Issue 35

O. Plekhov et alii, Frattura ed Integrità Strutturale, 35 (2016) 414-423; DOI: 10.3221/IGF-ESIS.35.47 416 а) b) Figure 2 : Initial structure of the iron sample (a) , fatigue crack on the polishing sample surface (b) , the depth of polishing 0.15mm. The fatigue tests were carried out using the Shimadzu USF-2000 ultrasonic testing machine, which provides the accelerated testing of the fatigue properties of materials at 20 kHz. The 20 kHz vibration generated by the piezo element is amplified by the booster and the horn and transmitted to the sample to generate repeated stress. In the ultrasonic fatigue testing system, the vibration system is constructed so that the longitudinal waves transmitted through the solid body are resonating. Consequently, stationary longitudinal waves are formed inside the vibration system (sample, horn and booster). From the physical point of view, the mass of the sample itself plays the role of a generating reactive force, and there is no need to immobilize one side of the sample. The generated strain distribution in the sample can be calculated by the measuring of the displacement of a free side of the sample. The uniform stress testing can be carried out by the controlling of the amplitude of the vibration. All the samples were tested at the constant stress amplitude up to the failure. The cooling of the sample was realized by an air stream. The temperature of the sample was controlled by the infrared camera. The temperature rise during the experiment is presented in Fig 3. Figure 3 : The temperature evolution of the sample during fatigue test. The Young`s modulus E (a characteristic of elastic properties) and amplitude independent decrement δ (a characteristic of reversible micro plasticity properties) were determined using acoustic resonance method . The main peculiarity of the acoustic experiments is small stress amplitude which doesn’t change the dislocation density in the sample. The measurements were carried out using sectional piezoelectric resonant vibrator. The longitudinal vibrations of samples have a frequency of about 100 KHz. The amplitude was varied in a wide range to investigate both a linear (amplitude- independent) and nonlinear (microplastic) areas. The essence of the technique is very close to the ultrasonic fatigue test. The sample is connected with piezo element. Stationary acoustic waves are formed inside the vibration system (sample and piezo element). The elastic and micro plasticity characteristics are calculated based of the measurement of a resonant frequency of the system. Eigen frequency of the longitudinal oscillations ν l of bar is connected with Young`s modulus as: