ESPI에 의한 레이저용접 조건에 따른 공진주파수특성 분석

Abstract

Speckle pattern interferometry (SPI) is essential method for researchers who want to measure vibration behavior of structures with optical methods. With this method, non-contact, non-destructive, and whole-field analysis of vibrating structures can be done in quasi real-time. Especially, electronic speckle pattern interferometry (ESPI) that the electronic processes were added to SPI is one of the very powerful method in the experimental vibration analysis field. And, additional advantages of ESPI are that both of the mode shapes and the natural frequencies of structures in oscillating can be measured simultaneously and cut down the cost. Several researchers introduced a number of optical methods in the analysis of the vibration mode shapes and the natural frequencies. One of the optical methods is the electronic speckle shearing interferometry (ESSI), known as the shearography. ESSI is an appropriate method to measure the slope contours of deformations of the structures for vibration mode shape analysis. Also, this method is able to obtain the vibration amplitude derivative directly and very robust method that can used in the practical work environment. But the ESSI is an optimized method for defect inspection in the objects and the shearing amount directly affects the results of the test. The other method is the phase shifting ESPI (stroboscopic-ESPI) that was introduced PZT for phase shifting of the reference beam in the interferometer. The ESPI is used to analyze out-of-plane displacements such as vibration and bending as well as in-plane displacements such as stress and strain. Researchers however can analyzed only the qualitative vibration mode shapes with the general ESPI methods. In general methods, the time-average ESPI is applied to measure the vibration mode shapes in real time and is a method that there is no effect by the circumstances around. But this method has some limitations that it is difficult to quantitatively analyze the interference fringes and fringe contrast and quality are somewhat low. Recently, quantitative modal analysis using the ESPI that a modal assurance criterion (MAC) calculation is applied in was reported. stroboscopic-ESPI is a quantitative analysis method while the general stroboscopic-ESPI methods are qualitative method. This paper describes the variations of resonance frequencies and vibration mode shapes of laser welded cold rolled carbon steel plate(SCP1) induced by thermal loading during laser welding processing are analyzed with stroboscopic ESPI. Stroboscopic ESPI to make it reliable are compared with theory and experimental method: Finite Element Method(FEM) and Scanning Laser Doppler Vibrometer(SLDV). The results of stroboscopic ESPI are well agreed with SLDV and also the comparison with theory give good agrement within 5%. It was confirmed that the Laser welded plate compared with standard specimen shows unexpected vibration mode shape in the 5th and the 6th, due to large localized thermal stress nearby welding zone within the narrow heat-affected zone.