Papers by Author: Young Tae Cho

Abstract: A nondestructive technique would be very useful for evaluating the CF/Epoxy composite laminates. It is found that a pitch-catch signal was more sensitive than normal incidence backwall echo of longitudinal wave to subtle flaw conditions in the composites (damages, fiber orientation, low level porosity, ply waviness, and cracks). The depth of the sampling volume where the pitch-catch signal came from was relatively shallow with the head-to-head miniature Rayleigh probes, but the depth can be increased by increasing the separation distance of the transmitting and receiving probes. Also, a method was utilized to determine the porosity content of a composite lay-up by processing micrograph images of the laminate. The porosity content of a composite structure is critical to the overall strength and performance of the structure. The image processing method developed utilizes a free software package to process micrograph images of the test sample. The results from the image processing method are compared with existing data. Beam profile was characterized in unidirectional CFRP (Carbon fiber reinforced plastics) with using pitch-catch Rayleigh probes and the one-sided and two-side pitch-catch technique was utilized to produce C-scan images with the aid of the automatic scanner. Also, it is confirmed that the pitch-catch ultrasonic signal was corresponding with simulated results assuming in unidirectional CFRP composites.

Abstract: Carbon/carbon(C/C) composite materials have obvious advantages over conventional materials, which consist of carbon fibers embedded in a carbon matrix. It’s low density, high thermal conductivity and excellent mechanical properties at elevated temperatures make it an ideal material for aerospace applications especially aircraft brake disks. Because of permeation of coupling medium such as water, it is desirable to perform contact-less nondestructive evaluation to assess material properties and part homogeneity. In this work, a C/C composite material was characterized with non-contact and contact ultrasonic methods using automated acquisition scanner. . Due to the acoustic impedance mismatch found between most materials and air, a major limitation for air-coupled transducers, through-transmission mode was performed. Especially ultrasonic images and velocities for C/C composite disk brake were measured and found to be consistent to some degree with the non-contact and contact ultrasonic measurement methods. 400 kHz frequency through-transmission scans based on both amplitude and time-of-flight of the ultrasonic pulse were used for mapping out the inhomogeneity in material property. Non-contact measured results were compared with those obtained by the motorized system using contact drycoupling ultrasonics and through transmission method in immersion. Results using a proposed peak-delay measurement non-contact method corresponded well to the ultrasonic velocities of the contact pulse overlap method.

Abstract: In particular, CFRP (carbon fiber reinforced plastics) composite materials have found wide applicability because of their inherent design flexibility and improved material properties. CFRP composites were manufactured from uni-direction prepreg sheet in this paper. However, impacted composite structures have 50-75% less strength than undamaged structures. It is desirable to perform nondestructive evaluation to assess material properties and part defect in order to ensure product quality and structural integrity of CFRP. In this work, a CFRP composite material was nondestructively characterized and a pitch-catch technique was developed to measure impacteddamaged area using an automated-data acquisition system. Also one-sided mode was performed to scan defect both manual contact measurement and an immersion tank. It is found a pitch-catch signal was found to be more sensitive than normal incidence backwall echo of longitudinal wave to subtle flaw conditions in the composite, including fiber orientation, low level porosity, ply waviness, and cracks. The paper describes the depth of the sampling volume with the head-to-head miniature Rayleigh probes and also ultrasonic C-scan images are acquired experimentally using one-sided measurement and a conventional scanner.

Abstract: Si-Mn spring steel (JIS SUP 9) is primarily employed in the construction of coil springs, particularly those used in soil cultivators deployed in harsh environments. This is due to its specific properties, which allow it to resist permanent deformation, and also provide rapid and effective vibration relief. Si-Mn spring steel (JIS SUP 9) for use in such projects is normally subjected to oil quenching and heat treatment, in order to strengthen its resistance to both impact and fatigue. Unfortunately, oil quenching is associated with environmental issues, most notably pollution and the contamination of wastewater with oil. Therefore, the objective of this study was to describe an alternative aqua quenching for use with this heat treatment. However, aqua quenching has traditionally conferred a lower tensile laod than does oil quenching. This problem of lesser tensile load was solved by properly controlling the density and temperature of the aqua quenching liquid, as well as careful mixing. As a result, it has proven possible to achieve results with aqua similar to those conferred by oil quenching, with regard to chemical components, tensile load, and surface hardness. The tensile load results achieved by oil quenching were 13,145 Pa and 17,177 Pa, and the tensile loads achieved with aqua quenching were 15,392 Pa, 16,804 Pa, 17,923 Pa, and 18,668 Pa. Therefore, the results of our experiments demonstrated the appropriacy and efficacy of this aqua quenching technique as a component of heat treatment for Si-Mn spring steel (JIS SUP 9).

