• Transactions of Materials Processing
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• v.14 no.6 s.78
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• pp.514-519
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• 2005

A tube hydroformability testing system was designed and fabricated enabling to apply the forming condition along arbitrarily pre-programmed internal pressure-axial feed path. The free-bulging and T-forming tests were carried out on the extruded aluminum (A6063) tube specimens with 40.6 mm outer diameter and 2.25 mm thickness. Nine different combinations of internal pressure and axial feed, yielding different strain paths from one another, were taken into consideration in order to induce bursting at various deformation modes. Major and minor strains were automatically measured from deformed grids around the fracture using a stereo-vision-based surface strain measurement system, named ASIAS. The forming limit diagram of the A6063 tube material was successfully obtained. Most of the data points acquired from free bulging and T-forming tests appeared in the range of negative minor strain on the FLD and are mostly located near the strain paths calculated from explicit finite element simulations. The forming limit obtained from tests after pre-tension was considerably lower than that from tests without pre-tension, which showed the strain path-dependency of the forming limit as well known in the sheet forming fold.

• Steel and Composite Structures
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• v.45 no.3
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• pp.331-348
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• 2022

The main goal of this article is to develop the finite element formulation based on the nonlocal strain gradient and the refined higher-order deformation theory employing a new function f(z) to investigate the static bending and free vibration of functionally graded porous (FGP) nanobeams. The proposed model considers the simultaneous effects of two parameters: nonlocal and strain gradient coefficients. The nanobeam is made by FGP material that exists in un-even and logarithmic-uneven distribution. The governing equation of the nanobeam is established based on Hamilton's principle. The authors use a 2-node beam element, each node with 8 degrees of freedom (DOFs) approximated by the C¹ and C² continuous Hermit functions to obtain the elemental stiffness matrix and mass matrix. The accuracy of the proposed model is tested by comparison with the results of reputable published works. From here, the influences of the parameters: nonlocal elasticity, strain gradient, porosity, and boundary conditions are studied.

• Computers and Concrete
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• v.2 no.4
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• pp.249-266
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• 2005

In the present paper a transient three-dimensional thermo-mechanical model for concrete is presented. For given boundary conditions, temperature distribution is calculated by employing a three-dimensional transient thermal finite element analysis. Thermal properties of concrete are assumed to be constant and independent of the stress-strain distribution. In the thermo-mechanical model for concrete the total strain tensor is decomposed into pure mechanical strain, free thermal strain and load induced thermal strain. The mechanical strain is calculated by using temperature dependent microplane model for concrete (O$\check{z}$bolt, et al. 2001). The dependency of the macroscopic concrete properties (Young's modulus, tensile and compressive strengths and fracture energy) on temperature is based on the available experimental database. The stress independent free thermal strain is calculated according to the proposal of Nielsen, et al. (2001). The load induced thermal strain is obtained by employing the biparabolic model, which was recently proposed by Nielsen, et al. (2004). It is assumed that the total load induced thermal strain is irrecoverable, i.e., creep component is neglected. The model is implemented into a three-dimensional FE code. The performance of headed stud anchors exposed to fire was studied. Three-dimensional transient thermal FE analysis was carried out for three embedment depths and for four thermal loading histories. The results of the analysis show that the resistance of anchors can be significantly reduced if they are exposed to fire. The largest reduction of the load capacity was obtained for anchors with relatively small embedment depths. The numerical results agree well with the available experimental evidence.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.36 no.5
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• pp.505-512
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• 2023

This study investigated crystal structures, microstructures, and electric-field-induced strain (EFIS) properties of Bi-based lead-free ferroelectric/relaxor composites. Bi_1/2Na_0.82K_0.18)_1/2TiO₃ (BNKT) as a ferroelectric material and 0.78Bi_1/2(Na_0.78K_0.22)_1/2TiO₃-0.02LaFeO₃ (BNKT2LF) as a relaxor material were synthesized using a conventional solid-state reaction method, and the resulting BNKT2LF powders were subjected to high-energy ball milling (HEBM) after calcination. As a result, HEBM proved a larger average grain size of sintered samples compared to conventional ball milling (CBM). In addition, the increased sintering time led to grain growth. Furthermore, HEBM treatment and sintering time demonstrated a significant effect on EFIS of BNKT/BNKT2LF composites. At 6 kV/mm, 0.35% of the maximum strain (S_max) was observed in the HEBM sample sintered for 12 h. The unipolar strain curves of CBM samples were almost linear, indicating almost no phase transitions, while HEBM samples displayed phase transitions at 5~6 kV/mm for all sintering time levels, showing the highest S_max/E_max value of 700 pm/V. These results indicated that HEBM treatment with a long sintering time might significantly enhance the electromechanical strain properties of BNT-based ceramics.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.32 no.3
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• pp.250-257
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• 2008

We calculate the variation of the surface stresses according to uniaxial and biaxial strains in face-centered cubic (FCC) metals. In our study, three mainly observed free surfaces of seven representative FCC metals are considered. Employed method is molecular mechanics, in which the interaction of atoms is described by empirical interatomic potentials. As uniaxial strain increases to tensile direction, the surface stresses on {100} and {110} free surfaces decrease monotonously, while those on {111} surface increase. These tendencies are the same regardless of the species of metals and interatomic potentials employed. However, when the system is under biaxial strain, surface stresses change different according to the surface directions, the species of metals, and even interatomic potentials. On {100} and {111} surfaces, heavy metals (Pt, Au) show the opposite variation to light metals (Ni, Cu). In the cases of Pd and Ag, the surface stresses reveal the opposite tendency, depending on interatomic potentials used.

