• 대한기계학회논문집B
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• 제27권6호
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• pp.701-713
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• 2003

The BEM is a highly efficient method in the sense of economical computation. However, boundary integration is not easy for the complex geometry and moving surface, e.g. a rotating blade. Thus, Kirchhoff surface is designed in an effort to overcome the difficulty resulting from complex boundary conditions. A Kirchhoff surface is a fictitious surface which envelopes acoustic sources of main concern. Acoustic sources may be distributed on each Kirchhoff surface element according to their acoustic characteristics. In this study, an axial fan is assumed to have unsteady loading noise as a dominant source. Dipole sources can be modeled to solve the FW-H equation. Acoustic field is then computed by determining Kirchhoff surface on which near-field is implemented, to analyze the effect of Kirchhoff surface on it. The optimal shape and the location of Kirchhoff surface are discussed by comparing with experimental data acquired in an anechoic chamber.

• 비파괴검사학회지
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• 제21권4호
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• pp.425-433
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• 2001

복합재료-금속 접착접합부가 사용 중 반복 하중을 받을 때 발생하는 피로 손상도를 음향초음파(acousto-ultrasonics; AU)법과 음향방출(acoustic emission; AE)법을 이용하여 평가하였다. 피로시험에는 단일겹치기(single-lap) 시험편을 사용하였으며, AU법을 통해 취득한 신호로부터 음향초음파변수(acousto-ultrasonic parameters; AUP)와 피로손상과의 상관관계 곡선을 얻고, AE법에서는 누적 AE events를 통한 피로손상과의 상관관계 곡선을 얻어, AU법과 AE법의 결과를 비교하였다. 이 곡선들은 피로손상에 의한 고분자기지 복합재료의 강성 저하$(E/E_0)$를 나타내는 곡선과 매우 유사하며, 이를 바탕으로 피로 손상도의 예측과 잔여 수명의 예측이 가능하다. 또한 피로 하중의 초기 단계와 피로손상의 누적에 의해 급격한 변화가 나타나는 단계의 파형 주파수 성분을 비교하여, 피로 파괴의 마지막 단계에서 나타나는 신호 중에는 본격적인 피로손상에 의해 발생하는 AE 신호 성분이 포함되어 있음을 확인할 수 있었다.

• Smart Structures and Systems
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• 제8권3호
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• pp.321-341
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• 2011

Acoustic emission (AE) monitoring was conducted for mortar specimens under three types of static loading patterns (cubic-splitting, direct-shear and pull-out). Each of the applied loading patterns was expected to produce a particular fracture process. Subsequently, the AEs generated by various fracture or damage processes carried specific information on temporal micro-crack behaviors of concrete for post analysis, which was represented in the form of detected AE signal characteristics. Among various available characteristics of acquired AE signals, frequency content was of great interest. In this study, cement-based piezoelectric sensor (as AE transducer) and home-programmed DEcLIN monitoring system were utilized for AE monitoring on mortar. The cement-based piezoelectric sensor demonstrated enhanced sensitivity and broad frequency domain response range after being embedded into mortar specimens. This broad band characteristic of cement-based piezoelectric sensor in frequency domain response benefited the analysis of frequency content of AE. Various evaluation methods were introduced and employed to clarify the variation characteristics of AE frequency content in each test. It was found that the variation behaviors of AE frequency content exhibited a close relationship with the applied loading processes during the tests.

• 소음진동
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• 제7권3호
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• pp.457-466
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• 1997

The experimental and analytical study was conducted to determine the noise transmission characteristics of acoustically loaded steel plate of rectangular enclosure and to investigate the sound radiation characteristics through out the enclosure. The vibrations of acoustically loaded plate give rise to sound radiations and generate the reverberant space that the sound field exists very close to a vibrating plate. Acoustic transmission loss is measured from the incident intensity into the plate and the transmitted intensity through out the plate. Sound radiation patterns are measured from both acoustic intensity technique and surface intensity technique. Those resultant patterns and vibrational modes are vital in understanding the relations between vibration and noise in the near field out of vibrating plate.

• 한국안전학회지
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• 제6권4호
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• pp.21-27
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• 1991

Acoustic Emission was monitored during tensile test and loading-holding-unloa-ding cycle test for two types (notched and unnotched) of CFRP specimens. AE activities showed that the fiber breakage during tensile tests depended upon the specimen geometry. We obtained new AE parameter such as the ration (damage ratio= AE events during unloading test / AE events during loading test) and the felicity ratio from which we investigated dynamic fracture process of CFRP specimens. The damage ratio of AE events was shown to be a good indicator to distinguish the generated fracture mechanism, such as fiber breakage and delamination. Also, ultrasonic testing results after loading-holding-unloading cycle test were good agreement with AE test results to detect defects or fiber breakage.

