PSI - Issue 18

Prokhorov A. et al. / Procedia Structural Integrity 18 (2019) 802–808 Author name / Structural Integrity Procedia 00 (2019) 000–000

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The specimens for VHCF tests were quenched and tempered after machining process in order to obtain a homogeneous microstructure. The heat treatment involves preheating at 1000° C and oil tempering with 600° C during 3 hours. After that the surface of specimens were polished with sand papers for decreasing manufacture roughness. There are different type of specimens were used for different type of tests. The straight cylindrical specimen (fig 1a) for tension test, the straight flat specimen (fig 1b) for cyclic tests for fatigue limit definition (Risitano-Luong test Risitano A.et al (2011), Luong M. et al(1993) and hourglass specimen (fig 1c) for VHCF tests. The mechanical properties of tested material were observed using electro-mechanical testing machine Shimadzu AGX-plus. The tests were performed with constant strain rate ~10 -3 s -1 .The stress-strain diagrams of row and quenched material presented at figure 2(a). The Young modulus of the both state of AISI 420 steel is 210 GPa.

(a)

(b)

Fig. 2. (a) Stress-strain diagram of AISI 420 row and quenched state. (b) Graphical determination endurance limit based on Luong test results.

Endurance limit of tested material were estimated based on Risitano-Luong method. According this method the endurance limit may be commonly determined by detecting a strong change of intrinsic energy dissipation regime under cyclic loading. The Risitano test were performed using servo-hydraulic testing machine with loading frequency 10Hz, with R=-1. The infrared thermography technique were used to detect the occurrence of intrinsic dissipation. IR camera Flir SC5000 were placed in front of specimen surface and detected the temperature increasing during each loading level from 250 MPa to 420 MPa amplitude value. After this test were built dependence between applied load and temperature changing on specimen surface (Fig. 2(b)). It is shown that figure has a bilinear dependence, according self-heating method endurance limit equal to value of applied stress at inflection point of this dependence. 3. Experimental setup Fatigue testing were performed based on ultrasonic testing machine Shimadzu USF-2000 with loading frequency 20kHz. The temperature stability on specimen surface controlled by thermocouple during the test. If temperature on surface exceed the 80° C loading were automatically stopped up to the temperature of a specimen decreased to the normal conditions. Applied stress for all specimens in this investigation was 305 MPa. The tests were performed in normal conditions and under alternating magnetic field influence. The setup for applying alternating magnetic field is shown on the figure 3. In the scheme, the sample was installed in the section of the magnetic circuit through the winding of which an alternating electric current with a frequency of 700 Hz flows. In the area of the working part of the sample, an inductance coil was installed, recording changes in the magnetic field. In this scheme, the sample is part of the