Search

Recently, the combination of Fringe Projection (FP) and 2D Digital Image Correlation (2D-DIC) has become a low-cost alternative for measuring deformations even in dynamic events such as vibration testing. FP and DIC are displacement measurement techniques, so high frequency vibration tests associated with low levels of displacement suppose a challenge. By means of Phase-Based Motion Magnification algorithm (PBMM), the periodic displacement observed in an image sequence can be magnified. This makes it possible to measure clear displacement maps by FP + 2D-DIC even when subtle displacement occurs. This methodology allows a better interpretation of the vibration behavior of mechanical components. In this work, the behavior of a beam excited at its natural frequencies has been studied, showing the potential of PBMM and FP + 2D-DIC

Phase-based motion magnification has been proposed as an effective methodology to visualise Operational Deflection Shapes (ODSs) using a single camera. In recent work, magnification allowed the observation of these ODSs by naked eye or more advanced measurements on magnified images by using 2D Digital Image Correlation (DIC). The combination of a full-field optical technique and motion magnification shows a high potential improving DIC measurement and providing qualitative and quantitative information for the interpretation of ODSs. However, both motion magnification and the combination with 2D-DIC is limited to a few cases where plane elements experiences in-plane motion, mainly beam-like structures. In order to provide a more general application, in this study measurements on couples of magnified images using stereoscopic 3D-DIC were performed. A cantilever beam was employed for validation purposes, comparing the results with numerical models. Afterwards, a measurement application on a large curved panel was carried out to show full capabilities of the proposed methodology for 3D characterisation. Visualisation in slow-motion video format enables an intuitive understanding of the deformation. The adopted methodology demonstrates to be highly useful in low amplitude tests, involving the use of little exciters or high frequency applications. [ABSTRACT FROM AUTHOR]

High speed video cameras provide valuable information in dynamic events. Mechanical characterisation has been improved by the interpretation of the behaviour in slow-motion visualisations. In modal analysis, videos contribute to the evaluation of mode shapes but, generally, the motion is too subtle to be interpreted. In latest years, image treatment algorithms have been developed to generate a magnified version of the motion that could be interpreted by naked eye. Nevertheless, optical techniques such as Digital Image Correlation (DIC) are able to provide quantitative information of the motion with higher sensitivity than naked eye. For vibration analysis, mode shapes characterisation is one of the most interesting DIC performances. Full-field measurements provide higher spatial density than classical instrumentations or Scanning Laser Doppler Vibrometry. However, the accurateness of DIC is reduced at high frequencies as a consequence of the low displacements and hence it is habitually employed in low frequency spectra. In the current work, the combination of DIC and motion magnification is explored in order to provide numerical information in magnified videos and perform DIC mode shapes characterisation at unprecedented high frequencies through increasing the amplitude of displacements. [ABSTRACT FROM AUTHOR]

Abstract: Recently, the development of flapping-wing micro air vehicles (FW-MAV) for operation in extreme environmental conditions has demanded properly designed, biologically inspired wings that can produce enough lift force to keep the vehicles aloft. The structural analysis of an artificial wing is carried out in the design of an FW-MAV. In this study, the dynamic characteristics of an artificial wing mimicking an Allomyrina dichotoma beetle''s hind wing were investigated by a non-contact measurement method. The natural frequencies, mode shapes, and damping ratios of the first three basic vibration modes in the operating frequency range were determined using a Bruel & Kjaer (B&K) fast Fourier transform analyzer, along with a laser sensor. The laser sensor was used to obtain the displacement history of the marked points on the wing to calculate the frequency response function. To confirm the results, a three-dimensional digital image correlation (3D-DIC) method, as well as high speed digital cameras, were employed to construct the mode shapes of the wing when it was vibrated at a pre-determined natural frequency. The mode shapes by the DIC method showed good agreement with those by the laser displacement sensor. These results provide a method for the modal analysis of a light weight structure like an insect wing as well as for the construction of the mode shapes using DIC. The high speed 3D-DIC method, used successfully in mode shape measurements, can also be used to study the wing deformation of an insect during flight, which is challenging in an insect study. [Copyright &y& Elsevier]

• High speed Digital Image Correlation are applied to study transversely isotropic rocks with five bedding angles under different high-speed impact load. • Three trends of indirect tensile strength with bedding angle are proposed by summarizing the existing quasi-static test results. • The failure criterion is established to calculate the dynamic Brazilian disc strength of transversely isotropic rocks by introducing the patchy weakness model. • It demonstrate experimentally and theoretically that the bedding plane angle of transversely isotropic rock is harder to be the priority direction as strain rate increasing. To explore the dynamic fracturing behaviours and failure mechanisms, transversely isotropic phyllite rocks with five bedding plane angles were investigated for full-field strain Brazilian disc tests under different strain rates using a split Hopkinson pressure bar and high-speed digital image correlation (DIC) technology. Two kinds of crack evolution processes: the cracks in specimens with low bedding angles activated near the non-loading point, and other cracks initiated at the centre of the disc prior to exceeding the peak stress were observed. Considering the four types of failure modes characterized by different mechanisms, the dynamic failure mechanism of transversely isotropic rock was discussed. The Brazilian disc strength of transversely isotropic rock subjected to impact loads was computed by introducing the patchy weakness model to reflect the dominant role of the rock matrix in the dynamic test. The DIC results demonstrated that additional random orientation defects are activated and the competition for evolving into cracks becomes intense, indicating that cracks develop less along bedding planes as the strain rate increases. Moreover, the fracture patterns of the transversely isotropic phyllite specimens changed under static and dynamic conditions. Therefore, it is necessary to study the properties of transversely isotropic rocks under dynamic impacts rather than simply using inferences from static results. [ABSTRACT FROM AUTHOR]