Mapping of Surface Residual Stress Field by Laser Interferometry Using Stress Relaxation Method
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Mapping of Surface Residual Stress Field by Laser Interferometry Using Stress Relaxation Method Dong-Won Kim, Nak-Kyu Lee1, Kyung-Hoan Na1 and Dongil Kwon National Research Lab. for NanoAssessment & MicroReliability, School of Materials Science and Engineering, Seoul National University, San 56-1, Shilim-dong, Kwanak-Gu, 151-742, Seoul, Korea. 1 Micro Machining Technology Team, Industrial Technology Center, Korea Institute of Industrial Technology, 539-1, Gajwa3dong, Seogu, 404-253, Incheon City, Korea.
ABSTRACT Based on the identification of the residual stress-free state using electronic speckle pattern interferometry (ESPI), we modeled the relaxed stress in annealing, the thermal strain/stress and the residual stress field in case of both single and film/substrate systems by using the thermoelastic theory and the relationship between relaxed stresses and displacements. Thus we mapped Au film by ESPI. In the surface residual stress fields on the indented bulk Cu and the 0.5 indented Cu, the normal and shear residual stress are distributed over -800 MPa to 700 MPa and –600 MPa to 600 MPa respectively around the indented point and in deposited Au film on Si wafer, the tensile residual stress is uniformly distributed on the Au film from 500 MPa to 800 MPa. Also we measured the residual stress by the x-ray diffractometer (XRD) for the verification of above residual stress results by ESPI.
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INTRODUCTION The residual stress has been evaluated mainly by destructive tests such as hole drilling and etching methods, and more recently by NDT techniques such as X-ray diffraction, Raman spectrophotometer and neutron diffraction methods or magnetic Barkhausen noise (MBN) testing methods [1-3]. However, so far the geometrical measured range by the detecting sensors for residual stress evaluation has been limited to the predicted interested points and discrete point profiles of system, and the residual stress fields in a certain areas have been mapped only via numerical simulation techniques such as finite element analysis (FEA) [4] i.e. Here we evaluated the stress-free states and mapped the surface residual stress fields of indented Cu and thin Au film on Si wafer by ESPI, which utilizes the interference theory and the phase shift technique of laser speckles with highly resolution of displacements, and the thermoelastic theory for analyzing the relaxed strains/stresses and residual stresses.
MODELING FOR EVALUATION OF SURFACE RESIDUAL STRESS FIELD BY ESPI Confirmation of free residual stress state by ESPI
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Assuming that a system is composed of a number of discrete elements, the system undergoing elastic and plastic deformation loses the compatibility among its elements because of the discontinuous transformation in each element. An imaginary stress from outside was applied to the system not to breakdown the system i.e. to maintain compatibility. Consequently, inside the system the characteristics of stress are altered to compensate for the imaginary stress: this is the origin of residual stress [5]. With a
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