## 4 Results and Discussion  
### Analysis of Excitation Duration on Detection Ability of ECPT for Surface Cracks on PDC Bit Matrix  
Due to the principle of ECPT that the effect of current obstruction by defect can cause a different temperature distribution of the specimen surface, excitation current intensity and excitation duration are two key parameters for ECPT testing. A larger excitation current intensity generally leads to more obvious thermal contrast between the defect and the sound area. Therefore, the excitation current intensity should be set as large as possible for different inspected objects. As a result, 350 A is chosen as the excitation current intensity during the experiments. Additionally, the excitation frequency used in the experiment is set to 200 kHz, at which the calculated penetration depth of the induced eddy current is about 0.49 mm. For a specific eddy current heating system, the influence of current excitation duration on the detection result differs with varied tested objects. Hence, it is necessary to conduct an experimental investigation on determining the optimized excitation duration for detecting surface cracks on PDC bits. In this paper, Position D in Figure 4b is selected as an example to optimize the excitation duration. With other detecting parameters unchanged, experiments with a series of excitation duration, including 100, 200, 300, 400, 500, and 600 ms, have been conducted respectively to detect the two cracks. Considering the infrared camera worked at the frame frequency of 25 Hz, 100 IR image frames were sequentially recorded for each position (the recording time of 4000 ms). Consequently, taking the heating duration of 500 ms as an example, each IR image sequence can be divided into three periods: frames from 1 to 10 (0-400 ms) as the stage before heating; frames from 11 to 23 (401-900 ms) as the heating stage and frames from 24 to 100 (901-4000 ms) as the cooling stage (after heating). According to the findings in previous works [12], the frame at the end time of heating excitation stage in each infrared sequence was chosen for IR image analysis to evaluate the influence of heat excitation duration on detection results. Figure 5 shows IR images recorded with different excitation duration when the coil was located at Position D. We can see the segments of two cracks can be easily recognized in Figure 5a-f despite the interference of excitation coil in visual field. To find the optimum heating excitation duration, a criterion is necessary to find the optimum heating excitation duration. It is well known that the crack is easier to detect when the thermal contrast between the defect and the surrounding sound area is higher. Hence, such thermal contrast can  
Figure 4: Tested specimen: (**a**) the PDC bit; (**b**) two cross cracks on the surface: Crack I and Crack II; (**c**) zoomed view of Crack I.  
be used to evaluate the effectiveness of each excitation duration setting for ECPT. The temperature response can certainly be used to calculate the thermal contrast. Additionally, there is a proportional relationship between the intensity of each pixel in the IR image and the temperature response of the corresponding position. Therefore, the intensity contrast of the IR image can be used to represent the thermal contrast. For simplified calculation, we convert the IR images to gray scale images and the gray contrast (intensity contrast gray scale-level images) between the defect and its surrounding area to represent that the thermal contrast is obtained as the criterion to assess the effectiveness of each excitation duration setting for ECPT. Here, we take Crack I as an example and the gray contrast between the crack and sound area are calculated based on the IR images shown as Figure 5a-f. The calculated gray contrast is shown in Figure 6. It can be seen from Figure 6 that the gray contrast rises with the increase of excitation duration from 100 ms to 500 ms. However, when the excitation duration exceeds 500 ms, the gray contrast decreases.