PSI - Issue 17

H. Lopes et al. / Procedia Structural Integrity 17 (2019) 971–978 Lopes et al./ Structural Integrity Procedia 00 (2019) 00 – 000

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(c) Fig. 3. Numerical modes of first, ninth, and fourteenth natural frequencies: (a) displacements in the direction, (b) rotations in the direction, and (c) rotations in direction. 3. Experimental modal rotations and validation of method The modal rotation of the laminated composite plate in the š and › directions were measured using shearography, being the shearing amount of 10 mm for both directions. The plate was suspended by two very flexible wires to create almost free-free condition. A continuous wave laser from Coherent model Verdi with power up to ʹ W at ͷ͵ʹ nm was used to illuminate the surface of the plate. To guarantee a uniform reflection of the laser light, the surface was covered by a very thin layer of white powder. An in-house built shearography system to measure the modal rotations, consisting on an optical Michelson interferometer and CCD camera with resolution of ʹ͵ͷʹൈͳ͹ʹͺ to capture the intensity pattern of the reflected laser light was used. The shearing amount of the interferometer is created by introducing a small tilt in one of the mirrors of the optical Michelson interferometer. A loudspeaker mounted behind the plate is used to excite it to its natural frequencies, which where were previously identified. In order to only capture the modal rotations at its maximum amplitude of vibration and to be able to apply the temporal phase modulation technique, stroboscopic laser illumination was created by passing the continuous wave laser through an acousto-optic modulator, being the loudspeaker harmonic excitation and the laser amplitude modulation pulse signals synchronized and controlled by using the Tektronix generator model AFG320. The tuning of the stroboscopic illumination is made by adjusting the phase difference between the harmonic and pulse signals and controlled by observing the number intensity fringes of interference due to the modal rotation. The focus of optical system is made by observing a test target mounted near the surface of the plate. The numerical aperture is adjusted so the intensity fringes of interference present the maximum contrast, meaning that the speckle is fully resolved. The output power of the laser was adjusted