• Journal of the Korean Society for Nondestructive Testing
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• v.25 no.2
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• pp.87-94
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• 2005

In recent, Lamb wave has been actively studied for non-destructive testing of plate. Among those studios, laser generation method of focused Lamb wave is expected to have high spatial resolution with advantages of non-contact testing. In this method, the laser beam is illuminated on the surface of object by through an arrayed-arc slit, and then the energy of the generated Lamb wave is concentrated at a focus of arc. This focusing effect improves the spatial resolution, which is dependant on the geometries of arrayed-arc slit. In this paper, the relationships between the parameters of arc-shaped slit and the focusing ability of the generated Lamb wave was investigated by the simulation based parametric study The results show that to improve the focusing effect, radius of illuminated laser, angle of arc and number of arcs must be increased, which minimum radius of arc and distance between arcs(=wavelength) must be decreased. These results are expected to be used as a guide to design the proper shape of slit.

• The Journal of Korean Academy of Prosthodontics
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• v.39 no.6
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• pp.611-624
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• 2001

A precise fit of the implant prosthesis is one of the most important factors in preventing mechanical complications. To analyze the degree of the misfit of implant prosthesis, a modal testing experiment was accomplished. And. to interpret the modal testing analysis mathematically, three-dimensional finite element models were established. In the experimental modal testing analysis, with a laser displacement meter, FFT analyzer, impact hammer, etc., natural frequencies of the models with various degree of prosthesis fit were determined after the frequency response function were calculated. In the finite element analysis, the natural frequencies and mode shapes of the models which simulated those of experimental modal testing were computed. The results were as follows: 1. Natural frequencies of the prosthesis-abutment were related to the contact state between components. 2. In the modal testing experiment, the natural frequencies increased from $50{\mu}m$ to $200{\mu}m$ gap and reached a plateau. 3. In the finite element analysis, the natural frequencies decreased gradually according to the in crease of the gap size. 4. In the finite element analysis, the mode shapes of model 1 with misfitting prosthesis showed different patterns from those without misfitting prosthesis. 5. The devices including a laser displacement meter used in this study were useful for measuring the natural frequencies of an implant prosthesis which had various degrees of fit.

• Journal of the Korean Society for Precision Engineering
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• v.13 no.12
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• pp.142-150
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• 1996

Nowadays, as the development paeiod of new products becomes even shorter, the importance of Rapid Prototyping Technology(RPT) has been rapidly increased. The major application of RPT is an early verification of product designs and quick production of prototypes for testing. Moreover, RPT is applied not only as a second tooling process such as mold making and investment casting but also as a creating some physical structure in medical field. Despite the remarkable progress of RPT, it is required to improve various problems resulting from application such as production time, accuracy and materials. This paper presents a laser scan path generation for accuracy of stereolithographicparts The methodology of laser scan path generation is discussed based on the stereolithography, The procedure of this research is as follows : 1) Input laser scanning conditions such as a laser beam diameter and a laser scanning interval, 2) Reconstruct original contours without self intersecting offset, 3) Calculate offset about reconstructed contours, 4) Calculate intersection points between horizontal or vertical lines and offset contours for internal hatch, 5) Decide laser shutter on/off points. The algorithm developed and programmed by C language is verified as an efficient method after testing a number of STL files of mechanical parts.

• Journal of the Korean Society for Nondestructive Testing
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• v.33 no.3
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• pp.257-263
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• 2013

Acoustic nonlinearity of surface waves is an effective method to evaluate the micro damage on the surface of materials. In this method, the $A_1$ (magnitude of the fundamental wave) and $A_2$ (magnitude of the second-order harmonic wave) are measured for evaluation of acoustic nonlinearity. However, if there is another source of second-order harmonic wave other than the material itself, the linear relationship between $A_1{^2}$ and $A_2$ will not be guaranteed. Therefore, the second-order harmonic generation by another source should be fully suppressed. In this paper, we investigated the initial second-order harmonic generation in narrowband surface waves by multi-line laser beams. The spatial profile of laser beam was considered in the cases of Gaussian and square-like. The temporal profile was assumed to be Gaussian. In case of Gaussian spatial profile, the generation of the initial second-order harmonic wave was inevitable. However, when the spatial profile was square-like, the generation of the initial second-order harmonic wave was able to be fully suppressed at specific duty ratio. These results mean that the multi-line laser beams of square-like profile with a proper duty ratio are useful to evaluate the acoustic nonlinearity of the generated surface waves.

