PSI - Issue 59

Olha Maksymiv et al. / Procedia Structural Integrity 59 (2024) 378–384 Olha Maksymiv et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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aqueous TF (curve 3 , Fig. 2). It is associated with the higher speed of specimens’ cooling in aqueous TF compared with the oil TF and air. The depth of the strengthened surface layer with NCS substantially affects the contact fatigue resistance, since buckling of the surface layer under that type of load. The contact durability of specimens tested in oil (Fig. 3a) is essentially higher than in tap water (Fig. 3b). The specimens with surface NCS generated by MPT in different TF are characterised by higher contact durability in the industrial oil compared with specimens without NCS (Fig. 3a); specimens tested in tap water had significantly higher contact durability just after MPT in air (Fig. 3b), whereas specimens after MPT in other TF had contact durability almost the same as after cementation and heat treatment (Fig. 3b). It should be noted that during contact durability tests, redistribution of the components of TF, which had penetrated into the surface NCS during MPT, took place on the rolling pass due to high contact loads.

Fig. 3. Contact durability of the steel specimens with preliminary cementation, quenching and low-temperature tempering in (a) industrial oil and (b) tap water: 1 – ground; 2 – 4 – after MPT in different TF: 2 – air; 3 – 10% aqueous solution of emulsol; 4 – industrial oil.

As was shown in a previous study by Kyryliv et al. (2016), the thin surface layer of material, plastically deformed by the previous load, not being supported from outside, loses the longitudinal stability and buckles (Fig. 4). The longitudinal force directed to the middle of the surface layer along the speed direction, compresses them and promotes the buckling. The longitudinal stability is limited by resistance to buckling, and it depends on the thickness of the strengthened layer. The tensile stresses and deformation appear between strengthened and underlying metal, which reach some critical value and cause the breaking of the integrity of the metal and initiation of the primary near-surface crack coming into progressive pitting.

Fig. 4. Scheme of buckling of the thin surface layer and initiation of the near-surface crack caused by tensile stresses.

Contact fatigue is characterized by the same features as other types of fatigue failure, but its difference is in a greater localization of stresses. The material's state under contact loads is characterized primarily by large stress gradients, their localization in relatively small volumes near the surface, and significant working stresses. Since such stresses near the surface often exceed the limit of elasticity and lead to the crack s’ nucleation, the quality of the surface layers significantly affects their working capacity as studied by Tricot et al. (1972) and Seo et al. (2011). For contact fatigue tests of specimens with surface NCS, the crack nucleation sites depend on the working environment