PSI - Issue 5

A. Prokhorov et al. / Procedia Structural Integrity 5 (2017) 555–561 A. Prokhorov et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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force, and there is no need to immobilize one side of the sample. Strain of the sample can be calculated by measurement of the displacement of a free side of the sample. The uniform stress testing can be carried out by controlling the amplitude of the vibration. The application of ultrasonic testing machines for fatigue experiments is described by Bathias (2004).. In the present study, all the samples were tested at the constant stress amplitude up to the failure. The cooling of the sample was realized by two different types air-cooling and water cooling. The experiment continued until the resonance frequency of the sample changed significantly (by more than 10%). Nucleation of the fatigue crack is detected by the potential drop method (four-point measurement of the electrical resistance). This method was applied to the experiments with air-cooling. Application of the potential drop method with water cooling is difficult task because of specifics of contact connection with specimens. In this method four contacts are placed on the sample, which is set inside the ultrasonic testing machine. A pair of external contacts are used for supplying of DC current of 5A to the specimen. A pair of the internal contacts are used to register the potential drop in the specimen. In addition, ADC is used to measure potential drop. Schematic diagram of the method is shown in the figure 2. The instrument accuracy of the electrical resistance is 30 u Ω. 3. The defect monitoring method

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Figure 2. (a)Schematic diagram of the experimental setup for the registration evolution of the material structure during fatigue testing. (b) specimen into testing machine

4. Results

4.1 Fatigue properties

There were carried out gigacycle fatigue tests of Armco-iron specimens. Fatigue curve (fig. 3) which illustrates decrease in fatigue limit of armco-iron has been obtained up to 160 MPa on the base of 10E+9 cycles. Loss of the resonant frequency of the system was used as a fracture criterion of the specimen. Initiation of macrocrack or degradation of elastic properties lead to the change of the frequency. Specimens could endure 10 Е +6 cycles with minimum amplitude any number of times after the fracture. Final rapture of specimen did not show the presence of macro-crack. It is assumed that the loss of the resonant frequency is due to the heating of the specimen because of the high-frequency impact which leads to the change in the elastic modulus. Infrared thermography was used for the measurement of the temperature field evolution on the surface of the specimen. Experiments have shown monotonous behavior of the temperature during the test and sharp increase of the temperature on the final stage. After that, fundamental frequency has been changed which reflects in loss of the resonant frequency. Air cooling was replaced by water cooling in the following experiments. Experimental results are shown in figure 3, where red dots denote