• Proceedings of the KSR Conference
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• 2007.11a
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• pp.913-916
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• 2007

In this paper, the possibility of time reversal phenomenon was investigated in damage detection of structure. In conventional lamb wave techniques, damage is identified by comparing the measured data (baseline signals) and the current data. But this method can lead to high false signal in the intact condition of structures due to environmental conditions of the structures. So in this studying, we investigate the possibility of damage detection in the aluminum plate using the time reversal phenomenon of lamb waves.

• Smart Structures and Systems
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• v.8 no.3
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• pp.303-320
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• 2011

Comparing to active damage monitoring, impact localization on composite by using time reversal focusing method has several difficulties. First, the transfer function of the actuator-sensor path is difficult to be obtained because of the limitation that no impact experiment is permitted to perform on the real structure and the difficulty to model it because the performance of real aircraft composite is much more complicated comparing to metal structure. Second, the position of impact is unknown and can not be controlled as the excitation signal used in the active monitoring. This makes it not applicable to compare the difference between the excitation and the focused signal. Another difficulty is that impact signal is frequency broadband, giving rise to the difficulty to process virtual synthesis because of the highly dispersion nature of frequency broadband Lamb wave in plate-like structure. Aiming at developing a practical method for on-line localization of impact on aircraft composite structure which can take advantage of time reversal focusing and does not rely on the transfer function, a PZT sensor array based phase synthesis time reversal impact imaging method is proposed. The complex Shannon wavelet transform is presented to extract the frequency narrow-band signals from the impact responded signals of PZT sensors. A phase synthesis process of the frequency narrow-band signals is implemented to search the time reversal focusing position on the structure which represents the impact position. Evaluation experiments on a carbon fiber composite structure show that the proposed method realizes the impact imaging and localization with an error less than 1.5 cm. Discussion of the influence of velocity errors and measurement noise is also given in detail.

• The Journal of the Acoustical Society of Korea
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• v.35 no.5
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• pp.389-394
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• 2016

MIMO (Multiple-Input-Multiple-Output) in underwater acoustic communication has distortion of received signal because of ISI (Inter-Symbol Interference) and crosstalk among transmitters. Time-reversal mirror was used for compensating of signal distortion, but it has a limit in eliminating crosstalk effectively. This paper proposes a time-reversal mirror based on GSC (Generalized Sidelobe Canceller) for removing crosstalk. The FAF05 (The Focused Acoustic Forecasting 05) experimental data has been used to verify the suggested method by comparison with the conventional time-reversal for communication performance, and it is demonstrated that the suggested method produces better communication performance results than conventional time-reversal.

• The Journal of the Acoustical Society of Korea
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• v.37 no.1
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• pp.21-30
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• 2018

A passive time-reversal technique can improve error performance of the underwater communication system by reducing influence of inter-symbol interferences, which is caused by a multipath channel response. The passive time-reversal communication system equipped with numerous receivers generally can obtain superior error performance since larger diversity gain can be obtained as the number of available received signal increased. In this paper, we analyze the optimal number and combination of receivers that can approximately achieve the best error performance when using the limited number of receivers. For this analysis, we use communication data collected during SAVEX15 (Shallow-water Acoustic Variability Experiment 2015) carried out in the south-western part of Jeju Island from May 14 to May 28, 2015. Analysis results show that there are depths of energy concentration due to the channel characteristics in which the underwater sound channel are present, and the passive time-reversal technique using the limited number of the receivers can derive near-optimal communication performance if the receivers for time-reversal processing are located at the depths where energy is concentrated.

• The Journal of the Acoustical Society of Korea
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• v.33 no.6
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• pp.392-399
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• 2014

The underwater acoustic communication is characterized by doubly spread channels, which are the delay spread due to multiple paths and the doppler spread due to environmental fluctuations or a moving platform. An equalizer is used to remove the inter-symbol interferences that the delay spread causes, but an equalizer doesn't use an acoustic environment such as a multipath. However, a passive time-reversal mirror is simpler than an equalizer because a matched filter is implemented numerically at the receiver structure along with one-way propagation. In this paper, a passive time-reversal mirror is applied to remove interferences due to a multipath in sea-going experimental data in East Sea in Oct. 2010 and improved communication performance is confirmed. The performance is verified by comparing the signal-to-interference plus noise ratio before/after passive time-reversal mirror. It is also performed independently of the passive time-reversal mirror and adaptive equalizer and the bit error rate is compared to verify the performance of underwater acoustic communication.

