Papers by Author: Chun Gon Kim

Abstract: This research investigated the ballistic impact characteristics of KM2 Kevlar fabrics and Dyneema UD for personnel body armour using two stage gas gun. The neat KM2 Kevlar fabrics were compared with those impregnated with shear thickening fluid(STF) which showed to absorb much energy up to a velocity of 300m/s. Also studied are the effects of projectile shapes on the energy absorbing in fabrics. We observed that the fiber pull out distance without breaking is important for ballistic energy absorption. We devised an efficient ballistic energy absorbing mechanism at high velocity range by adopting flattened roll design of Dyneema UD. The suggested hybrid design composed of flattened roll and KM2 fabric impregnated with STF with stitching had advantage of 15~20% areal density reduction compared with neat KM2 fabric design.

Abstract: The purpose of this study is to present the optimal design technology on the broad band radar absorbing composite laminates. The design concept is based on the 2-layer Dällenbach type radar absorber, in which the composite laminates act as the lossy layers. The radar absorbing function was achieved by controlling the electromagnetic property of the composite laminates by means of adding carbon nano materials into the matrix resin of the fiber-reinforced composite laminates. The laminates were fabricated with the E-glass fabric/epoxy prepregs and cured in an autoclave. The electromagnetic properties were measured for the frequency band of 0.5 ~ 18.0 GHz using a coaxial air line and a vector network analyzer. The radar absorbers were designed to have optimal absorbing performance in 10 GHz. The optimization was conducted to get the optimal number of the plies of the laminates and filler contents of the carbon nano materials in the respective layers. The performances of the fabricated absorbers were measured to be compared with the design results. The discrepancy between the design and the measurement was discussed.

Abstract: We carried out experiments to detect impact locations on a composite plate using two types of composite plates, a composite flat plate with a constant thickness of 5 mm and a composite stiffened panel with stringers. Four multiplexed FBG sensors were attached to the bottom surface of the composite plates to acquire impact signals. The FBG sensor wavelength shift data were collected at a sampling frequency of 40 kHz using a high-speed FBG interrogator (SFI-710, Fiberpro Inc., Korea). The arrival times of the impact signals at each FBG sensor were obtained using a signal processing procedure. The arrival times were affected by noise level and signal-to-noise ratio. In order to overcome this weakness, signal processing techniques such as wavelet decomposition, normalization using each noise level and filtering with a moving average were adopted. To calculate the impact locations of the composite plate, a neural network algorithm was applied.

Abstract: Most accelerometers are composed of a mass, a spring, and a damper. The quantification of the acceleration could be obtained from the equation of dynamic motion expressed in terms of the relative moving displacement. Therefore, if the moving displacement of a seismic mass is inferred from the output signal of a single-degree-of-freedom (SDOF) system, the acceleration could theoretically be obtained by a deformation response factor expressed in the relation between relative displacement and excited acceleration. A reflective grating panel-optical fiber sensor can measure the relative displacement by the movement of the grating panel that is attached to the seismic mass. Ultimately, this research is intended for application to grating panel-optical fiber sensors. However, in order to fabricate the adequate probe for this sensing principle, microscopic rotation (< 0.007 degree) should be prevented for stable reflectivity. Therefore, this paper describes the optimum design of the mass-spring structure which features a larger linear motion range of the leaf spring and no rotation of the seismic mass. Accordingly, finite element analyses (FEA) were accomplished for reflecting the grating panel-optical fiber sensor probe. The leaf spring was employed for the strict linear motion, and some parametric studies on the design of the leaf spring was conducted. First of all, in order to have flexibility in the leaf spring, the leaf spring is divided into N equal parts (N= 3, 4, 5). Furthermore, some parametric studies were carried out, including the length of the seismic mass, arm width, and length of the leaf spring, as well as the moment balancing design. Through the comparisons of each FEA result, the best design of the leaf spring was determined to obtain the optimum features over the measurement range of acceleration and frequency within the linear elastic of the leaf spring.

Abstract: In this paper, we have studied the permittivities of E-glass fabric/epoxy composite laminates containing three different types of carbon-based nano conductive fillers such as carbon black (CB), carbon nano fiber (CNF) and multi-wall carbon nano tube (MWNT). The measurements were performed for permittivities at the frequency band of 0.5 GHz ~ 18.0 GHz using a vector network analyzer with a 7 mm coaxial air line. The experimental results show that the complex permittivities of the composites depend strongly on the natures and concentrations of the conductive fillers. The real and imaginary parts of the complex permittivities of the composites were proportional to the filler concentrations. But, depending on the types of fillers and frequency band, the increasing rates of the real and imaginary parts with respect to the filler concentrations were all different. At the frequency of 10 GHz, the rates in the CNF filled composite and the MWNT filled composite were much larger then those of the CB filled composite. Between the CNF filled composite and MWNT filled composite, however, the former showed a little higher increasing rates than the other. These different rates can have great effect on the thickness in designing the single layer microwave absorbers. The effect of the different rates was examined by using Cole-Cole plots; the plot is composed of a single layer absorber solution line and permittivity lines of these three types of composites.

