Post-impact damage characterization of pultruded jute/glass hybrid composites using infrared vision and optical techniques

The use of an intelligent and integrated NDT approach applied to a hybrid jute composite (JC) made with natural fibers has been explored in this work in order to reveal emerging defects after an indentation test. In particular, the fiber content (wt.%) of the sample investigated was 40 jute + 25 glass with a stacking sequence of Surface veil/Chopped strand/Jute/J/C/S. The sample was analyzed by: a) near-infrared reflectography (NIRR) and transmittography (NIRT), b) short-wave infrared reflectography (SWIR), c) infrared thermography (IT), d) holographic interferometry (HI) and e) digital image correlation (DIC) techniques. The first two inspections, combined with the Canny edge detector and the distance transform (DT), were useful to highlight the fibers distribution and the areas with a low concentration of fibers, respectively. In the third inspection, two different methods of heating were applied: pulsed thermography (PT) and square pulse thermography (SPT). The first one linked to a mid-wave IR camera, while the second one combined to a long-wave IR camera. IRT data were processed using different algorithms, such as: principal component thermography (PCT) and thermographic signal reconstruction (TSR). Comparing the TSR and the optical results, two zones invisible to the naked eye and surrounding the indented area can be identified (Figure 2). These zones were quantitatively confirmed using the MOIRE Software package (DIC script) described in the final paper, illuminating the sample by means of a speckle pattern induced by a laser Nd : Yag 250 mW.