A nondestructive investigation of inserts in lightweight composite panels by ultrasonic C-scan and infrared thermography

In the aeronautical industry, there is an increasing need for lightweight composites above all for the internal design. In this regard, the final products should not contain any inclusion of foreign material in order to not compromise both the aesthetical beauty and the mechanical properties subjected to vibrational loads. In this work, the infrared thermography (IRT) and the ultrasonic C-scan (UT) methods were used to retrieve the unknown positions of sub-superficial inserts in five lightweight composite panels. They were randomly inserted during the manufacturing stage. In particular, the samples investigated herein are three glass fiber reinforced polymers (GFRP) and two carbon fiber reinforced polymers (CFRP) used as laminates or as skins for honeycombs. They were provided by the Società Elicotteristica Italiana (S.E.I.), Monteprandone (Italy). An example is reported in Fig. 1. It shows both the schematization of the cross-section and the front view of a honeycomb core (HC) covered with two skins of GFRP. A comparison between two different approaches centered on square pulse and flash thermographic scenarios is reported. The raw thermograms were processed by using advanced techniques such as the principal component thermography (PCT) and the partial least square thermography (PLST) which enhanced the visibility of the defects. In addition, the UT method applied both in reflection and in transmission modes were showed interesting advantages from a diagnostic point-of-view above all when its results are compared with the thermographic results. Fig. 2 shows the clear detection (see the blue area) of a defect realized in plastic material, having an unknown size, and positioned inside the HC. Pros and cons of both methods when applied on lightweight composite panels will be in-depth explained into the final manuscript.