• Proceedings of the Korean Society of Precision Engineering Conference
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• 2000.11a
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• pp.583-586
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• 2000

This work ethibits how susceptive the shear ultrasonic waves are to a little misoriented plies according to the angle variation of shear ultrasoic waves $0^\circ$ , $45^\circ$ and $90^\circ$. Also, it is shown that shear waves, particularly the transmission mode with the transmitter and receiver perpendicular to each other, have high sensitivity for detecting anomalies in fiber orientation and ply layup sequence that may occur in the manufacturing of composite laminates. Experimental results are agreed with modeling solutions which were based on decomposition of shear wave polarization vector as it propagates through the composite laminates. This wave appeared considerably to be sensitive to CFRP composites to the thickness direction along in-plane fibers.

• Composites Research
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• v.27 no.1
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• pp.7-13
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• 2014

For each construction material used, there is certain theoretical limit in sizes. For tall building construction, the reduction in slab weight is the first step to take in order to break such size limits. In this paper, the feasibility of such objective is proven and given by numerical analysis result. For a typical building slab, both concrete and advanced composite sandwich panels are considered. The concrete slab is treated as a special orthotropic plate to obtain more accurate result. For each panel, the deflection under the dead and live loads is compared, since both tensile and compressive strengths of the composites are far more higher than those of concrete. All types of sandwich panels considered, except one case, have self-weights less than one tenth of that of the reinforced concrete slab, with deflections less than that of the reinforced concrete slab.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2009.05a
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• pp.187-190
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• 2009

A multiple pulse rocket motor distributes the thrust energy more effectively compared to typical rocket motor as providing subsequent thrusts by the pulse motors of the missile. The pulse rocket motor is the advanced technology to improve an end game capability of the missile by increasing the range and final velocity. A pulse separation device is the core part of the pulse motor. The pulse separation device of bulkhead type was designed and developed. The elastic-plastic structural analysis of the bulkhead and rupture disc was conducted. Several air tests were also conducted to confirm the structural safety and acceptability about the design concept. Test results were compared with the analysis results, which showed reasonable agreements.

• Journal of the Korean Wood Science and Technology
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• v.37 no.1
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• pp.48-55
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• 2009

In the glulam beam deflection it is necessary to check the reliability of theory formula, because of wood anisotropy and wood qualities (knot, slop of grain). In this experiment, when bending stress occurred on glulam, practice deflection of glulam measuring with AICON DPA-Pro 3D system were compared with prediction deflection calculated as substituting MOE through non-destructive testing and static MOE through bending test in differential equation of deflection curve. MOE using ultrasonic wave tester of laminae, MOE using natural frequencies of longitudinal vibrations ($E_{cu}$, $E_{cf}$), MOE using ultrasonic wave tester of glulam ($E_{gu}$) and MOE using natural frequencies of longitudinal vibrations ($E_{gf}$) were substituted in this experiment. When practice deflection measured by 3D system was compared with prediction deflection calculated with differential equation of deflection curve, within proportional limit the ratio of practice deflection and prediction deflection was similar as 1.12 and 1.14, respectively. Deflection using ultrasonic wave tester was 0.89 and 0.95, Deflection using natural frequencies of longitudinal vibrations was 1.07 and 1.10. The results showed that prediction deflection calculated by substituting using non-destructive MOE of glulam having anisotropy in differential equation of deflection curve was agreed well with practice deflection.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.7 no.3
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• pp.384-392
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• 1997

Potassium lithium niobate(KLN) single crystal for a nonlinear-optic material, which changes the wavelength of lasers, has a ferroelectric tetragonal tungsten bronze structure at room temperature. It has been very hard to get single crystals of good quality due to the cracks during cooling process. In order to investigate the composition change due to the evaporation of solution during the growth, the thermogravimetric analysis was carried out. In atmospheric condition at $1000^{\circ}C$ which is about $10^{\circ}C$ higher than the crystal growing temperature, the weight change was negligible amount of $1.46{\times}10^{-5}$g/($\textrm{cm}^2$hr). By using both the Pt plate as the nucleation site and the slow cooling method with temperature fluctuation, KLN single crystal of good quality of size 1 cm could be obtained. The phase transition temperature was $490^{\circ}C$, which was higher than that reported by other researchers of the other composition. The optical anisotropy due to the absorption of OH-band exists in the range of IR.

