Damage detection on composite materials with active thermography and digital image processing A.P. Chrysafi, N. Athanasopoulos, N.J. Siakavellas * Department of Mechanical Engineering and Aeronautics, University of Patras, 265 00 Patras, Greece article info Article history: Received 19 May 2016 Received in revised form 23 February 2017 Accepted 27 February 2017 Keywords: Non-destructive testing Thermal image processing Carbon fiber reinforced plastics Wavelets Fourier transform Damage abstract This research is focused on the use of active infrared thermography as a non-destructive testing tech- nique for damage detection in carbon fiber reinforced plastics (CFRPs). The aim of this study is to examine the efficiency of various mathematical methods in thermographic data processing, with respect to the thermal excitation method and the type of artificial defect in the CFRP specimens. We applied two techniques of active infrared thermography to CFRP samples with artificial cracks and internal de- laminations at known locations. An infrared camera recorded the temperature field and generated a sequence of thermal images. To reveal the defects of the CFRP laminate, the thermograms were pro- cessed (a) as 2D images, and (b) as if each pixel was a 1D signal over time. We present representative experimental results, which illustrate that the depiction of the norm of the 1st spatial derivative of temperature and the 2D wavelet transforms proved to be most efficient for crack detection, whereas the 1D Fourier and 1D wavelet transforms did not yield clear results. In contrast, delamination damages could be identified through 1D techniques because the 1D Fourier transform as well as the 1D wavelet transform were very accurate. © 2017 Elsevier Masson SAS. All rights reserved. 1. Introduction Non-destructive testing (NDT) is an excellent method for the evaluation of structural integrity of materials or components, without interfering with their serviceability. Various NDT tech- niques have been studied and adopted, including ultrasonic testing, X-ray testing, eddy current thermography, and infrared thermog- raphy [1e4]. Infrared (IR) thermography has become a widely used NDT technique [5] for the detection of hidden subsurface defects in a plethora of structural elements and mechanical systems [6,7], such as aircraft parts [8,9], buildings [10], cultural heritage objects [11], electronic components [12], and even the human body [13]. Unlike other non-destructive methods, thermography is a fast non- contact method, suitable for testing large areas of complex geom- etry, and is capable of identifying multiple damages [8,14]. It is also applicable to a wide range of materials [15,16], including glass and carbon fiber composites [17e19], natural fibers [20], ceramics [21], and metallic materials [22,23]. The wide use of composite materials raised the need for NDT in order to ensure their reliability before and during their life. Thermography is widely used for the testing of CFRPs and other composite materials for damages and other defects [8,16,18]. The method of heating and the mathematical processing methods play a major role in the efficiency of thermography. Particularly in multilayer composite materials, thermography en- ables the detection of different types of defects/damages (cracks, delaminations, fiber debonding, and mixed mode damages). In most applications, where objects typically have similar tem- perature levels, the emitted electromagnetic radiationdalso called thermal radiationdranges between 0.1 and 100 mm (visible, infrared, and part of the UV light wavelength spectrum) [24]. An infrared camera equipped with changeable optics can record the emitted radiation, convert it to voltage values, and then to tem- perature values (T), resulting in a 2D map of the surface tempera- ture field. IR thermography is categorized into passive and active; both rely on the fact that different materials, or defects in a mate- rial, produce variations in the thermal field because of their different thermal properties. Passive thermography does not employ external heating sources, and the specimen temperature should differ from the ambient temperature owing to its operation; for example, testing an aircraft via passive thermography imme- diately after its landing, or a rotor blade in motion [25]. Active thermography uses external heating sources (halogen lamps, lasers, * Corresponding author. E-mail address: siakavel@mech.upatras.gr (N.J. Siakavellas). Contents lists available at ScienceDirect International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts http://dx.doi.org/10.1016/j.ijthermalsci.2017.02.017 1290-0729/© 2017 Elsevier Masson SAS. All rights reserved. International Journal of Thermal Sciences 116 (2017) 242e253