• Journal of Korean Association for Spatial Structures
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• v.18 no.2
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• pp.69-78
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• 2018

In current research, it was attempted a preliminary design and evaluation of non-uniform ultra high-strength concrete (UHSC) truss members. UHSC used here has the compressive strength of 180 MPa, the tensile strength of 8 to 20 MPa, and the tensile strain after cracks up to 2%. By the three-dimensional finite element stress analysis as well as strut-tie approach on concrete solid beams, the non-uniform truss shape of UHSC truss was designed with the architectural esthetic concept. In a series of examples, to compare with conventional concrete members, the proposed UHSC truss members have advantages in capabilities of the slender design with minimum weight with high performances under transverse loadings as well as the aesthetically non-uniform design for spatial structures.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2004.10a
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• pp.413-417
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• 2004

Recent experiments have shown the 'size effects' in micro/nano scale. But the classical plasticity theories can not predict these size dependent deformation behaviors because their constitutive models have no characteristic material length scale. The Mechanism - based Strain Gradient(MSG) plasticity is proposed to analyze the non-uniform deformation behavior in micro/nano scale. The MSG plasticity is a multi-scale analysis connecting macro-scale deformation of the Statistically Stored Dislocation(SSD) and Geometrically Necessary Dislocation(GND) to the meso-scale deformation using the strain gradient. In this research we present a study of nano-indentation by the MSG plasticity. Using W. D. Nix and H. Gao s model, the analytic solution(including depth dependence of hardness) is obtained for the nano indentation , and furthermore it validated by the experiments.

• Journal of Microbiology and Biotechnology
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• v.15 no.2
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• pp.247-253
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• 2005

The bacterial cellulose (BC) production by a wild­strain Acetobacter xylinum BPR2001 and that by its acetan­nonproducing mutant, EPI, were compared in a jar fermentor. EPI produced about $28\%$ less BC than the wild-strain. The apparent difference in the cultivation of the two strains was the viscosity increase in the culture broth that was closely associated with acetan production. Increasing the viscosity of the culture broth of EPI by adding agar led to the formation of relatively small and uniform BC pellets, and BC production consequently became two-fold higher than that in the absence of agar and was almost equal to that by BPR2001. Therefore, acetan has an important role in BC production by inducing physical changes in the culture broth of the wild-type strain.

• Structural Engineering and Mechanics
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• v.75 no.6
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• pp.659-674
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• 2020

The present paper employs nonlocal strain gradient theory (NSGT) to study buckling behavior of functionally graded magneto-electro-thermo-elastic (FG-METE) nanoshells under various physical fields. NSGT modeling of the nanoshell contains two size parameters, one related to nonlocal stress field and another related to strain gradients. It is considered that mechanical, thermal, electrical and magnetic loads are exerted to the nanoshell. Temperature field has uniform and linear variation in nanoshell thickness. According to a power-law function, piezo-magnetic, thermal and mechanical properties of the nanoshell are considered to be graded in thickness direction. Five coupled governing equations have been obtained by using Hamilton's principle and then solved implementing Galerkin's method. Influences of temperature field, electric voltage, magnetic potential, nonlocality, strain gradient parameter and FG material exponent on buckling loads of the FG-METE nanoshell have been studied in detail.

• Structural Engineering and Mechanics
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• v.88 no.4
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• pp.335-354
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• 2023

In this study, analysis of wave propagation characteristics for functionally graded carbon nanotube-reinforced composite (FG-CNTRC) nanoplates is performed using first-order shear deformation theory (FSDT) and nonlocal strain gradient theory. Uniform distribution (UD) and three types of functionally graded distributions of carbon nanotubes (CNTs) are assumed. The effective mechanical properties of the FG-CNTRC nanoplate are assumed to vary continuously in the thickness direction and are approximated based on the rule of mixture. Also, the governing equations of motion are derived via the extended Hamilton's principle. In numerical examples, the effects of nonlocal parameter, wavenumber, angle of wave propagation, volume fractions, and carbon nanotube distributions on the wave propagation characteristics of the FG-CNTRC nanoplate are studied. As represented in the results, it is clear that the internal length-scale parameter has a remarkable effect on the wave propagation characteristics resulting in significant changes in phase velocity and natural frequency. Furthermore, it is observed that the strain gradient theory yields a higher phase velocity and frequency compared to those obtained by the nonlocal strain gradient theory and classic theory.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2006.04a
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• pp.159-166
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• 2006

