Papers by Author: Kyung Young Jhang

Abstract: The bending fatigue of Al6061 alloy has been evaluated by the acoustic nonlinearity of laser-generated surface wave. The surface wave is very attractive for field applications since it enables to pulse and receive signals at the same surface of materials, and has strong acoustic nonlinear effects on the surface. A relative acoustic nonlinear parameter was measured successfully on the surface of fatigue-damaged aluminum 6061 alloy. The results show that the acoustic nonlinear parameter increased with fatigue damage accumulation in relation to dislocation evolution. Consequently, this study suggests that the acoustic nonlinearity technique of a laser-generated surface wave can be potentially used to characterize surface damage resulting from bending fatigue.

Abstract: This paper proposes an advanced signal processing technique for the precise estimation of a nonlinear ultrasonic parameter, based on power spectral and bispectral analysis. The power spectrum and bispectrum estimation of the pulse-like ultrasonic signal used in the commercial SAM (scanning acoustic microscopy) equipment is especially considered in this study. The usefulness of the proposed estimation is confirmed by experiments for a Newton ring with a continuous air gap and a real semiconductor sample with local delaminations. The results show that the nonlinear parameter obtained by the proposed method had a good correlation with the delamination.

Abstract: This paper proposes a non-contact single-mode guided wave technique, where the laser beam illuminated through the arrayed line slits is used as the transmitter, and the air-couple transducer is used as the receiver. The line-arrayed laser illumination has a wavelength-matching effect that can generate only a few modes. The air-coupled transducer detects the leaky wave of the propagated guided wave, and by tuning the detection angle of the transducer we can detect the selected single mode. The proposed technique is fully non-contact and is possible to avoid difficulty in the mode identification. The experimental results for a 1-mm thick aluminum plate prove the usefulness of the proposed method, and most especially, it shows that the method is powerful in the generation and detection of anti-symmetric modes of the lowest order.

Abstract: This study was aimed at identifying failure mechanism and strength of brazed copper pipe used in household air conditioner. For these ends, the brazed joints with various brazing length were manufactured according to manufacturer’s recommended methods. The 3-dimensional X-ray technique was used to identify internal flaws and their failure mechanism in brazed joints. And, strength of brazed joint was evaluated by static tests. To evaluate the effect of heat on the brazed joints, the micro-Vickers hardness is measured. Also, bursting tests were performed for burst pressure of brazed joints by using of self-designed internal pressure bursting tester.

Abstract: The detection of interfacial micro-delamination in the thin-layered structure such as the electronic package becomes very important as the electronic device becomes smaller and thinner. The conventional method used to detect the delamination in an electronic package is to use a scanning acoustic microscope (SAM). However, despite its high performance qualities, SAM is often faced with a tricky decision when a delaminated gap is too small. In this paper, a novel method based on ultrasonic nonlinearity is proposed to overcome this limit. The proposed method is integrated into the conventional SAM equipment, and its effectiveness is verified by experiments for the Newton Ring and the real semiconductor package that have micro-delaminations. The results showed that the nonlinear parameter had good correlation with the gap size of delamination. A method of imaging the nonlinear parameter is also proposed to assure the feasibility of the proposed method in the field application.

Abstract: High-tension bolts have been used widely for the clamping of many kinds of large structures. In these bolts, the estimation of clamping force has been regarded as the main issue in the evaluation of clamping condition. This paper proposes a method using ultrasonic wave, which is based on the dependency of sound speed on the stress. In order to verify the usefulness of the proposed method, two kinds of experiments are carried out. The first one involves the measurement of sound speed when the bolt is stressed by the tension tester, and here, the relationship between the exact axial force and sound speed is calibrated. The result shows good agreement with the expected linear relationship between sound speed and axial stress. The second experiment involves the measurement of axial stress by the proposed method when the bolt is stressed by the torque wrench. The results are coincident to the strain gage measurement. From these results, we can conclude that the proposed method is indeed useful in evaluating clamping force in high-tension bolts.

