Hyung-Ick Kim

Super-stretchable graphene oxide macroscopic fibers with outstanding knotability fabricated by dry film scrolling.

Graphene oxide (GO) has recently become an attractive building block for fabricating graphene-based functional materials. GO films and fibers have been prepared mainly by vacuum filtration and wet spinning. These materials exhibit relatively high Youngs moduli but low toughness and a high tendency to tear or break. Here, we report an alternative method, using bar coating and drying of water/GO dispersions, for preparing large-area GO thin films (e.g., 800-1200 cm(2) or larger) with an outstanding mechanical behavior and excellent tear resistance. These dried films were subsequently scrolled to prepare GO fibers with extremely large elongation to fracture (up to 76%), high toughness (up to 17 J/m(3)), and attractive macroscopic properties, such as uniform circular cross section, smooth surface, and great knotability. This method is simple, and after thermal reduction of the GO material, it can render highly electrically conducting graphene-based fibers with values up to 416 S/cm at room temperature. In this context, GO fibers annealed at 2000 °C were also successfully used as electron field emitters operating at low turn on voltages of ca. 0.48 V/μm and high current densities (5.3 A/cm(2)). Robust GO fibers and large-area films with fascinating architectures and outstanding mechanical and electrical properties were prepared with bar coating followed by dry film scrolling.

Natural Cork Agglomerate Employed as an Environmentally Friendly Solution for Quiet Sandwich Composites

Carbon fiber-synthetic foam core sandwich composites are widely used for many structural applications due to their superior mechanical performance and low weight. Unfortunately these structures typically have very poor acoustic performance. There is increasingly growing demand in mitigating this noise issue in sandwich composite structures. This study shows that marrying carbon fiber composites with natural cork in a sandwich structure provides a synergistic effect yielding a noise-free sandwich composite structure without the sacrifice of mechanical performance or weight. Moreover the cork-core sandwich composites boast a 250% improvement in damping performance, providing increased durability and lifetime operation. Additionally as the world seeks environmentally friendly materials, the harvesting of cork is a natural, renewable process which reduces subsequent carbon footprints. Such a transition from synthetic foam cores to natural cork cores could provide unprecedented improvements in acoustic and vibrational performance in applications such as aircraft cabins or wind turbine blades.

INVESTIGATION OF MECHANICAL DAMPING CHARACTERISTIC IN SHORT FIBERGLASS REINFORCED POLYCARBONATE COMPOSITES

The focus of this study is to experimentally investigate the effect of debonding stress, the interface between the fibers and the polymer matrix, on the damping properties of the short fiberglass reinforced polymer composites. In this study, short fiberglass reinforced polycarbonate composite materials were fabricated and characterized for their tensile properties by varying the fiberglass loading fraction. The debonding stress was evaluated by coupling the acoustic emission technique with the tensile testing. After the determination of the debonding stress was completed, dynamic cyclic testing was performed in order to investigate the effect of debonding on the damping properties of the polymer composites. It was experimentally observed in this study that the debonding can facilitate the stick-slip friction under cyclic loadings, which then gives rise to better damping performance in the fiberglass composites.

FATIGUE CRACK PROPAGATION BEHAVIOR ACCORDING TOFIBER ARRAYING DIRECTION FOR LOAD DIRECTION INWOVEN CFRP COMPOSITE

The fatigue crack propagation of CFRP (carbon fiber reinforced composite material) laminates is of current interest, particularly with regard to their durability under fatigue loading. Recently, carbon fiber reinforced composite materials (Woven fabric) are widely used in various fields of engineering because of its advanced properties. Then, many researchers have studied woven fabric CFRP materials but fatigue crack propagation behaviors for composites have not been still standardized . It shows the different crack propagation behavior according to load and fiber direction. Therefore, there is a need to consider fatigue crack propagation behavior in conformity with fiber arraying direction to load direction at designing structure using woven CFRP materials. In this study, therefore, the fatigue crack propagation for plain woven CFRP composite materials was investigated under two different fiber array direction (fiber arraying direction to load : 0°, 45°). Fatigue crack propagation tests of the woven CFRP composite were conducted under sinusoidal wave-form with stress ratios of 0.3 at a frequency of 10Hz, respectively. As a result of test, fatigue crack propagation rates (da/dN) were plotted against the stress-intensity factor amplitude (ΔK) and other factor. Also we compared ΔK with other factor that considering in-plain anisotropy. All of tests of fatigue crack propagation were carried out under mode I opening loading by using compact tension specimens.

