Papers by Author: Hak Joon Kim

Abstract: Ultrasonic phased array inspection has been paid a great attention in ultrasonic nondestructive evaluation since it can provide a benefit of focusing and/or steering ultrasonic beams. A time reversal technique is claimed to be very robust to focus beams through inhomogeneous media, using the phased array transducers. In this paper, the performance and limitations of a D.O.R.T. method, which is one of the time reversal techniques, is considered based on the experimental studies. In addition, a variation of the D.O.R.T, namely a pre-focused D.O.R.T., is proposed in order to overcome the limitations of the current D.O.R.T. method. The results of the newly proposed method are compared to those obtained by conventional focusing methods.

Abstract: One of the serious problems that make the flaw identification in a multi-layered thick composite panel more difficult is the interference effect of the upper layer. To take care of such a problem, here we propose an image enhancement approach that can get rid of such an interference effect to ultrasonic C-scan images by a normalization of the acquired signals by a reference signals, and demonstrate its performance in the experiments. Specifically, three specimens with artificial flaws are prepared and ultrasonic C-scan images are acquired experimentally to eliminate the undesired interference effect. Great successes are observed in the present study demonstrating the high potential of the proposed algorithm as a practical image enhancement tool in many practical situations.

Abstract: Chemical vapor deposit (CVD) diamond coating layer is expected to extend the lifetime of mechanical parts that are used severely abrasive conditions. However, one of the most severe problems is that the delamination between the CVD diamond coating layer and the silicon substrate occurs frequently due to large difference in the material properties. Therefore, the nondestructive evaluation of adhesive property of CVD diamond coating layer is needed. To address such a need, back-scattered Rayleigh surface wave is currently applied. However, the interpretation of the acquired signal is not easy at all. To take care of such a difficulty, we proposed the time trace angular scan (TTAS) plot and the frequency spectrum angular scan (FSAS) plot that can make possible of the systematic interpretation of the back-scattered signals from the diamond coating layer. In this paper, the concept of the TTAS and FSAS plots and the experimental results presented to demonstrate the effectiveness of the proposed approach.

Abstract: Carbon/phenolic composite (CPC) materials have obvious advantages over conventional materials, which are consisting of carbon fibers embedded in a carbon matrix. The CPCs have become to be utilized as parts of aerospace applications and its low density, high thermal conductivity and excellent mechanical properties at elevated temperatures make it an ideal material for 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/P(Carbon/phenolic) composite material was characterized with non-contact and contact ultrasonic methods using automated acquisition scanner. Also through transmission mode was performed because of the main limitation for air-coupled transducers, which is the acoustic impedance mismatch between most materials and air. Especially ultrasonic images and velocities for C/C(Carbon/Carbon) composite disk brake was measured and found to be consistent to some degree with the non-contact and contact ultrasonic measurement methods. Low frequency throughtransmission scans based on both amplitude and time-of-flight of the ultrasonic pulse were used for mapping out the material property inhomogeneity. Measured results were compared with those obtained by the motorized system with using dry-coupling ultrasonics and through transmission method in immersion. Finally, results using a proposed peak-delay measurement method well corresponded to ultrasonic velocities of the pulse overlap method.

Abstract: Titanium-nitride (TiN) ceramic coatings are widely utilized to extend the lifetime of mechanical parts under severely abrasive conditions and in lubrication environment. Since ceramic layers coated on machinery components inevitably experience the changes in their properties, it is necessary to evaluate the characteristics of ceramic coating layers nondestructively for the reliable use of coated components and the remaining life prediction. For this purpose, in the present study, the ultrasonic backward radiation technique is applied to evaluate very thin TiN ceramic layers coated on AISI 1045 steel and austenitic 304 steel substrates. In this study, very interesting behaviors are revealed from the interrogated TiN ceramic coated specimens. The sliding loading in the abrasive wear regime increases the peak amplitude of the profile. However, the sliding loading large enough to produce scuffing finally decreases the peak amplitude of the profile while the peak angle increases.

Abstract: Ultrasonic C-scan inspections have been used in various industrial fields. Usually, conventional ultrasonic C-scan systems have several mechanical manipulating axis controlled by a computer or an electronic device. These systems are suitable for the high resolution and inspection quality. But they are too heavy and bulky to carry in general, and many industrial fields do not require higher resolution than a hundred micron order. To take care of such a need, in this study we developed an ultrasonic C-scan system that does not have any mechanical manipulator to transport ultrasonic transducers. This system, named "M-Scanner", adopts an electro-magnetic position tracking sensor instead of mechanical manipulating axes in order to trace the transducer position. Furthermore, this new system is portable and has a reasonable construction cost along with practical inspection resolution required in many industrial field inspections.

Abstract: For the long range inspection of structures in nuclear power plant using array transducers, it is necessary to focus waves on defects under interrogation. To take care of such a need, in this study we adopt a time reversal technique that is claimed to be very robust to focus ultrasonic waves on defects. Specifically, we calculate the appropriate time delay using the time reversal technique and re-generate ultrasonic guided waves that are focusing to an interrogated defect with the calculated time delay. In this paper, we describe the principle of the time reversal technique briefly and present the performance enhancement obtained by the time reversal techniques.

Abstract: In this study, the expanded multi-Gaussian beam model is adopted to develop a model to calculate the ultrasonic beam fields radiated from an ultrasonic phased array transducer. Combining this beam model with three other components including time delays, a far-field scattering model and a system efficiency factor, we develop a complete ultrasonic measurement model for predicting the phased array ultrasonic signals that can be captured from a flat-bottom hole in a steel specimen in a pulse-echo set-up using an array transducer mounted in a solid wedge. This paper describes the complete model developed with its key ingredients.

Abstract: For the proper interpretation of ultrasonic measurement results from a side-drilled hole (SDH) using a rectangular transducer, it is very helpful to have a complete ultrasonic measurement model. A highly efficient ultrasonic beam model of a rectangular transducer and an accurate scattering model of a SDH are currently available. However, to develop such a complete measurement model, a reference model for the system efficiency factor is also needed. In this study a reference model suitable for a rectangular transducer is given and combined with existing models to develop a complete ultrasonic measurement model that can the predict ultrasonic signals from a SDH. Based on this model, we have calculated the ultrasonic signals from a SDH at different transducer orientations and compared the model-based predictions with experiments.

Abstract: This study was aimed at developing an effective method and a system for on-line health monitoring of pipes in nuclear power plants by using ultrasonic guided waves. For this purpose we developed a multi-channel ultrasonic guided wave system for a long-range inspection of pipes and a few techniques which can effectively find defects in pipes. To validate the developed system we performed a series of experiments and analyzed the results.