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Mode I and mode II fatigue crack growth resistance characteristics of high tempered 65G steel

Lenkovskiy T. M., Kulyk V. V., Duriagina Z. A., Kovalchuk R. A., Topilnytskyy V. H., Vira V. V., Tepla T. L.

Archives of Materials Science and Engineering

2017

Vol. 84, nr 1

34--41

Purpose: To investigate the fatigue crack growth at normal tension and transverse shear of 65G steel with the high tempered martensite microstructure and to build an appropriate fatigue crack growth rate curves. To determine the main and auxiliary fatigue crack growth resistance characteristics, which are necessary for machine parts life-time estimation at rolling contact fatigue conditions. Design/methodology/approach: For determination of fatigue crack growth resistance at normal tension a standard compact specimens with edge crack were tested using a hydraulic testing machine and fatigue testing at transverse shear were performed on the I-beam specimens with the edge longitudinal crack using the original testing setup. For crack growth measurement an optical cathetometer B-630 was used. The crack growth rate V was calculated as crack length increment during loading cycles. The stress intensity factor range K was determined by dependence "K = (1 – R)Kmax accordingly to the standard test methods. To establish crack faces friction factor at transverse shear fragments of fractured beam specimen containing crack faces were cut out and tested as a friction pair according to Amontons Coulomb's law. On the base of test results the fatigue crack growth rate curves in logarithmic coordinates "K vs. V were built. These graphical dependencies for normal tension and transverse shear were used for determination of fatigue crack growth resistance characteristics: fatigue threshold "Kth, fracture toughness "Kfc, "K1-2 and "K2-3 which indicates the beginning and the end of middle-amplitude region of curve, "K*, parameters C and n of Paris’s equation. Metallographic and fractographic analyses were performed on the scanning electronic microscope Zeiss EVO 40XVP. Findings: Empirical dependences of the stress intensity factor range on fatigue crack growth rate at normal tension and transverse shear of 65G steel with the high tempered martensite microstructure are obtained. Based on these graphical dependencies the fatigue thresholds and fracture toughness as well as the parameters of Paris’s equation are determined. Research limitations/implications: The fatigue crack growth on 65G steel under low-, medium- and high-amplitude cyclic loading at normal tension and transverse shear was investigated. The fatigue crack growth rate values for a wide range of stress intensity factor are estimated. On the base of fractographical analysis the features of fracture of high tempered martensite in 65G steel at transverse shear are studied. It is shown that the transverse shear crack faces friction factor for high tempered martensite structure is less than for low tempered martensite. Practical implications: Using the fatigue crack growth resistance characteristics of 65G steel at normal tension and transverse shear and related fatigue crack growth rate curves it is possible to predict the life-time of machine parts made of steels with high tempered martensite structure, working at rolling contact fatigue conditions. Originality/value: Complete fatigue crack growth rate curves of 65G steel with tempered martensite structure at normal tension and transverse shear are built and the fatigue crack growth resistance characteristics for both modes of fracture are determined for the first time.

Influence of 65G steel microstructure on crack faces friction factor under mode II fatigue fracture

Ivanytskyj Y. L., Lenkovskiy T. M., Molkov Y. V., Kulyk V. V., Duriagina Z. A.

Archives of Materials Science and Engineering

2016

Vol. 82, nr 2

49--56

Purpose: The aim of the paper is to evaluate the dependence of microstructure parameters, strength and plasticity of steel on crack faces friction factor. Design/methodology/approach: The specimens for the investigation were cut out from the 10 mm thick hot-rolled plate of 65G steel used as a model material for fatigue and durability testing of whole-rolled railway wheels. The mechanical characteristics of the steel were determined according to the state standard using cylindrical specimens of diameter 5 mm and effective length 50 mm. The specimens were heat-treated at the mentioned conditions. Fatigue testing under mode II loading was carried out on a special rigid loading machine in the standard laboratory conditions at symmetric sinusoidal cycle with a frequency of 12 Hz in the range of fatigue crack growth rates da/dN = 5∙10⁻⁸…5∙10⁻⁷ m/cycle until its reaches relative length l/b ≥ 0.8. The obtained microsections were investigated using the optical metallographic microscope Neophot 9 equipped with a digital camera Nikon D50 and electronic scanning microscope Zeiss EVO 40XVP. Hardness of the specimens with different microstructure was determined using durometer TK-2. The crack faces friction factor was determined using original device for fractured surfaces sliding under certain compression force realization. Findings: The dependences of microstructure parameters, strength and plasticity of steel on crack faces friction factor are obtained. Research limitations/implications: The investigation of the influence of microstructure parameters, strength and plasticity of real wheel steels on crack faces friction factor at the mode II fatigue crack growth will be carried out. Practical implications: The value of crack faces friction factor have strong impact on stress intensity at the crack tip and must be taken into account at crack growth rates curves plotting. Originality/value: Mode II fatigue crack faces friction factor of steel is firstly experimentally determined.