• 한국정밀공학회지
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• 제29권8호
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• pp.887-895
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• 2012

Lamb waves have received a great attention in the structural health monitoring (SHM) societies because they can propagate over relatively large distances in wave guides such as thin plates and shells. The time-of-flights of Lamb waves can be used to detect damages in a wave guide. However, due to the inherent dispersive and multi-mode characteristics of Lamb waves, one must decompose the Lamb wave modes into the symmetric and anti-symmetric modes for SHM applications. Thus, this paper proposes a decomposition method for the two-mode Lamb waves based on two rules: the group velocity ratio rule and the mode amplitude ratio rule. The group velocity ratio rule means that the ratio of the group velocities of fundamental symmetric and anti-symmetric modes is constant, while the mode amplitude ratio rule means that the magnitude of the fundamental symmetric modes of all measured response signals should be always larger than those of the anti-symmetric mode once the input signal is applied so that the magnitude of fundamental symmetric mode of excited Lamb-wave is larger than that of anti-symmetric mode, and vice versa. The proposed method is verified through the experiments ducted for an aluminum plate specimen.

• 한국군사과학기술학회지
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• 제13권1호
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• pp.126-132
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• 2010

A Lamb wave guided by a plate structure has dispersive characteristics because phase and group velocity change with the variation of frequency and thickness. The Lamb wave has two modes, symmetric and anti-symmetric mode, which propagates symmetrically and non-symmetrically with respect to centerline. In this paper, the derivation of Lamb wave equation with anisotropic material property is investigated. The phase velocity and group velocity dispersion curves are shown using the stiffness matrix of composite materials with the variation of angle.

• 한국철도학회:학술대회논문집
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• 한국철도학회 2008년도 춘계학술대회 논문집
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• pp.2066-2072
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• 2008

The time reversal was investigated for direct root between PZT and PZT, but in case of a circular PZT, lamb wave moves not only along the direct root but also another roots. The center frequency of lamb wave is kept when the lamb waves are reflected from damage. This paper presents experimental and theoretical results for the new structural health monitoring method by above features of lamb wave, and we can increase accuracy of the new structural health monitoring method by using STFT(Short Time Fourier Transform).

• 비파괴검사학회지
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• 제30권5호
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• pp.458-463
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• 2010

음향 비선형성은 재료 물성의 미세한 변화에 민감하기 때문에, 이를 측정하는 비선형 초음파 기술은 재료의 열화나 피로를 평가할 수 있는 기법으로 연구되어 왔다. 하지만 벌크파를 이용하는 일반적인 비선형 초음파 기법은 얇은 판재에 적용하는 것에는 여러 한계가 있다. 이와 같은 경우에는 비선형 Lamb 파의 사용을 생각할 수 있지만, Lamb 파는 벌크파와 매우 다른 전파 특성을 가지고 있어 그 비선형 특성에 대한 별도의 연구를 필요로 한다. 이를 위해 본 연구에서는 Lamb 파에서 비선형성에 의해 전파하면서 누적 성장할 수 있는 2차 고조파 모드의 발생 조건을 분석하였으며, 그 결과 네 가지 조건, 즉 (1) phase matching, (2) non-zero power flux, (3) group velocity matching, (4) non-zero out-of-plane displacement 를 제시하였다. 그리고 제시된 조건으로 알루미늄 판재에 대책 실험한 결과 이론 예측과 동일하게 전파 거리에 따라 2차 고조파 성분의 크기와 비선형 파라미터가 증가하였고, Al6061-T6 과 Al1100-H14에서 측정된 상대적인 비선형 파라미터의 비율이 이론적인 비율과 근접함을 보였다.

• 대한기계학회논문집A
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• 제40권11호
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• pp.919-925
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• 2016

본 연구에서는 비선형 체적파 혼합 기법의 선행연구를 토대로 램파 기법에 적용하여 램파 혼합 기법에 대한 연구를 수행하였다. 램파 혼합 기법의 타당성을 증명하기 위해 결함이 없는 시편과 부식에 의해 발생된 표면 결함이 있는 시편에 대해 실험을 진행하였다. 실험 대조군으로는 램파의 지배적인 면내변위와 면외변위를 가지는 모드 및 주파수로 선정하였다. 그 결과 램파 혼합 기법으로도 결함 검출이 가능하였고, 기존의 램파 기법의 경향성과 유사하게 나타났다. 그리고 이론과 동일한 지배적인 변위와 모드가 발생된 것을 확인 할 수 있었다. 그러나 결함 검출 성능은 측정 방법 및 단순 모드 선정에 따라 결정되는 것이 아니라 변위 비율에 따라 결정된다.

