PSI - Issue 33

R. Nobile et al. / Procedia Structural Integrity 33 (2021) 685–694 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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The scans are displayed on C-Scan maps, using a two-dimensional view of the displayed data corresponding to the planar surface on the component, monitoring the data in real time in the XY position. UT scans on the specimens were performed by contact technique using a water-based coupling gel. Before carrying out the C-scan scans it was necessary to perform a preliminary calibration of the instrument to establish the speed of the ultrasound in the material. The speed measured considering the surface echo and the background echo was 2700 m / s. Linear scans (0 °) were carried out maintaining a constant gain of 8 dB and using two wooden guides to move the probe and encoder system. The thermographic set-up was arranged according to the configuration shown in Figure 3b. The inspection was performed with set-up in reflection, placing two halogen lamps of 1000W each and the thermal imager on the same side with respect to the panel, using an infrared camera to acquire the thermographic signal model FLIR A655sc, four halogen lamps of 1000 W fixed on a rectangular aluminum frame and a PC connected to the thermal imager. The thermal imaging camera used in the tests is equipped with an uncooled, highly reliable micro-bolometric sensor, which produces thermal images at 640 × 480 pixels of high quality and measurement accuracy and stability. FLIR A655sc makes temperature differences of only 30mK visible. The FLIR A655sc produces 14-bit data at up to 50 frames per second with a resolution of 640 × 480 at full frame. Its high-speed windowing feature boosts the frame rate up to 200Hz, with 640 × 120 pixels per window. 3. Results and discussion 3.1. Test results of simulated impact damage For the impact damage simulation test, twelve AFS sandwich specimens were tested in displacement control with a constant speed of 110 mm / s, setting a depth of 4 mm as the impactor penetration into the specimen. The test data was recorded with a frequency of 100Hz. The comparison graphs of Figures 4a and 4b show the Load / displacement diagrams of the experimental impact tests relating to the different batches. A central impact was performed on each specimen both to avoid detachment of the skins near the edges and for better inspection convenience, as it was planned to correlate the extent of the damage impression by applying ND controls. ultrasound and thermographic. The visual analysis of the impact damage shows a clear area of deformation around the footprint of the impactor for AFS-20 specimens compared to AFS-30, as can be seen in Figures 5a and 5b. a b

Fig. 4. Load-displacement curves of AFS-20 (a) and AFS-30 (b) specimens.

Each AFS series shows an evident variability of the curves justifiable by the different density of the foams constituting the specimens grouped in the blue boxes, obtained from the edges of the laminated sandwich panels, and of the foams of the lots of the red boxes, obtained from the central part of the panels. In fact, the latter have a lower and more homogeneous density than the edge areas, motivating the choice to obtain the final specimens in this