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장파장 산란 근사를 이용한 구형 개재물 문제의 유효 탄성적 성질
정현조,김진호,Jeong, Hyun-Jo,Kim, Jin-Ho 대한기계학회 1999 大韓機械學會論文集A Vol.23 No.6
The effective elastic properties of materials containing spherical inclusions were calculated by the elastic wave scattering theory. In the formulation additional scattering fields by the presence of random multiple scatterers that affects the effective properties were found by the single scattering approximation. In calculating the scattering fields the ensemble average on the displacements and strains inside the scatterer was found from the static approximation at long wavelength limit. The displacements were assumed to be equal to the incident field, while the strains were calculated by Eshelby's equivalent inclusion principle on the single inclusion problem. Four different models were considered and they reflected different degrees of multiple scattering effects based on the approximation introduced in the process of embedding the inclusion in the matrix. The expressions for the effective elastic constants were given in each model, and their relations to the results obtained from other scattering theory and elasticity theory were discussed. The theoretical predictions were compared with experimental results on the epoxy matrix composites containing tungsten particles of different sizes and volume fractions
유한요소법을 이용한 이방성 재료에서의 초음파 전파 거동 해석
정현조,박문철,Jeong, Hyun-Jo,Park, Moon-Chul 대한기계학회 2002 大韓機械學會論文集A Vol.26 No.10
The accurate analysis of ultrasonic wave propagation and scattering plays an important role in many aspects of nondestructive evaluation. A numerical analysis makes it possible to perform parametric studies, and in this way the probability of detection and reliability of test results can be improved. In this paper, a finite element method was employed for the analysis of ultrasonic wave propagation in anisotropic materials, and the accuracy of results was checked by comparing with analytical predictions. The element size and the integral time step, which are the critical components for the convergence of finite element solutions, were determined using a commercial finite element code. Some differences for wave propagation in anisotropic media were illustrated when plane waves are propagating in a unidirectionally reinforced composite materials. When plane waves are propagating in nonsymmetric directions in a symmetric plane, deviation angles between the wave vector and the energy vector were found from finite element analyses and the results agreed well with analytical calculations.
정현조,김진호,박문철,Jeong, Hyun-Jo,Kim, Jin-Ho,Park, Moon-Cheol 대한기계학회 2002 大韓機械學會論文集A Vol.26 No.11
Numerical modeling of a nondestructive testing system plays an important role in many aspects of quantitative nondestructive evaluation (QNDE). The ultimate goal of a model is to predict test results for a specific flaw in a material. Thus, in ultrasonic testing, a system model should include the transducer, its radiation pattern, the beam reflection and propagation, and scattering from defects. In this paper attention is focused on the scattering model and the scattered fields by defects are observed by an elastodynamic boundary element method. Flaw types addressed are void-like and crack-like flaws. When transverse ultrasonic waves are obliquely incident on the flaw, the angular distribution of far-field scattered displacements are calculated and presented in the form of A-scan mode. The component signals obtained from each scattering problem are identified and their differences are addressed. The numerical results are also compared with those obtained by high frequency approximate solutions.
압전필름센서에 의한 충격하중을 받는 보의 응력파의 측정
권일범 ( Kwon Ii-bun ),최만용 ( Choi Man-yong ),김진기 ( Kin Jin-ki ),정현조 ( Jeong Hyun-jo ) 한국구조물진단유지관리공학회 2000 한국구조물진단유지관리공학회 학술발표대회 논문집 Vol.4 No.1
충격하중을 받는 구조물의 응력파를 측정함에 의하여 구조물의 손상을 감시할 수 있다. 이러한 충격하중에 따른 응력파를 측정하기 위한 압전 센서의 적용 가능성을 점검하기 위하여 강재 보에서 충격하중으로 인하여 전파되는 응력파를 PVDF 필름 센서와 재래식 초음파 및 음향방출 (AE) 센서를 사용하여 수신하였다. 실험에서 수신한 시간 영역의 신호 해석을 위하여 분산성 파의 시간-주파수 분석이 가능한 wavelet 변환(WT)을 적용하였다. 보에서 충격하중을 모사하기 위하여 연필심을 파단시켰으며, PVDF 필름, 초음파 및 AE 센서의 출력 전압은 변형률 게이지로 측정한 변형률에 비례하였다. 보에서 굽힘파의 속도는 전단 변형과 회전관성을 고려한 Timoshenko 보 이론과 잘 일치하였다.