• Geomechanics and Engineering
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• 제31권4호
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• pp.365-373
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• 2022

In order to investigate the damage evolution law of rock specimens under cyclic loading, cyclic loading tests under constant loads with different amplitudes were carried out on limestone specimens with high strength and brittleness values using acoustic emission (AE) technology and the energy evolution and AE characteristics were evaluated. Based on dissipated energy density and AE counts, the damage variable of specimen was characterized and two damage evolution processes were analyzed and compared. The obtained results showed that the change of AE counts was closely related to radial deformation. Higher cyclic loading values result in more significant radial strain of limestone specimen and larger accumulative AE counts of cyclic loading segment, which indicated Felicity effect. Regarding dissipated energy density, the damage of limestone specimen was defined without considering the influence of radial deformation, which made the damage value of cyclic loading segment higher at lower amplitude loads. The damage of cyclic loading segment was increased with the magnitude of load. When dissipated energy density was applied to define damage, the damage value at unloading segment was smaller than that of AE counts. Under higher cyclic loading values, rocks show obvious damage during both loading and unloading processes. Therefore, during deep rock excavation, the damages caused by the deformation recovery of unloading rocks could not be ignored when considering the damage caused by abutment pressure.

• The Journal of the Acoustical Society of Korea
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• 제19권1E호
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• pp.27-37
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• 2000

This paper presents a simplified analytical formulation for computing acoustic power radiation from a rectangular plate exposed to random forces such as turbulent boundary layer pressure fluctuations and arbitrary mechanical force in a subsonic flow field. The expression for the acoustic power is derived using modal expansion method and light fluid loading is assumed on the plate. In order to simplify the formulation for acoustic power due to combined excitations of mechanical forces and turbulent pressures, it is assumed that the structural damping of the plate is small and excitations are broadband random forces having frequency spectra above the convective coincidence. Under these assumptions, an approximate solution for the broadband acoustic power radiation from a plate excited by both turbulent pressures and arbitrary mechanical forces is obtained and evaluated considering the effect of modal coupling on the radiated acoustic power. An efficient method is also suggested to compute modal acoustic impedance in a moving fluid medium by using averaged Green function.

• 대한조선학회논문집
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• 제60권1호
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• pp.20-30
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• 2023

In this paper, the Energy Flow Finite Element Analysis (EFFEA) was performed to predict the acoustic and vibrational responses of complicated plate structures considering improved Fluid-Structure Interaction (FSI). For this, a new power transfer relationship was derived at the area junction where two different fluids are in contact on both sides of the plate. In order to increase the reliability of EFFEA of complicated plate structures immersed in a high-density fluid, the corrected flexural wavenumber and group velocity considering fluid-loading effect were derived. As the specific acoustic impedance of the fluid in contact with the plate increases, the flexural wavenumber of the plate increases. As a result, the flexural group velocity is reduced, and the spatial damping effect of the flexural energy density is increased. Additionally, for the EFFEA of arbitary-shaped built-up structures, the energy flow finite element formulation for the acoustic tetrahedral element was newly performed. Finally, for validation of the derived theory and developed software, numerical applications of complicated plate structures submerged in seawater or air were successfully performed.

• 한국소음진동공학회논문집
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• 제25권1호
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• pp.33-39
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• 2015

This paper investigates the underwater acoustic radiation from a periodically stiffened cylindrical shell excited resiliently mounted machinery. Underwater acoustic radiation is important to a submarine. Generally, submarine structure can be modeled as stiffened cylindrical shell immersed in water. Analytical model is derived for the far-field acoustic radiation from machinery installed inside cylindrical shell. The analytical model includes the effect of fluid loading and interactions between periodic ring stiffeners. Transmitted force from machine to a shell through isolator can be different by the impedance of shell. In this paper the effect of a shell impedance for acoustic radiation is investigated. Impedance of a shell should be considered if thickness of a shell is thin.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집A
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• pp.193-198
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• 2001

The feasibility of the acoustic emission technique in predicting the residual fatigue life of STS304 stainless steel is presented. Acoustic emission was continuously monitored during the fatigue tests. Considerable acoustic emission occurred during the first few cycles. Acoustic Emission increased rapidly at about 90% of the fatigue life, clear and ample warning of impending fatigue failure was observed. Fatigue damage accumulation was evaluated in terms of an AE cumulative counts. The AE cumulative counts may be taken as an indicator of fatigue cumulative damage. Fatigue damages corresponding to 20, 40, 60 and 80% of the total life were induced at a cyclic stress amplitude. The specimens with and without fatigue damage were subjected to tensile tests. In tensile tests, the total cumulative counts were reduced with increasing fatigue damage. It was observed that the residual tensile strength of material did not change significantly with prior cyclic loading damages.