PSI - Issue 64

Lukasz Scislo et al. / Procedia Structural Integrity 64 (2024) 2246–2253 Lukasz Scislo et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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measurement area. Using the light as the signal carrier is advantageous as it behaves in a very predictable manner. If an instance of light (a ray) is observed reflecting off of a flat mirror, then the light's behaviour would follow a well known law of reflection. The surface (mirror, calm body of water) is smooth compared to the wavelength. It allows for precise alignment of the vibrometer or/and the object, thus obtaining maximal optical signal strength. On opposite sites for surfaces that are rough in comparison with the wavelength, the incident light is scattered over a larger angular area. Fig. 5 presents the use of the mirror to measure the top surface of the carbon fibre sample. This surface is normally not fully visible by all heads of the 3D LDV, and only the front surface of the sample can be measured (Fig.5a). Due to the low mass of the object, its fragility, as well as its black surface, two previously mentioned techniques have been used together with the mirror (Fig.5b) to obtain the Frequency Response Functions (FRFs) results of the top surface (Fig.5c). Using the FRF graph, it is possible to visualise each of the natural frequencies, e.g. 2 nd bending mode (Fig.5d).

Fig. 5. Carbon fibre sample (CERN- Mechanical Measurement Lab): a) Measurement of the front surface; b) Measurement of the top surface with the use of the mirror; c) Frequency response function graph; d) Visualisation of results after the surface enhancements and the use of the mirror- 2 nd bending mode at 1746 Hz. Due to the straight visibility of the front surface by the vibrometer measuring heads, good visibility, and the signal strength of the top surface using a mirror, an additional technique called "stitching" can be utilised to combine such surfaces. In principle, the stitching method can be used to connect the spatial shapes of two or more surfaces (Chen et al., 2023). However, this technique is not presented in this paper as it is mostly used to improve and present advanced visualisation of the mode shapes and not enhance the measurement side of the process itself. 3. Conclusions The article presented the optical approach to vibration measurement with the use of a 3D Laser Doppler Vibrometer and a few possibilities for overcoming typical problems in measuring fragile objects, such as ultralightweight structures and historical or cultural objects that have substantial restrictions on mounted sensing