13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- the abrasive paper (at the final stage of polishing the grit size does not exceed 3 μm). Before starting the experiment, the polished surface was covered by a thin layer of amorphous carbon. The temperature evolution was recorded by infrared camera CEDIP Silver 450M. The spectral range of the camera is 3-5 mm. The maximum frame size is 320×256 pixels; the spatial resolution is 10-4 meters. The temperature sensitivity is 25 mK at 300 K. Calibration of the camera was made based on the standard calibration table. Mechanical tests were carried out at 100 kN servo-hydraulic machine Bi-00-100. The test conditions comply with the conditions of the experiment was described in [3]. The process of crack propagation was studied at 5 Hz loading frequency. The selected frequency of loading provides a close to adiabatically condition at crack tip. At low frequency (less that 5 Hz) the heat transfer process plays a great role and doesn’t allow one to calculate the right value of heat source. The investigation of high loading frequency requests the high frame rate and treatment of large amount of infrared data. Figure 1. Geometry of sample (all sizes are in millimeters). 3. Experimental data processing and determination of the stored energy At the beginning of data processing procedure, the first frame was subtracted from the film to eliminate the influence of infrared radiation from the camera lens on the determined temperature field. Due to the relative motion of the specimen and infrared camera lens under cyclic tests, there is the problem of motion compensation in order to obtain the correct temperature data at a given point on specimen surface. Compensation of relative motion was made based on the algorithm described in details in [4]. The main idea of this algorithm is the selection of marker-zone on the studied surface and searching this area on the surface in each next time step. Farther, displacement of each point on the surface is calculated for each time step. As a result of data processing, we obtained the temperature increment field (Fig. 2) based on which the heat sources field was determined. A B Figure 2. Infrared image of sample before data processing (A) and obtained temperature increment field (B). To calculate the specific power of the heat source, we have used a finite difference scheme of the equation for heat sources evolution (Eq. 1).
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