• The Journal of the Acoustical Society of Korea
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• v.43 no.2
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• pp.184-199
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• 2024

Residual crosstalk has been considered as a major drawback of conventional time-reversal processing in the case of simultaneous multiple focusing. In this paper, the Gram-Schmidt process is applied to time-reversal processing to mitigate crosstalk in ocean waveguides for multiple probe sources. Experimental data-based numerical simulations confirm that nulls can be placed at multiple locations, and it is shown that different signals can be simultaneously focused at different probe source locations, ensuring distortionless responses in terms of active time-reversal processing. This focusing property is also shown to be much less affected by a reduction in the number of receivers than the adaptive time-reversal mirror method. The proposed method is shown to be effective in eliminating crosstalk in passive multi-input multi-output communications using sea-going data.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.26 no.7
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• pp.1453-1460
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• 2002

Creep-fatigue crack growth models have been proposed employing $(C_t)_{avg}$ as a crack tip parameter characterizing the time-dependent crack growth. The basic assumptions made in these previous models were ideal creep reversal conditions such as no creep reversal and complete creep reversal condition. Due to this assumption, the applicability of the models was limited since they did not consider partial creep reversal condition which is usually observed in many engineering metals at high temperature. In this paper the creep reversal parameter, Temperature;$C_R$, which was defined by Grover, is critically evaluated to quantity the extent of partial creep reversal at the crack tip. This approach does not rely on any simplifying assumptions regarding the extent of the amount of creep reversal during the unloading part of a trapezoidal fatigue cycles. It is shown that the $(C_t)_{avg}$ value calculated for 9Cr steel agrees well with the experimentally measured one. It is argued that the extent of improvement is not significant when the result is compared with that of the conventional model which has an assumption of full creep reversal behavior.

• Journal of the Korean Society for Nondestructive Testing
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• v.30 no.6
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• pp.569-577
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• 2010

Nonplanar surface geometries of components are frequently encountered in real ultrasonic inspection situations. Use of rigid array transducers can lead to beam defocusing and reduction of defect image quality due to the mismatch between the planar array and the changing surface. When a flexible array is used to fit the complex surface profile, the locations of array elements should be known to compute the delay time necessary for adaptive heam focusing. An alternative method is to employ the time reversal focusing technique that does not require a prior knowledge about the properties and structures of the specimen and the transducer. In this paper, a time reversal method is applied to simulate beam focusing of flexible arrays and imaging of point-like defects contained in specimens with nonplanar surface geometry. Quantitative comparisons are made for the performance of a number of array techniques in terms of the ability to focus and image three point-like reflectors positioned at regular intervals. The sinusoidal profile array studied here exhibits almost the same image quality as the flat, reference case.

• The Journal of the Acoustical Society of Korea
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• v.25 no.5
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• pp.239-245
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• 2006

Time-reversal processing (TRP) has been shown as an effective way to focus in both time and space. The temporal focusing properties have been used extensively in underwater acoustics communications. Recently. adaptive time-reversal processing (ATRP) was applied to the simultaneous multiple focusing in an ocean waveguide. In this study. multiple focusing with ATRP is extended to the underwater acoustic communication algorithm for multiple receiving locations. The developed algorithm is applied to the underwater acoustic communication to show, via simulation and real data, that the simultaneous self-equalization at multiple receiving locations is achieved.

• Journal of Information Display
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• v.12 no.3
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• pp.129-134
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• 2011

The universal current reversal method is used for the simultaneous measurement of response time (${\tau}$), azimuthal angle (${\varphi}_o$), spontaneous polarization ($P_S$), and rotational viscosity (${\gamma}_{\varphi}$) of two ferroelectric liquid crystals (FLCs). The application of AC field in FLCs results in reorientational current, which is further analyzed to obtain various parameters. The variation in the parameters with temperature follows the typical trend predicted by the theory. The theoretical curve fits well into the experimental data. Its comparison with traditional current reversal method is confirmed to address certain limitations of that method.