behavior of underground building structure was thoroughly analyzed and evaluated.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2006.09b
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• pp.718-719
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• 2006

Two different commercial aluminium powder grades have been densified by direct hot extrusion. The extrusion temperature was $425^{\circ}C$, with an extrusion ratio of 1:16. Prior to extrusion, some green compacts were pre-sintered ($500^{\circ}C$). The evolution of the extrusion load during the process and the hardness of the final products have been investigated. Additionally, microstructural characterization by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Electron Backscattered Diffraction (EBSD) was carried out. The obtained results evidence grain refinement. Additionally, inter-metallic precipitation, dynamic recovery and geometric dynamic recrystallization take place depending on some process variables, powder composition, heat treatment, strain $\ldots$

• Journal of the Korean Society for Precision Engineering
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• v.13 no.3
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• pp.80-93
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• 1996

A 2-dimensional FEM/GEM program was developed under the plane strain assumption using linear line elements for analyzing stretch/draw forming operations of an arbitrarily shaped draw-die. FEM formulation adopted a new algorithm for solving force equilibrium as well as non-penetration condition simultaneously. Also, a rigid-viscoplastic material model with Hill's normal anisotropic yield condition and rate-sensitive hardening law is assumed, along with the Coulomb friction law in the contact regions. For the case of numerical divergence at nearly final forming stages, geometric force equilibrium method(GEM) is also introduced. The developed program was tested by simulating the forming processes of cylindrical punch/open die, and the drawing processes of automotive oilpan and hood inner panel in order to verify the usefulness and validity of FEM/GEM formulation. The numerical simulation verified the validity and robustness of developed program.

• Transactions of Materials Processing
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• v.33 no.2
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• pp.87-95
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• 2024

The current study aims to prove that high-speed forming has better formability than conventional low-speed forming. Experimentally, the quasi-static forming limit diagram was obtained by Nakajima test, and the dynamic forming limit diagram was measured by electrohydraulic forming. For the experiments, the LS-DYNA was used to create the optimal specimen for electrohydraulic forming. The strain measurement was performed using the ARGUS, and comparison of the forming limit diagrams confirmed that EHF showed better formability than quasi-static forming. Theoretically, the Marciniak-Kuczynski model was used to calculate the theoretical forming limit. Swift hardening function and Cowper Symonds model were applied to predict the forming limits in quasi-static and dynamic status numerically.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.26 no.5
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• pp.924-931
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• 2002

Vacuum interrupters in order to be used in various switch-gear components such as circuit breakers, distribution switches, contactors, etc. spread the arc uniformly over the surface of the contacts. The electrodes of vacuum interrupters are made of sinter-forged Cu-Cr materials for good electrical and mechanical characteristics. Since the closing velocity is 1-2m/s and impact deformation of the electrode depends on the strain rate at that velocity, the dynamic behavior of the sinter-forged Cu-Cr is a key to investigate the impact characteristics of the electrodes. The dynamic response of the material at the high strain rate is obtained from the split Hopkinson pressure bar test using disc-type specimens. Experimental results from both quasi-static and dynamic compressive tests are Interpolated to construct the Johnson-Cook model as the constitutive relation that should be applied to simulation of the dynamic behavior of the electrodes. The impact characteristics of a vacuum interrupter are investigated with computer simulations by changing the value of five parameters such as the initial velocity of a movable electrode, the added mass of a movable electrode, the wipe spring constant, initial offset of a wipe spring and the virtual fixed spring constant.

• Transactions of Materials Processing
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• v.28 no.6
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• pp.361-367
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• 2019

Microstructure evolution and tensile properties of Al-8mass%Mg alloy casting billet by biaxial alternative forging were investigated in this study. An alternative forging system tailored in this study was used to allow continuous strain accumulations on the alloy workpiece. A finite element (FE) simulation results revealed that the strain was mainly concentrated in the core and that the shear bands developed into a form with an X shape in the cross-section of workpiece after the alternative forging using octangular rod shaped dies. With increasing the forging passes, it was observed that the Al-8mass%Mg alloy workpieces were significantly deformed, and cracks began to form and propagate on the both ends of the forged workpieces after five passes at room temperature. In as-forged microstructures taken by microscopes, twins, clustering of dislocations, and fine subgrains were found. Tensile strengths of the forged specimens showed significant increases depending on the number of forging passes, and a trade-off relationship was observed between the elongation and strength. At room temperature and 100℃ the workpieces showed similar behaviors in microstructural evolution and tensile properties depending on forging passes, while the increase range in strength was reduced at 200℃.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.41 no.5
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• pp.337-343
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• 2017

As a shop practice, a strength estimation method for die cast parts is suggested, in which various defects such as pores can be allowed. The equivalent porosity is evaluated by combining the stiffness data from a simple elastic test at the part level during the shop practice and the theoretical stiffness data, which are defect free. A porosity equation is derived from Eshelby's inclusion theory. Then, using the Mori-Tanaka method, the porosity value is used to draw a stress-strain curve for the porous material. In this paper, the Hollomon equation is used to capture the strain hardening effect. This stress-strain curve can be used to estimate the strength of a die cast part with porous defects. An elastoplastic theoretical solution is derived for the three-point bending of a die cast beam by using the plastic hinge method as a reference solution for a part with porous defects.

• Steel and Composite Structures
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• v.26 no.6
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• pp.673-691
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• 2018

The main goal of this research is to examine the in-plane and out-of-plane forced vibration of a curved nanocomposite microbeam. The in-plane and out-of-plane displacements of the structure are considered based on the first order shear deformation theory (FSDT). The curved microbeam is reinforced by functionally graded carbon nanotubes (FG-CNTs) and thus the extended rule of mixture is employed to estimate the effective material properties of the structure. Also, the small scale effect is captured using the strain gradient theory. The structure is rested on a nonlinear orthotropic viscoelastic foundation and is subjected to concentrated transverse harmonic external force, thermal and magnetic loads. The derivation of the governing equations is performed using energy method and Hamilton's principle. Differential quadrature (DQ) method along with integral quadrature (IQ) and Newmark methods are employed to solve the problem. The effect of various parameters such as volume fraction and distribution type of CNTs, boundary conditions, elastic foundation, temperature changes, material length scale parameters, magnetic field, central angle and width to thickness ratio are studied on the frequency and force responses of the structure. The results indicate that the highest frequency and lowest vibration amplitude belongs to FGX distribution type while the inverse condition is observed for FGO distribution type. In addition, the hardening-type response of the structure with FGX distribution type is more intense with respect to the other distribution types.