Lateral heat conduction based eddy current thermography for detection of parallel cracks and rail tread oblique cracks Ruizhen Yang a,⇑ , Yunze He b,⇑,1 , Bin Gao c , Gui Yun Tian c , Jianping Peng d a Department of Civil Engineering, Changsha University, Changsha 410022, PR China b College of Mechatronics Engineering and Automation, National University of Defense Technology, Changsha 410073, PR China c School of Automation Engineering, University of Electronic Science and Technology of China, Chengdu 610054, PR China d School of Physical Science and Technology, Southwest Jiaotong University, Chengdu 610031, PR China article info Article history: Received 9 September 2014 Received in revised form 6 December 2014 Accepted 28 January 2015 Available online 7 February 2015 Keywords: Eddy current thermography Lateral heat conduction Rail oblique crack Parallel crack abstract Rail tread oblique crack, initiated by rolling contact fatigue (RCF) damage, is one of the most significant phenomena and has serious influence on rail industry. Electromagnetic non-destructive testing (EM NDT) methods are usually used in rail regular inspection. However, the conventional EM NDT methods based on eddy current field distribution are difficult to detect the cracks parallel to the inductive coil (parallel cracks) and natural oblique cracks. This paper studied lateral heat conduction (LHC) induced by eddy current for detection of these defects. The proposed method was verified through both numerical and experimental studies as well as the investigation of characteristic of LHC. Due to significant temperature gradient in the direction of lateral heat conduction, the spatial derivative and gradient were proposed to improve the defect detectability on the thermo- grams. Finally, the test of natural oblique cracks on a rail was conducted to validate the proposed methods. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Rail tracks are subjected to intense stresses, deforma- tion and wear. With the tendency of the railway transpor- tation into heavy haul and high speed, the problem of surface damage due to wheel-rail rolling contact fatigue (RCF) becomes more and more serious [1]. In 2000, the Hatfield derailment accident was an outstanding example of the possible consequences of RCF, which resulted 39 injuries and huge economic fallout in United Kingdom [2]. RCF is a group of rail damages due to overstressing of the rail material, which occurs on the rail surface in form of shelling, squats and gauge corner cracks or within the subsurface in form of deep seated shells. The rail tread oblique crack, which belongs to RCF initiated damage, is one of the most significant phenomena which will cause failure. After initiation, oblique cracks prolong to the cen- tre of the rail tread with a 45° angle along rail side and facing the direction of traffic, as shown in Fig. 1(a). At the same time, the cracks extend to inside of rail with an acute angle (15–40°) along rail tread, as shown in Fig. 1(b). As the deep increasing, the acute angle has a gradually increase. Up to 8–10 mm in depth, they would turn into the transverse crack and lead to a fracture of rail, as shown in Fig. 1(c). Research on the application of NDT methods for the detection of defects in rails began as early as 1877 [3]. The regular inspection of the rail infrastructure was initiated in the late 1920s in the USA when Dr. Elmer Sperry developed the Sperry Car to detect fissures in the rail. By now, more than ten NDT techniques can be used http://dx.doi.org/10.1016/j.measurement.2015.01.024 0263-2241/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +86 731 84261208. E-mail addresses: xbaiyang@163.com (R. Yang), hejicker@gmail.com (Y. He). 1 Tel.: +86 13467698133. Measurement 66 (2015) 54–61 Contents lists available at ScienceDirect Measurement journal homepage: www.elsevier.com/locate/measurement