Evaluation of the Plastic η-Factor Considering Strength Mismatch in a Narrow Gap Welding Part (II)

Abstract This study would like to evaluate the influences with the strength mismatch and the variation of the welding width of the narrow gap welding for the plastic parameter, the major constant determining the plastic -factor of J -Integral, using 3-D FEM. For this, we evaluate the plastic -factor according to the variation of the strength mismatch of weldment with same materials and welding width through FEM. Also, we proposed the equation of plastic -factor considering the variation of the strength mismatch of weldment with similar materials and welding width. 1. 서 론 최근 원자력 발전소 배관과 같은 대형 구조물 배관에 대한 협계용접(narrow-gap welding)이 증가되고 있다. 협계용접은 기존 용접법에 비해 용접부의 면적과 폭이 감소하며, 이에 따라 용접시간이 단축될 뿐만 아니라, 용접부의 수축 및 변형, 그리고 이에 따른 잔류응력이 감소되는 장점이 있다. 현재 이러한 협계용접부에 대한 대부분의 연구는 협계용접부의 용접 건전성 평가에 집중되어 있고, 원전 배관 설계 및 수명관리에 중요한 LBB(leak before brake) 해석을 협계용접부에 적용하는 연구가 일부 수행되고 있다. (1,2) †책임저자, 회원, 성균관대학교 기계공학부 E-mail : seok@skku.edu TEL : (031)290-7446 FAX : (031)290-7482 * 성균관대학교 기계공학부** 성균관대학교 대학원 기계공학과

THE MICROSTRUCTURAL ANALYSIS OF THE EFFECT OF FIC ON GAS TURBINE BLADES

Advancements in superalloys permit hot gas path components to operate for many thousands of hours under severe centrifugal, thermal and vibratory stresses. The blade of a gas turbine must withstand the most severe combination of temperature, stress, and environment. After a certain period of operation, the blade is damaged by the turbine operation mode. To recover the similar initial mechanical properties, the blade of nickel-based superalloy component undergoes a replacement repair process. A fluoride ion cleaning (FIC) process is used to assist in the successful welding repair of nickel-based superalloy components. This work is to study the FIC influence on a gas turbine blade that was serviced 25,748 equivalent operating hour (EOH) after first overhaul. The blades, including the fractured regions were obtained from a plant. The conditions of blade samples were observed by optical microscope and SEM, and the chemical composition of the flaws in the blade was investigated by EDS. γ′ phase morphology is also an important indicator of the mechanical properties of blades at the high temperatures. The blade condition was examined from morphology and size of the γ′ phase, because the γ′ phase coarsening rate depends largely on the exposure time and temperature.

Toughening mechanisms of thermoplastic particulate polycarbonate composites

Toughness of a polymer is a key material property for energy absorbing capability for various engineering applications. Significant effort has been made to improve toughness of a polymer and hence increase the energy absorbing capability; typically rigid-particles in thermoplastics or rubbery modifiers in a brittle polymer matrix. The focus of this study is to investigate toughening mechanisms of a thermoplastic polymer composite. Micron-size thermoplastic particle reinforced polycarbonate (PC) composite materials was fabricated via a solution mixing method. The mechanical properties of the polymer composites were characterized in tensile testing while the acoustic emission was monitored to assess the material failure modes during the tensile test. Substantial improvement in tensile toughness was observed for the polymer composites and the toughening mechanisms responsible for the improvement were identified and quantified for each contribution to the observation.

Determination of local debonding stress and investigation of its effect on mechanical properties of glass short fiber reinforced polycarbonate composites

Thermoplastic polymers are often reinforced by adding short fibers to improve mechanical properties including Youngs modulus and tensile strength of the polymers. In many engineering applications, energy absorbing characteristics in such particulate polymers is known to be a very important property to be considered in composite designs, and meanwhile debonding at the interface between fiber and matrix in the composites may affect the energy absorption properties. Here, the focus of this study is to employ a semi-empirical approach to determine the debonding stress and investigate the effect of the debonding stress on energy absorbing properties of short glass fiber reinforced polycarbonate composites. Glass short fiber reinforced polycarbonate composites are fabricated via a solution mixing technique. Tensile testing and acoustic emission measurement are simultaneously performed for the polycarbonate composites. The test results including toughness are compared for the composites over neat polycarbonate. Also the local debonding stress in the vicinity of each glass fiber in composites is estimated by combining modeling and experiments. A finite element model is developed to determine local debonding stress at the interface between the fiber and matrix. The local debonding stress appears to considerably affect the toughness of the composites.

Spherically shaped micron-size particle-reinforced PMMA and PC composites for improving energy absorption capability

The focus of this study was to experimentally investigate spherically shaped micron-size particles reinforced polymethyl methacrylate (PMMA) and polycarbonate (PC) polymer composites for improving energy absorbing capabilities such as toughness and low-velocity impact resistance. In this study, a solution mixing method was developed to fabricate both PMMA and PC polymer composites with spherically shaped micron-size polyamide- nylon 6 (PA6) particles inclusions. The morphology of the fracture surfaces of polymer composites was examined by using optical microscopy and scanning electron microscopy. Strain-rate dependent response of both PMMA and PC polymer composites was investigated by characterizing tensile and flexural properties. Low-velocity penetration testing was performed for both polymer composites and the key results observed for energy absorption capabilities are discussed in this study.