Papers by Author: Young Nam Kim

Abstract: In this research, we proposed nondestructive evaluation method of a flaw in an optical glass lens by resonant ultrasound spectroscopy (RUS). An optical glass lens is used for improving signal coupling between fibers, emitters and detectors. The shape of optical glass lens is an aspherical lens with 3mm in length and 6.35 mm in diameter. For nondestructive evaluation of an aspherical lens, we made of the measuring system by RUS. For flaw evaluation by the RUS to verify the data obtained from the experiment, we carried out simulation by explicit finite elements method, and compared the experimental results with simulation results. Also, we compared the resonant frequencies of specimens with some defect with those of acceptable specimens. The histogram drawn from these data and the available resonance mode surveyed were used to classify the acceptable specimens in the plant.

Abstract: Carbon fiber reinforced plastics, among the advanced composite material watched with keen interests today, is widely used as structural components requiring light weight property because of its high specific strength as well as high specific rigidity. However, this material has a drawback of weakness against a transverse impact loading acting toward the direction of its stacked thickness, which requires different design parameters other than those used for general metal products in actual application. In connection with this point of view, this study utilized specimen of laminated composite material shell having certain radius of curvature considering actual structural component made of laminated CFRP composite material. Penetration experiment was conducted by measuring time of penetration of steel ball between two points where ballistic-screen sensors were attached on front and reverse side of the specimen with the air gun under air pressure that is adjusted by the pressure gauge attached to. Critical penetration energy was found by measuring kinetic energies of the steel ball before and after the penetration. In order to identify crack pattern and penetration mode generated inside the specimen after the experimental penetration, this study used digitalized optical microscope. Through this study, therefore, penetration characteristics by changes in number of layers, by different stacking sequences as well as penetration mode have been observed.

Abstract: Because the layup of composite laminates influences there properties, the strength of composites depends on layup sequence of CFRP laminates. It is very important to detect ply error before the laminate is cured for both manual procedure and fiber placement procedure. An ultrasonic technique would be very beneficial, which could be used to test the part after and before curing laminates and requires less time than the optical test. Scanners were set out for different measurement modalities for acquiring ultrasonic signals as a function of in-plane azimuthal angle. The first motorized scanner was utilized for making transmission measurements using a pair of normal-incidence shear wave transducers. A scanner was built for the acousto-ultrasonic configuration using contact transducers. And a ply-by-ply vector decomposition model has been utilized for evaluating layup errors in composite laminates fabricated from unidirectional plies. We have compared the test results with model data. It is found that high probability shows between experimentations and the decomposition model in characterizing cured and uncured laminates with defect angles.

Abstract: An aluminum or CFRP (Carbon Fiber Reinforced Plastics) tube is representative light-weight materials but its axial collapse mechamism is different from each other. The aluminum tube absorbs energy by stable plastic deformation, while the CFRP tube absorbs energy by unstable brittle failure with higher specific strength and stiffness than those in the aluminum tube. In an attempt to achieve a synergy effect by combining the two members, aluminum/CFRP square tubes were manufactured, which are composed of aluminum tubes wrapped with CFRP outside aluminum square tubes with different fiber orientation angle and thickness of CFRP, and axial collapse tests were performed for the tubes. The crushing behavior and energy absorption capability of the tubes were analyzed and compared with those of the respective aluminum square tubes and CFRP square tubes. Test results showed that the collapse of the aluminum/CFRP square tube complemented unstable brittle failure of the CFRP square tube due to ductile characteristics of the inner aluminum square tube. The collapse modes were categorized into four modes under the influence of the fiber orientation angle and thickness of CFRP. The absorbed energy per unit mass, which is in the light-weight aspect, was higher in the aluminum/CFRP square tube than that in the aluminum square tube or the CFRP square tube alone.

Abstract: CFRP (Carbon Fiber Reinforced Plastics) of the advanced composite materials as structural materials for vehicles, has a wide application in light weighs structural materials of airplanes, ships and automobiles because of high strength and stiffness. However, there is a design variable to be considered in practical application of the laminate composite material, that is, this material is vulnerable to transverse impact. This paper aims at evaluating the effects of curvature and oblique impact on the penetration characteristics of composite laminate shells. They are stacked to [02/902]S, [03/903]S and [02/903/0]S, [0/902/0]S and their interlaminar number two and four. Those specimens were prepared to varied curvature radius (R=100, 150, 200mm and ∞). They are cured by heating to the appropriate hardening temperature (130oC) by means of a heater at the vacuum bag of the autoclave. Test specimens were prepared with dimensions 100mm×140mm. When the specimen is subjected to transverse impact by a steel ball, the velocity of the steel ball was measured both before and after impact by determining the time for it to pass two ballistic-screen sensors located a known distance apart. In general, the critical penetration energy increased as the curvature increased, interface decreased and slope angle on the impact surface increased.