• Journal of the Korean Society for Nondestructive Testing
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• v.34 no.3
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• pp.226-232
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• 2014

In the present study, the characteristics of laser ultrasound in the ablation regime are investigated using simulations and experiments. The laser ultrasonic technique has been recognized as a noncontact method in the field of nondestructive tests (NDTs). In hostile environments (such as hot temperatures), this method has various advantages over the conventional contact ultrasonic method. In particular, in the ablation regime, the laser ultrasonic technique is suitable for inspecting internal defects because of the high amplitude and directivity of the longitudinal wave. In this paper, a simulation model for laser ultrasound in the ablation regime was developed. This model was subsequently applied to a defective specimen using the B-scan method to locate defects. Finally, we performed an experimental test to verify the simulation results. Consequently, the simulation demonstrated good agreement with the experimental test.

• Journal of the Korean Society for Nondestructive Testing
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• v.33 no.2
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• pp.160-164
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• 2013

A laser ultrasonic inspection system is a non-contact inspection device which generates and measures ultrasonics by using laser beam. A laser ultrasonic inspection system provides a high measurement resolution because the ultrasonic signal generated by a pulse laser beam has a wide-band spectrum and the ultrasonic signal is measured from a small focused spot of a measuring laser beam. In this study, galvanic corrosion phenomenon was measured by non-destructive and non-contact method using the laser. The case of mixed foreign material on the part of corrosion was assumed and laser ultrasonic experiment was conducted. Ultrasonic was generated by pulse laser from the back side of the specimen and ultrasonic signal was acquired from the same location of the front side using continuous wave laser and Confocal Fabry-Perot Interferometer(CFPI). The characteristic of the ultrasonic signal of exist foreign material part was analyzed and the location and size of foreign material was measured.

• Proceedings of the Korea Concrete Institute Conference
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• 2009.05a
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• pp.247-248
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• 2009

Testing standards both nationally and abroad for high flowing concrete are presented about slump flow and the time to reach 500 mm. However, the slump flow testing method to measure flow properties of high flowing concrete results in measurement error due to the differences in each person's perception of the test. This study introduced laser diode to reduce such testing errors from personal differences in experimental procedure.

• Journal of the Korean Society for Nondestructive Testing
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• v.28 no.4
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• pp.372-376
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• 2008

We report experimental results on the correlations between welding penetration depth and the frequencies of the radiation from the welding pool. Various welding samples such as SUS304, brass, SUS316, etc. have been investigated with 2 kW CW Nd:YAG laser welding machine. The radiation signals from the plume generated by the interactions between the welding sample and laser with respect to the defocusing length was measured with fiber system collecting the plume signal. Analysis of the frequencies by using fast Fourier transform (FFT) shows that the penetration depth is deep as plume signal frequencies are low, shallow penetration depth for high frequencies. Frequencies up to 250 Hz for obtained signals can be analyzed with the discrete FFT. This is the useful method fur closed loop control of the laser power with respect to the welding penetration depth and is used for real time inspection of the welding quality.

• Journal of the Korean Society for Nondestructive Testing
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• v.34 no.6
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• pp.457-466
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• 2014

This paper presents an analytic and numerical simulation of the generation and propagation of pico-second ultrasound with nano-scale wavelength, enabling the production of bulk waves in thin films. An analytic model of laser-matter interaction and elasto-dynamic wave propagation is introduced to calculate the elastic strain pulse in microstructures. The model includes the laser-pulse absorption on the material surface, heat transfer from a photon to the elastic energy of a phonon, and acoustic wave propagation to formulate the governing equations of ultra-short ultrasound. The excitation and propagation of acoustic pulses produced by ultra-short laser pulses are numerically simulated for an aluminum substrate using the finite-difference method and compared with the analytical solution. Furthermore, Fourier analysis was performed to investigate the frequency spectrum of the simulated elastic wave pulse. It is concluded that a pico-second bulk wave with a very high frequency of up to hundreds of gigahertz is successfully generated in metals using a 100-fs laser pulse and that it can be propagated in the direction of thickness for thickness less than 100 nm.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 2002.10a
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• pp.85-89
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• 2002

This paper used the Laser Speckle Interferometry to present the strain analysis of Nd:YAG Laser welding zone. Previous TIG welding to sheet plate, which welds only high-skilled engineer, produces residual stress nearby welding zone due to thermal effect. However, Laser welding makes sheet pate welding easy and thermal effect minimum. Thermal effect zone is measured by strain analysis of the laser-welding zone by ESPI under tensile testing of sheet plate welded by Nd:YAG laser. The ESPI results, which compared with strain gage method, are agreed within error 3 %.