Contents lists available at ScienceDirect Composite Structures journal homepage: www.elsevier.com/locate/compstruct An improved method of eddy current pulsed thermography to detect subsurface defects in glass fiber reinforced polymer composites Changhang Xu ⁎ , Wuyang Zhang, Changwei Wu, Jing Xie, Xiaokang Yin, Guoming Chen College of Mechanical and Electronic Engineering, China University of Petroleum, Qingdao 266580, China ARTICLE INFO Keywords: Glass fiber reinforced polymer (GFRP) Eddy current pulsed thermography (ECPT) Subsurface defects detection Non-destructive testing (NDT) ABSTRACT Due to its induction heating mechanisms, eddy current pulsed thermography (ECPT) is always used as Non- destructive testing (NDT) technique only for conductive materials. This work explores the feasibility of ECPT to detect subsurface defects in glass fiber-reinforced polymer (GFRP) composites, a typical non-conductive mate- rial. The proposed method introduces a metal part as the heat-generating component to generate the induction heat. Then the induction heat in the metal part can be transferred to GFRP, which makes it possible to detect subsurface defects in GFRP. The detection mechanism of the proposed method is firstly investigated, and the influence factor analysis is then performed to obtain the optimal test condition. Experiments were finally per- formed on GFRP specimens with various subsurface defects to verify the effectiveness of the method. The results demonstrate that the proposed method is effective in subsurface defects detection of GFRP, which provides a promising way to extend the application of ECPT to non-conductive materials. 1. Introduction As one kind of composite materials, glass fiber-reinforced polymer (GFRP) has attracted increasing attention in recent years due to its super properties including high strength-to-weight ratio, excellent corrosion resistance, electrical insulation [1–4]. GFRP has been widely used in a variety of applications, such as civil engineering [5], trans- portation industry [6], naval structures [7], aerospace components [8], and others [9]. However, GFRP is vulnerable to various types of defects induced during manufacturing, service, and maintenance. These defects can be divided into two major categories: surface defects (e.g., scrat- ches) and subsurface defects (e.g., delamination) [3]. Such defects could significantly affect the performance of GFRP. Hence, it is critical to detect the defects in an effective and timely manner by using non- destructive testing (NDT) techniques. Compared with surface defects, subsurface defects bring more challenges for NDT techniques due to their invisibility. Consequently, there is an increasing demand for de- veloping effective NDT techniques to detect subsurface defects in GFRP [10]. Several NDT techniques have been used to detect subsurface defects in GFRP, such as ultrasonic testing (UT) [11], X-ray testing [12], acoustic emission (AE) [13] and electromagnetic method [14]. How- ever, each of these techniques faces some challenges for the inspection of GFRP. UT requires the operators with a range of skills and expertise due to the complicated detection process. X-ray testing requires bulky equipment, and its high radiation energy may endanger the safety of the operators. AE requires complicated data processing methods to re- duce the influence of background noise and is only effective for dy- namic defects. Consequently, it is still necessary to develop novel NDT techniques to inspect subsurface defects in GFRP effectively. As an NDT technique, infrared thermography (IRT) has attracted increasing attention in recent years since it can achieve a rapid, non- contact, and non-invasive inspection on a variety of materials [4]. IRT has been widely used for condition monitoring of electrical equipment [15], crack detection of weld or metal [16], damage detection of composites [17], among others. In general, IRT can be divided into two categories: passive thermography and active thermography. Active thermography employs a variety of external heating sources to generate heat in the materials, such as light [18], eddy current [19], ultrasound [20,21], microwave [22]. Using eddy current as the heating source leads to the technique termed as eddy current pulsed thermography (ECPT) [23]. The heating mechanism of ECPT involves inducing eddy current and then generating Joule heat in conductive materials, which makes it possible to detect defects within skin depth accurately and efficiently [24]. Previous research has demonstrated the effectiveness of ECPT to detect cracks in metallic materials [24–26] and various types of defects in CFRP [27–30]. Meanwhile, investigations on ECPT were also performed on the detection mechanisms [31] and the probability of https://doi.org/10.1016/j.compstruct.2020.112145 Received 27 May 2019; Received in revised form 1 February 2020; Accepted 26 February 2020 ⁎ Corresponding author. E-mail address: chxu@upc.edu.cn (C. Xu). Composite Structures 242 (2020) 112145 Available online 29 February 2020 0263-8223/ © 2020 Elsevier Ltd. All rights reserved. T