PSI - Issue 59

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

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V. Kovalov et al. / Structural Integrity Procedia 00 (2019) 000 – 000 V. Kovalov et al. / Structural Integrity Procedia 00 (2019) 000 – 000

bending deformation. As a result of the conducted experimental studies, the experimental dependences of fatigue crack growth rate under the action of cyclic loads depending on the structural state have been established. It is established that the rate of fatigue crack growth in steels with ferrite-perlite structure directly depends on the amount of excess ferrite. The results of experimental studies indicate that the greater the volume of excess plastic component (ferrite), the lower the rate of fatigue crack development. © 2024 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of DMDP 2023 Organizers Keywords: electric contact surfacing, surface layer, fatigue tests, velocity of the crack growth. 1. Introduction The structural strength of parts in the final machined state depends on the structure of the surface layer. The structure of the surface layer can be changed within a wide range by means of heat treatment as it shown in Krauss (1999), Shi et al. (2012) and Nasir et al. (2006). Obtaining the required strength is determined by whether the resulting microstructure is optimal in terms of resistance to acting stresses, including residual stresses that affect fatigue strength. Favourable residual stresses can occur in welded parts as a result of cooling at an increased rate. In this case, compressive residual stresses in the surface layers reduce the sensitivity to stress concentration and thus increase fatigue resistance. Tensile residual stresses reduce fatigue strength and favour the initiation and development of fatigue cracks, resulting in failure. It is important to determine the structures formed during heat treatment of welded shafts made of structural steels, which are prone to fatigue strength reduction under conditions of stress concentration, and on this basis to determine the optimal structure of the surface layer as shown in Shi et al. (2012). Fatigue damage is preceded by damage and crack development and then brittle fracture occurs without visible plastic deformations as shown in Shi et al. (2012), Nasir et al. (2006), Suominen et al. (2013). Therefore, when selecting materials for operation under cyclic loads, for choosing a rational technology of restoration and hardening of parts, not only the characteristics of standard mechanical properties and endurance limit, but also the resistance to crack development are of great importance. The propensity to crack inhibition is one of the most important characteristics of the surface layer. Taking into account that reduction of crack growth rate increases service life and reliability of parts in operation, evaluation of efficiency of a particular technology is relevant. Nomenclature t The duration of the test from the time of loading to the time of failure or until the test is terminated; n The nominal number of revolutions of the test piece per minute; М Bending moment constant along the entire working surface of the specimen; W Resistance moment of the specimen section; l Distance from the load application point to the nearest support; d Sample diameter; Р Load applied to the sample during test; Q Loading; L Distance between supports, L =200 mm; l i Current total length of the notched crack; b, w Height and thickness of the specimen, respectively; P max Maximum load applied to the specimen at a given moment; P min Accordingly, the minimum loading; С 0 , n Integral constants bending deformation. As a result of the conducted experimental studies, the experimental dependences of fatigue crack growth rate under the action of cyclic loads depending on the structural state have been established. It is established that the rate of fatigue crack growth in steels with ferrite-perlite structure directly depends on the amount of excess ferrite. The results of experimental studies indicate that the greater the volume of excess plastic component (ferrite), the lower the rate of fatigue crack development. © 2024 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of DMDP 2023 Organizers Keywords: electric contact surfacing, surface layer, fatigue tests, velocity of the crack growth. 1. Introduction The structural strength of parts in the final machined state depends on the structure of the surface layer. The structure of the surface layer can be changed within a wide range by means of heat treatment as it shown in Krauss (1999), Shi et al. (2012) and Nasir et al. (2006). Obtaining the required strength is determined by whether the resulting microstructure is optimal in terms of resistance to acting stresses, including residual stresses that affect fatigue strength. Favourable residual stresses can occur in welded parts as a result of cooling at an increased rate. In this case, compressive residual stresses in the surface layers reduce the sensitivity to stress concentration and thus increase fatigue resistance. Tensile residual stresses reduce fatigue strength and favour the initiation and development of fatigue cracks, resulting in failure. It is important to determine the structures formed during heat treatment of welded shafts made of structural steels, which are prone to fatigue strength reduction under conditions of stress concentration, and on this basis to determine the optimal structure of the surface layer as shown in Shi et al. (2012). Fatigue damage is preceded by damage and crack development and then brittle fracture occurs without visible plastic deformations as shown in Shi et al. (2012), Nasir et al. (2006), Suominen et al. (2013). Therefore, when selecting materials for operation under cyclic loads, for choosing a rational technology of restoration and hardening of parts, not only the characteristics of standard mechanical properties and endurance limit, but also the resistance to crack development are of great importance. The propensity to crack inhibition is one of the most important characteristics of the surface layer. Taking into account that reduction of crack growth rate increases service life and reliability of parts in operation, evaluation of efficiency of a particular technology is relevant. Nomenclature t The duration of the test from the time of loading to the time of failure or until the test is terminated; n The nominal number of revolutions of the test piece per minute; М Bending moment constant along the entire working surface of the specimen; W Resistance moment of the specimen section; l Distance from the load application point to the nearest support; d Sample diameter; Р Load applied to the sample during test; Q Loading; L Distance between supports, L =200 mm; l i Current total length of the notched crack; b, w Height and thickness of the specimen, respectively; P max Maximum load applied to the specimen at a given moment; min Accordingly, the minimum loading; С 0 , n Integral constants © 2024 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of DMDP 2023 Organizers