Abstract: Optical NDE (Nondestructive Evaluation) techniques such as ESPI (Electronic Speckle Pattern Interferometer) and shearography that are non-contact and real-time method are used to detect the defects in material. However, ESPI measurement is affected by disturbance such as rigid body motion, and has difficulty in recognizing the real shape and size of the defect. Shearography also involves several problems like it needs several operator-dependent factors including the amount of shearing and shearing direction for the quantitative analysis of internal defect, and it does not show the exact shape of the defect. In order to overcome these problems and evaluate the internal defect quantitatively in this study, the imaging of the inner defect based on the ESPI technique is proposed. In this method, the external perturbative load such as thermal load is induced to the specimen, and the deformation distribution of the specimen is measured by ESPI. Then the distribution of the shear strain energy is obtained by the strain analysis of the ESPI measurement result. On the edge of the defect, the shear strain energy is concentrated, so we can obtain the outline of the internal defect by following the peak point of the shear strain energy distribution. This method makes it possible to evaluate the size and shape of the defect quantitatively without operator-dependent factors.

Abstract: Ultrasonic guided wave has been widely used for the pipe inspection. The conventional method is to use piezoelectric transducers that should be contacted to the target surface. However, in order to automatically inspect pipes in the manufacturing stage, non-contact inspections are strongly required. In this paper, we propose a new non-contact method for the automation of pipe inspection, which generates the ultrasonic guided wave by laser and receives by air-coupled transducer. Also we have developed a rotational canning system to show the inspection result as a 2-D image, which makes the operator find out defects easily. Then, in practical applications of guided wave techniques, it is preferred to generate and detect a single or less dispersive mode. So this paper considers the enhancement of mode-selectivity, where a laser beam illuminated through arrayed line slits is used as the transmitter and the air-coupled transducer is used as the receiver. The line arrayed laser illumination is a wavelength matching technique that can generate only a few modes. The air-coupled transducer detects the leaky wave of the propagated guided wave, and by tuning its detection angle we can detect the selected single mode. This method can generate and receive the guided wave of a specific mode, which makes the interpretation of received signal clearer and resultantly improves the accuracy of inspection. The proposed method was applied to the stainless pipe specimen with artificial defects and its inspection performance was verified. The experimental results showed that the fundamental axial-longitudinal mode was dominantly generated and received, and the location and the size of pipe defects could be displayed well in the 2 dimensional scanning images in real time.

Abstract: The hinge, which is used in various folders of electronic communication devices, damages easily due to the repeated impact-like stress that occurs when the folder is opened. The hinge damage not only affects the lifetime of devices, but it may cause to lose the reputation of the brand name of the company. In this paper, the total process for the life evaluation and failure analysis of this part is proposed. For this, the mechanism of the hinge motion was analyzed first to calculate the force acting on the hinge. Also an automatic durability test system was developed to repeatedly open and shut the folder. Then the change in deformation around the hinge was monitored using an Electronic Speckle Pattern Interferometer (ESPI) method, as the repetition number increased for the opening of the folder. ESPI results showed that the deformation was concentrated at the hinge and it increased according to the increase of repetition number of folder opening. Finally, we evaluated the life of the hinge through an accelerated life test (ALT). ALT results showed that the life of hinge was dependent on the force acting on the hinge

Abstract: The linear ultrasonic technique has been extensively used as a powerful, non-destructive test tool for reliability testing and failure analysis of electronic packaging. This is used most often in the inspection of defects such as delaminations, voids, or cracks through use of a SAM (Scanning Acoustic Microscope). Then, as the reliability level that is required of electronic packaging becomes higher and the thickness of package becomes thinner, the possible defect which needs to be detected becomes smaller. In the conventional SAM, however it is very difficult to detect small defects less than m µ 1 . 0 , such as micro-delaminations. In order to solve such a problem, this paper proposes a nonlinear ultrasonic method, where the nonlinearity caused by the effect of crack-face interactions is considered. The basic concept of this method involves harmonic frequencies that are generated in the transmitted ultrasonic wave due to the partial contact at the interface of micro-delamination. As an evaluation index, the nonlinear parameter dependent on the amplitude of the second order harmonic frequency component is obtained by spectral analysis of the transmitted signal. Experimental results show that the nonlinear parameter has good correlation with the micro-gap and the proposed method can detect the micro-delamination even less than nm 1 .