• Microbiology and Biotechnology Letters
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• v.37 no.4
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• pp.405-407
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• 2009

A Bacillus subtilis mannanase gene was subcloned into an Escherichia coli- Corynebacterium lactofermentum shuttle vector pHE83, and the resultant plasmid pHE83M was transferred into an endogenous plasmid-free strain of C. lactofermentum. Mannanase produced by the recombinant C. lactofermentum (pHE83M) was secreted extracellulary at the level of 86%, and the remaining activity of mannanase was detected in the cell-free extract. The maximum mannanase productivity of the recombinant strain reached 8.1 unit/mL in the culture filtrate of LB medium. According to the zymogram of mannanase on SDS-PAGE, it was found that the mannanase produced by the recombinant C. lactofermentum could be maintained stably with a migration identical to the mannanase produced by the parental strain, B. subtilis WL-3.

• Journal of the Korean Society of Safety
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• v.25 no.6
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• pp.14-21
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• 2010

A miniature single solder ball joint is designed to mimic the actual solder joints used in the micro-electric industries. Shear tests were conducted to evaluate the mechanical behavior of miniature single solder joints at intermediate strain rates from $0.019\;s^{-1}$ to $2.16\;s^{-1}$ at room temperature. The shear fracture strength of the present solder ball joints generally increased with increasing shear strain rate, ranging from 32 to 51MPa. This behavior is affected by the sensitivity of bulk solder strength to strain rate. Shear fracture mode changed from brittle to partial ductile (failure inside the bulk solder) with an increase of shear speed. The unloading shear fracture toughness is generally consistent with the measure of the amount of bulk solder on the fractured surface.

• Structural Engineering and Mechanics
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• v.81 no.4
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• pp.461-479
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• 2022

A finite element method analysis framework is introduced for the free vibration analyses of functionally graded porous beam structures by employing two variables trigonometric shear deformation theory. Both Young's modulus and material density of the FGP beam element are simultaneously considered as grading through the thickness of the beam. The finite element approach is developed using a nonlocal strain gradient theory. The governing equations derived here are solved introducing a 3-nodes beam element. A comprehensive parametric study is carried out, with a particular focus on the effects of various structural parameters such as the dispersion patterns of GPL reinforcements and porosity, thickness ratio, boundary conditions, nonlocal scale parameter and strain gradient parameters. The results indicate that porosity distribution and GPL pattern have significant effects on the response of the nanocomposite beams.

• Steel and Composite Structures
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• v.44 no.4
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• pp.557-567
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• 2022

Nonlinear free vibration analysis of a functionally graded beam resting on the nonlinear viscoelastic foundation is studied with uniform temperature rising. The non-linear strain-displacement relationship is considered in the finite strain theory. The governing nonlinear dynamic equation is derived based on the finite strain theory with using of Hamilton's principle. The Galerkin's decomposition technique is utilized to discretize the governing nonlinear partial differential equation to nonlinear ordinary differential equation and then is solved by using of multiple time scale method. The influences of temperature rising, material distribution parameter, nonlinear viscoelastic foundation parameters on the nonlinear free response and phase trajectory are investigated. In this paper, it is aimed that a contribution to the literature for nonlinear thermal vibration solutions of a functionally graded beam resting on the nonlinear viscoelastic foundation by using of multiple time scale method.

• Korean Journal of Materials Research
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• v.15 no.4
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• pp.240-245
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• 2005

An oxygen free copper was severely deformed by accumulative roll-bonding (ARB) process for improvement of its mechanical properties. Two copper sheets 1 m thick, 30 mm wide and 300 m long are first degreased and wire-brushed for sound bonding. The sheets are then stacked to each other, and roll-bonded by about $50\%$ reduction rolling without lubrication at ambient temperature. The bonded sheet is then cut to the two pieces of same dimensions and the same procedure was repeated to the sheets up to eight cycles $(\varepsilon-6.4)$. TEM observation revealed that ultrafine grains were developed after the third cycle, and their size was slightly increased at higher cycles. Tensile strength of the copper increased with the strain at low strain levels, but it hardly increased from 3 cycles $(\varepsilon>2.4)$ due to occurrence of dynamic recovery, even if the imposed strain increased.