An Investigation on Eddy Current Pulsed Thermography to Detect Surface Cracks on the Tungsten Carbide Matrix of Polycrystalline Diamond Compact Bit  
[PERSON]  
1 College of Mechanical and Electronic Engineering, China University of Petroleum, Qingdao 266580, China; [EMAIL_ADDRESS] (W.Z. ); [EMAIL_ADDRESS]. employed to detect surface or internal defects in drilling bits in recent years. [PERSON] et al. conducted nondestructive inspection for internal defects on a rotary drill bit using an ultrasonic technique [3]. [PERSON] et al. proposed a method based on an ultrasonic phased array to identify and characterized internal defects in a rotary drill component [4]. [PERSON] et al. demonstrated that a scanning acoustic microscope (C-SAM) can be used to detect defects at the PDC cutter of a drill bit [5]. [PERSON] [6] proposed a method of acoustic emissions for testing the toughness of PDC inserted in a drill bit. These testing techniques showed sound inspection capabilities for PDC bits. However, there are still disadvantages in these methods, such as the difficulties of using acoustic couplant on the irregular surface of PDC bits, low detection efficiency, or indirect detection results. Therefore, it is necessary to develop other effective and suitable methods for detecting surface cracks on PDC bits. Combining high sensitivity of eddy current testing and visualization of IR thermography, eddy current pulsed thermography (ECPT), a kind of active thermography, has been widely applied to detect surface defects on conductive materials. Previous research has demonstrated the efficacy of ECPT to detect surface cracks on steel wires [7], metal compressor blades [8], and rail track due to rolling contact fatigue [9], etc. In addition, [PERSON] et al. detected steel corrosion and investigated the influence of testing parameters on detection accuracy of ECPT [10; 11]. [PERSON] et al. investigated the feasibility of ECPT for hidden crack detection on corroded metal surfaces [12]. [PERSON] et al. stated that multiple defects on ferromagnetic or non-ferromagnetic metallic surfaces can be detected using ECPT [13]. ECPT has also been used to detect various defects in carbon fiber composites [14; 15]. However, there has not been any report on the application of ECPT to detect surface defects on PDC bits. The main possible difficulties for using ECPT to detect surface defects on PDC bits come from its large volume and weight. Firstly, it is very difficult to excite the whole bit one time because the required power of heating excitation will be extremely high. Additionally, the size of the excitation coil needs to be large enough to generate sufficient heat for the entire surface of PDC bits. Therefore, it is not feasible to raise the temperature of the tested object with one or several heating excitations to detect surface defects on PDC bits using conventional ECPT. Hence, it is necessary to develop a new detection mode for ECPT to detect surface cracks on PDC bits. Some researchers have proposed scanning thermography techniques, such as line scanning thermography, scanning induction thermography, and so on [16; 17; 18]. These scanning thermography techniques can achieve defect detection on a large area or cylinder. For example, [PERSON] et al. demonstrated that scanning induction thermography is effective for defect detection of composite components with flat surfaces [18]. Such achievements motivate us to develop a new scanning mode for ECPT to detect surface cracks on PDC bits with more complicated geometry. We aim to investigate the feasibility of using ECPT to detect surface cracks on the tungsten carbide matrix of PDC bits in this work. A new scanning detection mode for ECPT is proposed according to the surface features of the bit's matrix. The influence of excitation duration on the detection result is analyzed, and principal component analysis (PCA) is then used to process the IR image sequences to further improve detection sensitivity. Finally, the whole crack is reconstructed by integrating IR images of varied crack segments.