Abstract: Nowadays, researches for replacing material systems for cryogenic propellant tanks by composites have been being performed for the purpose of light weight of a launch vehicle. In this paper, a type III propellant tank, which is composed of the composite developed for cryogenic use and an aluminum liner, was fabricated and tested considering actual operating environment, that is, cryogenic temperature and pressure. For this aim, liquid nitrogen (LN2) was injected into the fabricated tank and in turn, gaseous nitrogen (GN2) was used for pressurization. During this test procedure, strains and temperatures on the tank surface were measured. ESG (electric strain gage) and thermocouple were used for the measurement of strain and temperature, respectively. Delamination between hoop layer and helical one, was detected during the experiment. Finally, Tsai-Wu criterion for the tank and microscopy for the composite/aluminum ring specimen were carried out to investigate the reason why delamination had happened.

Abstract: The improvement of crack resistance is essential to the application of composites for cryogenic use such as structures storing liquid oxygen or liquid hydrogen. In this study, an effort to improve the crack resistance of a carbon/epoxy composite was made by adding MWNTs (Multi-Walled Carbon Nanotubes) to the resin formulation. Ahead of the investigation of MWNT effect, an epoxy matrix system was developed by mixing two kinds of epoxy resins and adding additives for high toughness at cryogenic temperature. The MWNT-added carbon/epoxy unidirectional prepregs were fabricated by way of a filament winding method with different concentrations of MWNTs (0.2wt% and 0.7wt%). The mechanical tests were performed inside an environmental chamber at room temperature and -150°C. The developed material system has little influence on interlaminar shear strength but resulted in higher fracture toughness at -150°C than those of baseline material. Microcrack densities after thermo-mechanical cycles were measured through an optical microscope.

Abstract: Composite pressure tanks are rapidly expanding in their range of use. However, for high pressure flammable or toxic gases, there is less confidence about their reliability. In this study, fiber Bragg grating (FBG) sensors embedded into a composite pressure tank monitored strain and detected damage. The sensor heads and fiber optic lines were protected with an acrylate recoating, adhesive films, PVC tubes and Teflon
film to survive under the harsh environment of the filament winding process. During the quasi-static loading test, the internal strain was measured with embedded FBG sensor arrays. In order to detect damage occurred during the test, impacts were applied on three different positions in each side by an impact hammer. The difference between damaged and undamaged sides in impact response was analyzed. An interrogation system using a wavelength-swept fiber laser (WSFL) was used for measuring strain, and an erbium-doped fiber amplifier (EDFA) laser source with a tunable Fabry-Perot filter was used for measuring impact response.

Abstract: Because of their good multiplexing capabilities, fiber Bragg grating (FBG) sensors are being studied more actively than any of other fiber optic sensors. The application fields of FBG sensors have been mainly focused on composite materials through embedding rather than through surface attachments. However, there are many limitations on the embedding of FBG sensors into composite materials because of the birefringence that is induced when FBG sensors are not embedded parallel to the reinforcing fibers. This study investigates the fabrication of FBG sensors that have various grating lengths, good multiplexing capabilities, better stability from birefringence, and ease in production. The signal characteristics of FBG sensors are also verified through the cure monitoring of two composite laminates.

Abstract: Multi-walled carbon nanotube (MWNT)/poly (methyl methacrylate) composites were fabricated with the variation of the concentration rate of nanotubes by the solution casting. SEM images showed that the nanotubes were dispersed well throughout PMMA. Assuming that MWNTs in MWNT/PMMA composites were randomly oriented, the Tsai-Pagano equation, which can give the moduli of short fiber reinforced composites, was used to evaluate that of the MWNT/PMMA composite. For investigating mechanical properties of the MWNT/PMMA composite, tensile loading tests were performed, varying the concentration rate of the MWNTs. For each concentration rate of the MWNTs, at least 5 specimens of MWNT/PMMA composites were made and tested. As the concentration rate of the MWNTs increased from 0 to 0.15wt%, tensile strength and modulus of the MWNT/PMMA composites were improved by about 20% and 32%, respectively. However, the experimental results were not in agreement with what we estimated. Here are two reasons supposed. First, the MWNTs used in this research were not stretched straightly but entangled. It means that MWNTs cannot be assumed to be short fibers. Second, the concentration rate of the MWNTs is too small to be considered as short fiber composites.