Abstract: Composite laminates help to lighten components because they have the advantage of higher strength-to-weight ratio. So, they have come to be utilized as parts of advanced areas. An NDE technique for stacking orientation determination would be very beneficial because of layup orientation influence to the laminate stiffness. In this study, an investigation of shear wave ultrasonic technique was carried out in order to detect stacking orientation error for quasi-isotropy composite laminates. During testing, the most significant problem is that the couplant conditions do not remain the same because of changing the viscosity of the couplant. Therefore, the design and use of a shear wave transducers would greatly aid in alleviating the couplant problem. A pyramid with an isosceles triangle with two 45o was made of aluminum to generate shear waves using two longitudinal transducers based on ultrasonic-polarized mechanism. Also, the signal splitter was connected to the pulser jack on the pulser/receiver and to the longitudinal transducers. The longitudinal transducers were mounted with mineral oil and the shear transducer was mounted on the bottom as a receiver with burnt honey. It is found that the shear wave was generated at a maximum and a minimum based on the ultrasonic-polarized mechanism. To apply this method into a practical inspection tool, the quasi-isotropy composite laminates were nondestructively evaluated for detecting error of the laminates.

Abstract: General structures or machines are firmly connected by bolts so that they have many contact surfaces. Between these contact surfaces, there is a movement that vibrates with tiny amplitude, which causes friction, leading to fretting damage. This damage causes a remarkable reduction in the fatigue strength of the structures sub-materials, leading to remarkable fall-down of life span of machines and structures. The particle reinforced composites under research in the aerospace and automobile industries, theoretical analysis and research on fretting fatigue involved with contact are needed, but there are few domestic studies. Therefore, this study is to identify cracking damage, its initiation, growth direction and location by considering the strength load and contact (surface/surface) in particle reinforced composites. So it performed a study on ellipsoidal particle reinforced composites that have contact force under tensile load. This study performed a theoretical analysis and a fretting test to identify damage mechanism to industrial structures as well as aerospace and automobile industries according to expanded coverage of application and development of the composites.

Abstract: Since layup orientation of the laminates greatly influences its properties, an NDE technique for layup orientation determination would be very beneficial. It is desirable to perform contact-less nondestructive evaluation to assess material properties and defects of composites because of permeation of coupling medium such as water. An ultrasonic technique would be very useful, which could be used to test the part after and before curing laminates and requires less time than the optical test. First of all, conventional ultrasonic experimentations were conducted using water as a transmitting medium. In contrast, air-coupled ultrasound is non-contact and has clear advantages over water-coupled testing. Therefore ultrasonic systems for air-coupled and conventional ultrasound 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 orthotropic composite laminates. With the transmitter and receiver on the same side of a laminates, Lamb waves were generated and used for the amplitude magnitude. As a result, it was confirmed that the influence of fiber direction in the laminates could be detected by the non-contacting or contacting inspection from one-side and the detectability was improved by using shaped tube for reducing the specular reflection on transmitting. Furthermore, a vector decomposition model was utilized for lay-up error of the orthotropic laminates. Finally, aircoupled results well corresponded somewhat to those of contact ultrasonic examination in the orthotropic laminates.

Abstract: We developed a device that makes possible the normal generation of low-temperature plasma under atmospheric pressure. For plasma generation, a radio frequency of rf (13.56 MHz) was used, for plasma gas, helium (He), and for material gases, acetylene (C2H2) and toluene (C6H5CH3) were used. As a result of measuring Fourier transform infrared (FT-IR) after adding the CO2 gas to the generated plasma, the absorption of C=O (carboxyl group) was observed around 1715 cm-1. When the flow rates of the added CO2 increased, the absorption peak increased at the same time, and we knew that this originated from the CO2 molecules.

Abstract: Recently, the demand for Ultra-precision products is rapidly increasing, especially in such industries as semi-conductors, computers, aerospace, and precision machinary. Ultra-precision processing is also extremely needed for NT in the field of mechanical engineering. Lately, together with drastic advancement of electronic and photonics industry, the need of Ultra-precision processing is increased in the manufacturing of various kernel parts, which are connected with these industrial fields. Especially, motion accuracy to nanometers is required in order in stroke of hundreds millimeters according as the diameter of the processed object great and the processing accuracy rises. In this case, the response speed absolute delay due to inertial mass of the moving parts is very large. Therefore, real-time motion error compensation becomes the barest necessity. In this paper, we used ultra-precision cutting unit (UPCU) to cope with the problem. A special UPCU was designed and tested to obtain sub-micrometer from accuracy in diamond turning of flat surfaces. The thermal growth spindle error was compensated in real time using the UPCU driven by piezoelectric actuator along with a laser encoder displacement sensor.