Contents lists available at ScienceDirect NDT and E International journal homepage: www.elsevier.com/locate/ndteint An investigation of the stepped thermography technique for defects evaluation in GFRP materials Davide Palumbo ∗ , Pasquale Cavallo, Umberto Galietti Department of Mechanics, Mathematics and Management (DMMM), Politecnico di Bari, Viale Japigia 182, 70126, Bari, Italy ARTICLE INFO Keywords: GFRP Stepped thermography Lock-in thermography Non-destructive testing ABSTRACT The ability of thermography to detect defects in composite materials has been demonstrated and showed in various works and in many applications. In this regard, various NDT techniques are currently used for defect detection in composites such as Lock-in Thermography (LT), Pulsed (PT) Stepped Thermography (ST/SH), all of which have their own peculiarities and capabilities. A critical aspect concerns the overall lengthy time required for testing and analysis of thermographic data above all, for large structure where it is necessary a scanning approach. In this work, two algorithms based on the stepped thermography approach were investigated in quantitative way with the aim to optimizing the testing parameters and data analysis in terms of testing time and signal to noise ratio. In particular, several tests were carried out on a sample specimen with simulated defects and the well-established lock-in thermography technique has been used as comparison. 1. Introduction Composite materials, thanks to their high strength to weight ratio, are widely used for manufacturing large structures and components and in this regard, many examples can be found in different engineering fields from boating-yachting to aeronautical or aerospace. As clearly demonstrated in the literature [1], the presence of defects can drama- tically change the strength of the structure and so, non-destructive techniques play a key role for estimating the residual strength of structures or components. Many NDT techniques are currently used for defect detection in composites such as X-ray [2], ultrasound [3–6], shearography [7], vi- bration testing [8] and electrical potential technique [9] and in many applications a combination of them is necessary to identify different kinds of defects to quantify. In the case of large structures, a rapid and easy inspection of com- ponents is required in order to reduce the time of the ordinary main- tenance and thus to limit the costs. In this regard, it is very important to develop automatic procedures and algorithms for data analysis in order to obtain more quickly the quantitative characterization of defects. Stimulated thermography presents peculiarities suitable for in- vestigation of large areas since it does not require the coupling with the component, is easily automatable and the testing time is relatively shorter with respect to other traditional well-established NDT techni- ques. With regards of testing setup and data processing, the most diffused thermographic techniques for the non-destructive evaluation of com- posite materials are: Lock-in Thermography (LT), Pulsed (PT) and Stepped Thermography (ST/SH) [10–15]. All of them uses a surface or volumetric [16] heat source to stimulate with thermal waves the ma- terial in order to induce a heat flux in the material and analyse the temperature behaviour on the surface of the component. The main idea, common with all thermographic techniques is that a defect inducing different thermophysical local properties in the material, will induce also an anomaly in the thermal diffusion and then a different surface temperature of the component. In fact, these techniques look for areas of different thermal behaviour that can be correlated to a defect and that are typically dependent by different thermal-physical properties involved in the heat transmission phenomena such as the thermal conductivity, the specific heat at constant pressure and the density of material [10]. All the cited IR techniques have been used in various works and in many applications, demonstrating the ability of thermography to detect defects in CFRP and GFRP composite materials. In particular, these techniques are effective in the evaluation of interlaminar delaminations or debonds, and any kind of flaws parallel to the external surface. Thermographic techniques have been used in combination with other NDT techniques to investigate defects in adhesively bonded components [17–21] while, many applications concern the investiga- tion of composite material subjected to impact damage [12,22] or https://doi.org/10.1016/j.ndteint.2018.12.011 Received 20 June 2018; Received in revised form 27 December 2018; Accepted 27 December 2018 ∗ Corresponding author. E-mail addresses: davide.palumbo@poliba.it, dpalumbo@email.it (D. Palumbo). NDT and E International 102 (2019) 254–263 0963-8695/ © 2018 Elsevier Ltd. All rights reserved. T