Abstract: This study evaluates cracks in a ceramic ferrule. When an optical fiber is connected to machinery, a ferrule is used to precisely set the optical axes of an optical fiber. A ferrule is a cylinder of 10.5mm in length and 2.5mm in diameter. The crack size of a ferrule can range from 10.40
m to 32.35
m. An acceptable ferrule must have no cracks. In this paper, we predicted and examined defects by Resonant Ultrasound Spectroscopy (RUS). To verify the data obtained from the experiment, we carried out simulation by explicit finite elements method, and compared the experimental results with simulation results. The average error range is 1~2 percent between the experimental resonant frequency of the acceptable specimens and the simulated FEM resonant frequency. Also, we compared the resonant frequencies of specimens with a crack with those of acceptable specimens. The histogram drawn from these data and the available mode surveyed were used to classify the specimens in the plant as either cracked or non-cracked specimens.

Abstract: The layup of a CFRP (carbon-fiber reinforced plastics) composite laminates affects the properties of the laminate, including stiffness, strength and thermal behavior. It is very important to detect ply error before the laminate is cured for both manual procedure and fiber placement procedure. An ultrasonic technique would be very beneficial, which could be used to test the part after and before curing laminates and requires less time than the optical test. Also cross-polarized scan is very sensitive to the layup errors and ply misorientations. Scanners were set out for different measurement modalities for acquiring ultrasonic signals as a function of in-plane azimuthal angle. Firstly, a manual scanner was built for making transmission measurements using a pair of normal-incidence shear wave transducers to find the effect of fiber misorientations of composite laminates. Also a method for nondestructively determining the ply layup in a composite laminate is presented. The method employs a normal-incidence longitudinal ultrasound to perform C-scan of ply interfaces of the laminate. And a ply-by-ply vector decomposition model has been utilized for evaluating layup errors in composite laminates fabricated from unidirectional plies. Test results between after and before curing laminates with model data were compared for a fiber orientation of the laminates.

Abstract: In this work, a wood material was nondestructively characterized and a technique was developed to measure ultrasonic velocity in wood using automated data acquisition software. It is desirable to perform nondestructive evaluation (NDE) to assess material properties and absorption of wood because wood is a natural composite and often inhomogeneity. Several ultrasonic techniques were applied to wood for the evaluation of variations material properties. The variation of ultrasonic velocity was measured and found to be somewhat consistent with those in infiltrated area in the course of nature absorption. Low frequency (1MHz) through-transmission scans based on both amplitude and time-of-flight of the ultrasonic pulse were used for mapping out the material property inhomogeneity. These results were compared with those obtained by dry-coupling ultrasonics. A good correlation was found between ultrasonic velocity (a pulse overlap and peak-delay measurement method) and C-scan image signal based on peak-to-peak amplitude.

Abstract: CFRP composite materials widely used as structural materials for airplanes, ships and aero space vehicles because of their high strength and stiffness. This study aims to investigate the effects of curvature and stacking sequence on the penetration characteristics of CFRP laminated shell. They are stacked to [02/902]S, [03/903]S and [02/903/0]S, [0/902/0]S, and their interlaminar numbers are two and four. We are manufactured to composite laminated shells with curvature radii of 100, 150, 200mm and ∞ (flat-plate), respectively. They were cure to the appropriate hardening temperature (130°C) at the vacuum bag of an autoclave. Composite laminated shells of dimensions 100㎜×140㎜ were prepared. The velocity of the steel ball that transversely impacts a specimen was measured both before and after impact by determining the time for the ball to pass two ballistic-screen sensors located at a known distance apart. In general, the kinetic energy after impact-kinetic energy before impact increased in all specimens. Absorbed energy increased as the curvature increased [03/903]S and [02/902]S which is small interlaminar number, was higher than four interlaminar specimen.