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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pointed out that this surface is up to 2000 times brighter than a perfect diffusing white surface. However, there are two significant disadvantages: the maximal operating temperature is ≈ 80 ° C, and even more significantly, the tape must be glued to the surface. This last element is problematic in the case of many surfaces and objects. Especially small in surface, fragile and lightweight (additional problem of adding a mass) and surfaces of expensive objects like art, historical or heritage objects. To avoid this problem, the speckle pattern technique can be applied to particular measurements. Speckle patterns are produced when a coherent light is focused on a rough surface. If possible, the surface can be covered with black and-white paint dots (random pattern), similar to the surface preparation for another optical measurement technique called Digital Image Correlation (DIC), where this step is a required element of the measurement procedure. However, in the case of the LDV measurement system, this step is often not possible as the object may be used for a specific task and cannot be covered in paint or, for example, historical monuments, where this is not allowed. Thus, additional solutions must be evaluated. In the example underneath (Fig.4a), the problem of enhancing the single by covering the surface with contrasting colours was resolved by covering the surface with the removable powder (FLUXO R175 Developer). This operation has not changed the dynamics of the structure (no noticeable mass was added). The developer powder is usually used as the non-invasive method that enables visual detection of sheet metal surface defects in the form of cracks (30 to 50µm). It has a few significant advantages. It covers the surface very well, creating a thin, fine-grained layer. It also allows for the quick drying of the solvent contained in the developer, and it is effortless to apply to the object's surface. It can be used in various tested materials: steel, ceramics, plastics, and glass. In the case of transparent materials, it allows the laser to measure the object's response and not allow it to go through.

3 rd bending mode 220.6 Hz

Speckle pattern with developer powder

a)

b)

Fig. 4. Carbon fibre composite sandwich (CERN- Mechanical Measurement Lab): a) Surface enhancement using FLUXO R175 Developer; b) Visualisation of results after the surface enhancements- 3 rd bending mode at 220.6 Hz.

The additional advantage of using the mentioned developer powder or similar products is that they can be easily removed, e.g., using air pressure. Thus, the object's surface is intact. In the case of the disadvantages of such a technique, it must be pointed out that most targets are not perfectly stationary. The detector will see different dark and bright speckles, which can result in noise in the vibrometer output. If, additionally, the surface is rough, a speckle pattern can lead to dropouts. However, in the case of 3D LDV measurements, when the surface is covered with numerous measurement points, these eventual problems are limited by software algorithms, which disregard some local surface effects that are reflected in incorrect measurements. Fig.4b shows the results of the measurements (3rd bending mode) after the application of surface enhancement. 2.3. Enhancements in the surface visibility access with mirrors The problem of numerous measurement tasks is the lack of visibility of the surface. Thus, the object must often be removed from its original placement to perform measurements. This is unnecessary in the case of the LDV, as using specific signals, a mirror, or multiple mirror instances can be used to direct the laser beam to the particular