13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- equilibrium. At each loading cycle is observed area of temperature drop caused by the thermoelastic effect, which goes to the site of temperature increase caused by the local transition through the proportional limit and the formation of zones of plastic deformation. During decrease stress in the crack tip heat generation are continues. The geometry of the zones of plastic deformation is shown in Figure 6. With decreasing stress at the crack tip heat increases and the temperature reaches its maximum at practically zero stress value. Then, at the beginning of the next cycle, the temperature drops due to thermoelastic effect and the process repeats. The main goal in investigation of cylindrical specimens was to understand principles of crack initiation and propagation inside of specimen i.e. Fish eye. For achieve this goal specimens were tested in 2 regimes – multi-cycle fatigue with using infrared camera and giga-cycle fatigue. In giga-cycle fatigue regime rise of temperature was very high and not able to control without cooling – such way infrared camera was helpless in analyzing of structure parameters. During multi-cycle fatigue was determine fatigue strength of titanium specimens by Luong method [1]. For materials with usual structure it is work well, but for nano-grain structures it is not so accurate, because damage accumulated during the formation of such a structure start to heat much earlier than usual structure, but we still can to evaluate fatigue strength limit on sharp rise temperature point (fig. 6) Fig6. The dependence of the surface temperature of the sample from the applied stress Summarizing results of high cycling test and infrared camera, using Resitano-Luong technic, we can conclude that fatigue limit of Ti Grade 4 in initial stage is approximately 375 Mpa, in “Stage 1” 575 MPa, in “Stage 2” 600 MPa. Infrared thermography methodic can high accuracy visualize zone of intense energy dissipation in the tip of fatigue crack (Fig. 7). The distribution of temperature in the crack tip during deformation may differ from the form of zones of plastic deformation due to the processes of heat conduction, so to analyze the geometry of the intense heat caused by plastic deformation, it is logical to use only the first cycle of deformation. Figure 7 shows a comparison of the observed shape of zones of intense energy dissipation in the first cycle of deformation and classical solutions of (3.4) to form zones of plastic deformation in the crack tip (in the calculations we used the following data: half crack length 4 mm, the applied stress 300 MPa). Analysis of the results suggests only a qualitative agreement forms zones of plastic deformation in the tip of fatigue crack propagation to models Mises and Tresca-Saint Venant. Temperature, °C Stress, MPa
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