University of Naples Federico II Infrared thermography to evaluate carbon/epoxy composites Carosena Meola, Simone Boccardi, Giovanni Maria Carlomagno, Natalino Daniele Boffa, Fabrizio Ricci Department of Industrial Engineering/Aerospace Division – University of Naples Federico II Introduction Carbon fiber reinforced polymers (CFRPs) are increasingly used in aircraft primary structural components; however, they exhibit different problems when compared to metallic materials. A main weakness is their vulnerability to low velocity/energy impact. In particular, important damage may arise inside the material thickness without any visible perception on the impacted side. In addition during fabrication, they are susceptible to formation of defects, like voids, fibers misalignments and non uniform resin distribution, which may affect their performance in service. Therefore, the availability of any effective non-destructive evaluation technique, to get information on the presence of buried defects, and of any probable failure starting point, is of great importance. In the present work we use infrared thermography which allows in a remote, non-invasive and fast way to assess material conditions. In particular, the same infrared camera is used for both non-destructive evaluation and for monitoring of impact events. with a twofold function:  non-destructive evaluation (Lockin thermography) before and after impact;  monitoring online thermal effects (thermal signatures), which develop over the surface of the material under impact. Nondestructive evaluation with lock-in thermographyDescription of specimens: Some phase images By varying the heating frequency f is possible to assess the material conditions at different layers through the specimen thickness. The used infrared camera SC6000 (Flir Systems) Detector QWIP LW 8-9 μm 640 x 512 pixels full frame Impact tests with a modified Charpy pendulum Specimen lodge with a window 15 cm x 7.5 cm to allow for the contact with the hammer from one side and optical view (by the infrared camera) from the other side. The hammer has hemispherical nose 12.7 mm in diameter Infrared camera Charpy pendulum The infrared camera views the surface opposite to impact and gets thermal images in time sequence at frame rate f r = 96 Hz. The first image (t = 0) of the sequence, i.e. the specimen surface (ambient) temperature before impact, is subtracted to each subsequent image so as to generate a map of temperature difference ∆T: Some ∆T images Conclusions CFRP-1 16 plies 1D parallel fibers s  2.60 mm Sound f = 0.70 Hz f = 0.26 Hz Specimen CFRP - 2 f = 0.88 Hz f = 0.15 Hz f = 0.05 Hz Phase images taken from the impacted surface Phase images taken from the rear surface CFRP-2 NCF- 5 HSW s  7.8 mm Impacted in four points E = 50, 60, 65, 70 J CFRP-3 [+45°/-45°/ 0°/ 90°] s s  1.60 mm Teflon disc; D = 10mm CFRP-4 [0°/ +45°/90°/-45°] s s  2.4 mm one impact E = 9.8 J  T = T(i,j,t) – T(i,j,0) i and j representing lines and columns of the surface temperature map. Specimen CFRP - 4 after impact at E = 9.8 J Specimen CFRP-1 f = 0.22 Hz f = 0.70 Hz f = 0.88 Hz Specimen CFRP-3 Specimen CFRP-2 with impact at E = 70 J From one side the Teflon disc appears of better contrast, at the same f value, because is shallower (0.4 mm deep) than from the other side (1.2 mm deep); decreasing f the contrast improves. From phase images is possible to evaluate the extension of the damaged area A D and its depth p f = 0.88 Hz f = 0.36 Hz f = 0.15 Hz misalignment of fibers and non-uniform distribution of epoxy resin The largest damage extension is appraised and is found to have lower dimensions D H and D V than the warmer signature appearing during impact monitoring The usefulness of lockin thermography to visualize defects produced during manufacturing processes, like fibers misalignments, non-uniform distribution of either fibers or matrix as well as slag inclusions, has been shown. In addition, lockin thermography exhibits to be able to visualize also impact damage. In particular, post processing of phase images supplied information on the damage extension at different layers through the sample thickness. However with this approach, the overall delamination may be underestimated because it propagates between fibers and matrix in a rather tortuous way and in a very thin delaminated zone, so that the variation of the phase angle gets confused with the background. Thus, if the investigation regards the material performance for design purposes, it is better to evaluate the overall delamination by referring to the thermal signature visualized by the infrared imaging device while monitoring of the impact event. ∆T distribution with time