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

558

4

specimens with air cooling, blue dots – with water. Fatigue cracks were obtained on the fracture surfaces with water cooling.

190

185

180

175

170

165

Stress (MPa)

160

155

10 6

10 7

10 8 N

10 9

10 10

Figure 3. The Weller curve for Armco iron, red – tests with air cooling, blue – tests with water-cooling.

4.2 Potential drop method

As a result of tests more than 20 fractured specimens were obtained. The authors suppose that fracture occurs when the difference between natural frequency of vibrations of the specimen and testing machine oscillation reaches more than 0.5 kHz. Direct electric signal of power supply of PDM transforms into alternating signal due to the high frequency oscillation of specimen (fig. 4a). Low frequency vibration appears on the different stages of test process. The reason of this oscillation is non-linear effects in vibration system. These oscillations are caused by non-linearity of the oscillation system. The emergence of non-linearity can be related to the initiation and the development of discontinuities in the material volume and also a change in the elastic properties of the material Betechtin (2016). It should be noted, that the peak in the figure 4b corresponds to the inflection point of the graph presented in figure 5. The authors suggest that this moment corresponds to the transition from the particulate accumulation of defects in the volume of the material to the macroscopic fracture. Thereby, defects dispersed in volume localize in the area of the smallest diameter of the sample and a macroscopic crack is formed.

a

b

Figure 4. (a)The signal which detected by PDM method. (b) The signal filtered by the Chebyshev bandpass filter.

The subsequent increase in the direct component of the signal corresponds to the propagation of fatigue crack in the material volume. Due to the feature of the testing machine and physics of the loading process, the fatigue crack