PSI - Issue 59

Viktor Kovalov et al. / Procedia Structural Integrity 59 (2024) 771–778 V. Kovalov et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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2. Experimental device and methodology Theoretical analysis of the influence of the surface layer structure on the crack growth rate should be carried out taking into account modern ideas about the mechanism of fatigue crack initiation. The fatigue process in parts and structures always starts and is accompanied by the appearance and progressive development of fatigue cracks, one or several, in those places of these elements in which the cyclic stresses reach the highest values. The formation of fatigue cracks is a consequence of the appearance and development of plastic deformation processes in micro volumes. The use of the crack growth rate indicator as a characteristic of mechanical properties makes it possible to determine the most effective technology for the restoration of worn parts of the corresponding purpose. For this purpose not only qualitative but also quantitative analysis of the fatigue crack growth process is necessary. The duration of the fracture process, as well as the ratio between the crack size and residual strength give an idea of the survivability of the material and the propensity of the damaged part to brittle fracture. In the experimental evaluation of fatigue crack growth rate, the initial data are the relationship between the change in crack length and the number of cycles of applied loads. With the occurrence of a crack in the specimen after stresses, it undergoes significant changes: near the crack tip there is a concentration of stresses. To characterise this phenomenon, the stress intensity parameter K 1 is used, which represents the joint effect of applied loads and crack length on the stress state near the crack tip. The K 1 coefficient reflects the redistribution of stresses in the specimen due to crack formation and characterises the magnitude of forces transmitted through the area near the crack tip. The fatigue crack propagation rate was investigated on model specimens from steel 40 and 40X, which simulated the structural state of the surface layer with a certain content of plastic phases regulated by combined heat treatment. Samples from steel 40 were heated to a temperature of 820...840°C, kept at this temperature and cooled in a heat insulating mixture or in air until complete cooling as shown in Berezshnaya et al. (2020) and Kassov et al. (2021). Samples of 40X steel were heated to a temperature of 830...850°C, followed by cooling in a heat -insulating mixture or in air as shown in Berezshnaya et al. (2020) and Kassov et al. (2021). After heat treatment, the blanks were chiselled and ground to final dimensions, and then the notch was applied (Fig.1) using a special cutter. A chevron notch was adopted, performed with a mill sharpened to a radius of 0.3 mm. The method used includes two stages: fatigue crack formation, during which it is possible to measure the crack growth rate, and crack propagation until the specimen fracture under the action of applied static forces. The essence of the experiment is that the model specimen is loaded with bending and tensile forces. At the same time, a fatigue crack is pre-formed from the notch made by mechanical treatment. In the process of crack growth, the rate of crack development is determined, which requires changing the number of cycles and loading conditions when the crack passes certain distances. Prior to testing, the specimens were polished as a metallographic slurry over the area of the notch. Near the top of the notch, approximately every 0.9...1.0 mm, a sharp scribe was used to draw perpendicular to the presumed crack path (Fig. 1).

Fig 1. Specimen prepared for testing with markings applied.

The fatigue crack was formed from the notch. The loading scheme was concentrated loading, creating a bending deformation in the specimen. The specimens were mounted on a special support fixture with rollers freely rotating in the supports relative to the centres. This minimised friction losses during testing. The diameter of the support rollers was 25 mm. During testing, the support rollers were equidistant from the axis of load application (in the notch plane) to with in ± 1 mm. The specimens were placed so that the plane was parallel to the axis of the rollers. The non- parallelism did not exceed 2°. The minimum loading was chosen so that it was about 10% of the maximum value. The asymmetry coefficient was 0.1. The tests were carried out at a frequency of the applied loading of 11 Hz.