Although the strain distribution along the length of a beam in buildings or infrastructures is non-uniform, most fiber optic sensors are point sensors that can measure the strain only at a local point of a beam. Long gage fiber optic sensors that measure integrated strain over a relatively long length can consider strain variation. This type of sensor was found to be efficient and useful for monitoring large-scale structures. On the other hand, the maximum strain or stress in a beam can not be measured with long gage optic sensors. However, for the assessment of the safety of multi-span steel beams subjected to various vertical loads, the maximum strain or stress measured during monitoring is required for comparison with the allowable stress of the beam calculated by a design code. Therefore, in this paper, mathematical models are presented for determination of the maximum values of strains in more three-span steel beams based on the average strains measured by long gage optic sensors.

• Journal of the Korean Society for Nondestructive Testing
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• v.22 no.6
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• pp.609-620
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• 2002

It is well known that during tensile testing, a part of the mechanical work done on the specimen is transformed into heat energy. However, the ultimate temperature rise and the rate of temperature rise is related to the nature of the material, conditions of the test and also to the deformation behaviour of the material during loading. The recent advances in infrared sensors and image/data processing techniques enable observation and quantitative analysis of the heat energy dissipated during such tensile tests. In this study, infrared imaging technique has been used to characterise the tensile deformation in AISI type 316 nuclear grade stainless steel. Apart from identifying the different stages during tensile deformation, the technique provided an accurate full-field temperature image by which the point and time of strain localization could be identified. The technique makes it possible to visualise the region of deformation and failure and also predict the exact region of fracture in advance. The effect of thermal gradients on plastic flow in the case of interrupted straining revealed that the interruption of strain and restraining at a lower strain rate not only delays the growth of the temperature gradient, but the temperature rise per unit strain decreases. The technique is a potential NDE tool that can be used for on-line detection of thermal gradients developed during extrusion and metal forming process which can be used for ensuring uniform distribution of plastic strain.

• Structural Engineering and Mechanics
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• v.18 no.3
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• pp.377-388
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• 2004

Based on the basic equations of elasticity, free-edge effects on stresses in cross-ply laminated plates are found by using the state space method. The laminates are subjected to uniaxial-uniform extension plate, which is a typical example of general plane strain problem. The study takes into account material constants of all individual material layers and the state equation of a laminate is solved analytically in the through thickness direction. By this approach, a composite plate may be composed of an arbitrary number of orthotropic layers, each of which may have different material properties and thickness. The solution provides a continuous displacement and inter-laminar stress fields across all material interfaces and an approxiamte prediction to the singularity of stresses occurring in the boundary layer region of a free-edge. Numerical solutions are obtained and compared with the results obtained from an alternative numerical method.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.29 no.11 s.242
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• pp.1542-1550
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• 2005

This paper presents the numerical procedure for calculating non-uniform residual stresses based on relieved displacements obtained from incremental hole drilling. The relationship between the in-plane displacement produced by introducing a blind hole and the corresponding residual stress is established. Finite element calculations are described to evaluate the relieved coefficients required for the determination of non-uniform residual stresses. Validity of the proposed method has been tested through three axisymmetric test examples and two three-dimensional examples. As a result of . simulation on the test examples, it is found that this numerical procedure is well adopted to measuring non-uniform residual stress in the full hole depth range of the hole diameter from the surface. The accuracy of the hole drilling method with displacement measurement is discussed, comparing tile method with strain measurement

• Transactions of Materials Processing
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• v.7 no.6
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• pp.610-619
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• 1998

The superplastic blow forming technology has advantages of cost reduction and low material consumption. compared to the conventional sheet metal forming technology due to the capability of precisely forming with high elongation and low flow stress. however it has a disadvantage that its partial thickness distribution is non-uniform. A processing technology like diaphragm forming has been developed even though it is difficult to prepare materials for superplastic blow forming. in this study a hemisphere forming of sheet before superplastic forming. It was found that the rotary forming material was less in quantity of cavitation at pole than that of hemisphere part that was superplastic formed without rotary forming treatment. Also discussed are the critical strain which is closely related to cavity shape and size.