Mr. Chul Min Yeum Profile

Chul Min Yeum

at KAIST

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Proceedings Article | 20 April 2012 Paper

Laser ultrasonic imaging of a rotating blade

Byeongjin Park, Troung Thanh Chung, Chul Min Yeum, Hoon Sohn

Proceedings Volume 8348, 83481A (2012) https://doi.org/10.1117/12.918007

KEYWORDS: Ultrasonics, Ultrasonography, Sensors, Wave propagation, Pulsed laser operation, Signal to noise ratio, Aluminum, Laser development, Nd:YAG lasers, Wind energy

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Although there are many laser ultrasonic imaging techniques developed so far, it still remains challenging to create such images from a rotating object. In this study, an advanced laser ultrasonic imaging technique is developed so that wavefield images can be constructed from a rotating blade using an embedded piezoelectric sensor and a scanning excitation laser system. Here, the biggest challenge is to precisely estimate and control the exact excitation point when the wind blade is rotating with additional ambient vibration and having complex shapes. In this study, the laser excitation point is precisely estimated by computing the correlation values between the measured response signal and the ones in the training data sets. First, training ultrasonic signals are measured at the fixed sensing point by scanning the excitation laser over the target surface of the blade when the blade is in a stationary condition. Once the training is complete, an ultrasonic signal is generated for the rotating blade using the excitation laser and measured by the sensor. The correlation between the measured response and a training response is maximized when they correspond to the same excitation point. Finally, ultrasonic images are generated by scanning the excitation laser over the target surface of the blade. The effectiveness of the proposed imaging technique is investigated through experimental tests performed on a rotating blade specimen.

Proceedings Article | 31 March 2011 Paper

Delamination detection in a composite plate using a dual piezoelectric transducer network

Chul Min Yeum, Hoon Sohn, Jeong Beom Ihn

Proceedings Volume 7984, 798406 (2011) https://doi.org/10.1117/12.880942

KEYWORDS: Ferroelectric materials, Composites, Transducers, Wave propagation, Signal detection, Damage detection, Feature extraction, Environmental sensing, Structural health monitoring, Solids

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In this study, a new damage detection technique is developed so that delamination in a multilayer composite plate can be detected by comparing multi-path pitch-catch Lamb wave signals in a piezoelectric transducer network rather than by comparing each signal with its corresponding baseline signal obtained from the pristine condition. The development of the proposed technique is based on the premise that the fundamental anti-symmetric mode (A0) slows down when it passes through a delamination area while the speed of the fundamental symmetric mode (S0) is invariant. First, the delay of the A0 mode in each path is used as a delamination sensitive feature and extracted using a proposed mode extraction technique. This mode extraction technique uses dual piezoelectric transducers composed of a concentric ring and circular piezoelectric transducers, and it is capable of isolating the A0 mode in any desired frequency without frequency or transducer size tuning. Once the time delays of the A0 mode are computed for all pitch-catch paths in the transducer network, an instantaneous outlier analysis is performed on these features to identify wave propagation path(s) affected by the delaminated region(s). Because the time delays of the A0 mode are instantaneously computed from existing multiple paths, it has been demonstrated that robust delamination detection can be achieved even under varying temperature conditions.

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