• Journal of the Korea institute for structural maintenance and inspection
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• v.14 no.3
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• pp.83-90
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• 2010

Composite materials can be used economically and efficiently in civil engineering applications when standards and procedure for analysis, design, construction and quality control are to be established. Bridge systems, including the girders and cross-beams, and concrete decks behave as the specially orthotropic plates. For such systems with sections, boundary conditions other than Navier solution types, it is very difficult to obtain its analytical solution. To design the bridge made by the composite materials, cross-section was used as the form-core shape for economical reason and finite difference method was used for output of the stress value. The Tsai-Wu failure criterion for stress space is used. In this paper, the rate of tensile strength reduction due to increased size was considered. And also numerical study is made for these cases.

• Composites Research
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• v.12 no.5
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• pp.31-39
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• 1999

Glass-fiber reinforced polymeric composites provide the desitable properties of high stiffness and strength as well as specific weight. Hence, they have become some of the most important materials in several industries. These composites can be grouped into thermoplastic and thermoset composites, with thermoplastic composites having several advantages over thermoset composites in mechanical properties and processing. As a result, the study of the material behavior and forming techniques of such composites has attracted considerable attention in recent years. When the continuous fiber-reinforced polymeric composites are molded by flow molding, the molded parts leads to be nonhomogeneity and anisotropic because of the separation and orientation of fibers. As the characteristics of the products are greatly dependent on the separation, it is very important to clarify the separation in relarion to molding conditions, fiber mat structures and mold geometry. In this study, the effects of the mold geometry and the fiber mat structure on the compression moldability are studied using the cup-type molding.

• Composites Research
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• v.17 no.4
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• pp.53-60
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• 2004

This paper describes a method for a falling weight impact test to estimate the impact energy absorbing characteristics and impact strength of CFRP laminate plates based on considerations of stress wave propagation theory. The absorbed energy of T300 orthotropic composites is higher than that of quasi-isotropic specimen over impact energy 6.8J, but in case of using T700 fiber, much difference does not show. Also, absorbed energy of T300 orthotropic composites, which are composed of the same stacking number and orientation became more than that of T700 fiber specimen; however there was no big difference in case of quasi-isotropic specimens. The delamination areas of the impacted specimen were measured with the ultrasonic C-scanner to find correlation between impact energy and delamination area. The fracture surfaces were observed by using the SEM (scanning electron microscope) through a low-velocity impact test in order to confirm the fracture mechanism.

• Journal of Korea Foundry Society
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• v.27 no.6
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• pp.255-257
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• 2007

최근에는 탄소섬유복합재료(CFRP)는 우주 및 민간 항공산업분야에 널리 활용이 되고 있는 실정이다. CFRP 복합재는 적층구성에 따라 기계물성치 및 강성에 크게 영향이 미치므로 가장 대표적인 직교이방성 적층재의 적층배향을 비파괴 탐상하는 것은 중요하다. 본 연구에서는 CFRP 적층재의 섬유배향에 가장 민감한 초음파 전단파를 활용하기 위해 2개의 종파 초음탐촉자를 이용하는 새로운 기법을 제안하였다. 또한 초음파 전단파를 발생하기 위해서는 탐촉자 밑면에 접촉매질인 태운꿀을 사용하는데 초음파시험하는 도중에 물성치중에 하나인 즉 점성이 상당히 변하게 된다. 이러한 문제를 해결하기 위해 2개의 종파용 초음파탐촉자를 이용해 전단파를 발생시킴으로써 접촉매질문제를 상당히 경감할 수 있는 것으로 나타났으며 여기에서 발생한 전단파가 CFRP 복합적층판의 섬유배향에 매우 민감함을 알 수 있었다.

• Transactions of the Korean Society of Mechanical Engineers
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• v.12 no.3
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• pp.397-408
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• 1988

The study is concerned with the theoretical and experimental investigation of the elliptical bulge test. The elliptical bulge test is analyzed by using a rigid-plastic finite element method incorporating large deformation and normal anisotropy. Thin elliptical diaphragms of mild steel are bulged for three aspect ratios. The contact problem the die round and the sheet is successfully solved by using a skew boundary condition. It is shown that the proper consideration of die radius and normal anisotropy is very significant. The relation between bulging pressure and deformation is obtained. It has been found that the pole is nearly under proportional straining during deformation. The instability criterion by maximum load condition enables the effective prediction of instability pressure. The computional results are in good agreement with experimental results and to be very useful for a better understanding of the elliptical bulge test.