• 한국음향학회지
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• 제29권8호
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• pp.483-490
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• 2010

본 논문에서는 판 형상의 구조물에 발생한 균열의 형상과 개수 등을 진단할 수 있는 Inter-Digital Transducer (IDT)형 Lamb 파 센서를 제안하고 그 타당성을 실험적으로 검증하였다. IDT형 Lamb 파 센서는 기존의 패치형 압전 센서보다 작동 주파수의 조절이 용이하고, finger의 형상에 의해 지향성을 조절할 수 있다는 등의 장점이 있다. 본 논문에서는 무지향성을 가지는 환상형 IDT형 센서와 높은 지향성을 가지는 사각형 IDT형 센서를 설계, 제작하고, 이들에 의해 알루미늄 판을 전파하는 Lamb 파의 전파시간과 진폭의 변화를 측정, 분석하여 판 내에 발생한 균열의 길이, 개수 및 각도를 평가하고, 제안한 IDT 센서의 효용성을 고찰하였다.

• 비파괴검사학회지
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• 제36권3호
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• pp.175-180
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• 2016

Nondestructive evaluation for material health monitoring is important in aerospace industries. Composite laminates are exposed to heat cyclic loading and humid environment depending on flight conditions. Cyclic heat loading and moisture absorption may lead to material degradation such as matrix breaking, debonding, and delamination. In this paper, the moisture absorption ratio was investigated by measuring the Lamb wave velocity. The composite laminates were manufactured and subjected to different thermal aging cycles and moisture absorption. For various conditions of these cycles, not only changes in weight and also ultrasonic wave velocity were measured, and the Lamb wave velocity at various levels of moisture on a carbon-epoxy plate was investigated. Results from the experiment show a linear correlation between moisture absorption ratio and Lamb wave velocity at different thermal fatigue stages. The presented method can be applied as an alternative solution in the online monitoring of composite laminate moisture levels in commercial flights.

• Smart Structures and Systems
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• 제3권2호
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• pp.201-217
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• 2007

An active damage detection technique is introduced to locate damage in an isotropic plate using Lamb waves. This technique uses a time-domain energy model of Lamb waves in plates that the wave amplitude inversely decays with the propagation distance along a ray direction. Accordingly the damage localization is formulated as a least-squares problem to minimize an error function between the model and the measured data. An active sensing system with integrated actuators/sensors is controlled to excite/receive $A_0$ mode of Lamb waves in the plate. Scattered wave signals from the damage can be obtained by subtracting the baseline signal of the undamaged plate from the recorded signal of the damaged plate. In the experimental study, after collecting the scattered wave signals, a discrete wavelet transform (DWT) is employed to extract the first scattered wave pack from the damage, then an iterative method is derived to solve the least-squares problem for locating the damage. Since this method does not rely on time-of-flight but wave energy measurement, it is more robust, reliable, and noise-tolerant. Both numerical and experimental examples are performed to verify the efficiency and accuracy of the method, and the results demonstrate that the estimated damage position stably converges to the targeted damage.

• Smart Structures and Systems
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• 제1권3호
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• pp.257-266
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• 2005

In Lamb wave detection of damages in smart structures, the excitation pulse is usually designed as a narrow band burst wave for the convenience of analysis and recognition. However, the wideband excitation can excite more modes in plate/shell structure and thus provides extra information for changes of the structure. This paper presents a method that can extract information in wideband Lamb wave signals. By transforming the detected signals into various sub-frequency band, the measured signal can be converted to its equivalences of narrow band excitations, therefore, the information in different frequency bands can be acquired from a single test and in the same time the complicity of wideband signal can be simplified. Some test results are provided to verify this method.

• Smart Structures and Systems
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• 제20권6호
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• pp.759-767
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• 2017

Wear and aging associated damage is a severe problem for safety and maintenance of engineering structures. To acquire structural operational state and provide warning about different types of damage, research on damage identification has gained increasing popularity in recent years. Among various damage identification methods, the Lamb wave-based methods have shown promising suitability and potential for damage identification of plate-type structures. In this paper, a comprehensive study was presented to elaborate four remarkable aspects regarding the Lamb wave-based damage identification method for plate-type structures, including wave velocity, signal denoising, image reconstruction, and sensor layout. Conclusions and path forward were summarized and classified serving as a starting point for research and application in this area.