Issue 21

A. Yu Fedorova et alii, Frattura ed Integrità Strutturale, 21 (2012) 46-53; DOI: 10.3221/IGF-ESIS.21.06 47 M ATERIALS AND CONDITIONS OF EXPERIMENT xperimental study of temperature evolution at the fatigue crack tip was carried out on the plane specimens of titanium Ti-6Al-4V. The specimens were manufactured from a commercial pure titanium sheet 3 mm thick. The chemical composition of material presented in Tab. 1. Fe C Si V N Ti Al Zr O H 0.3 0.1 0.15 4.2 0.03 other 5.9 0.2 0.2 0.015 Table 1 : Chemical composition of Ti-6Al-4V. Mechanical properties of material are modulus of elasticity - 113 GPa, yield stress - 800 MPa, ultimate stress 900 MPa, fatigue limit – 460 MPa, fracture toughness – 75.6 МРа  m. The geometry of specimen is shown in Fig. 1. The specimens were weakened by holes to initiate fatigue crack at the specimen center. The fatigue crack (about 10 mm) was initiated at the initial stage of the experiment by high amplitude cyclic loading of the specimens at the average stress of 215MPa, stress amplitude of 238 MPa and loading frequency of 20 Hz. Then the load was decreased to slow down the rate of crack propagation, which allows a detailed analysis of the heat generation processes at the crack tip. The surface of the specimens was polished in several stages by 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 [9]. The process of crack propagation was studied at 5 Hz and 10 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. It was shown that for selected values of loading frequency the value of determined parameters (stress intensity factor) depends on the applied stress and crack length, only. Figure 1 : Geometry of specimen. All sizes are in millimeters. T HE PROCESSING OF THE EXPERIMENTAL DATA t 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 following algorithm. E A