2 Fatigue Analysis of 7075 Aluminum Alloy by Optoacoustic Method 9 Prefatigue load Fig. 2.2 Maximum load vs. number of cycle plot in the fatigue test Fig. 2.3 Acoustic measurement Vzz Vzy Vzx x y z Mirror Beam splitter Laser (Interferometer x) Laser (Interferometer y) Specimen Beam expander Illuminated area Tensile load Stationary end CCD camera Stepping motor Glass window Fig. 2.4 Experimental arrangement of 2D-ESPI [6] direction (x) and vertical direction (y) to the tensile machine (interferometer x, and interferometer y). Each interferometer was sensitive to in-plane displacement along its horizontal direction. Semiconductor laser with the wave length of 660 nm and the power of 50 mW was used for light source. The individual displacement fields in each sensitive direction were obtained by switching light sources of the two interferometers. The laser beam was expanded by a beam expander and split into two paths by a beam splitter and converged to the surface of specimen via two mirrors. The incident angels to the surface were 36.3ı for the interferometer x, and 48.0ı for the interferometer y. The speckle pattern was captured with a CCD camera with a frame rate of 15 frames per a second. The speckle intensity received by each pixel changes depending on the displacement in the sensitive direction due to optical path difference between the two interferometric arms. Thus, the displacement field on the measured surface can be obtained by computing the intensity difference as fringe contours.
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