PSI - Issue 42

Structural Integrity Procedia 00 (2022) 000 – 000 ^ĐŝĞŶĐĞ ŝƌĞĐƚ

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Structural Integrity Procedia 00 (2022) 000 – 000

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Procedia Structural Integrity 42 (2022) 919–926

© 2022 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 the scientific committee of the 23 European Conference on Fracture – ECF23 Abstract In the present study, very high cycle fatigue (VHCF) characteristics and properties of a direct quenched and partitioned (DQ&P) medium carbon (0.4 wt.% C) steel having ultrahigh-strength (UTS > 2 GPa) was investigated using an ultrasonic-fatigue testing technique (~19 kHz frequency) up to ~10 10 cycles at the load ratio ( R ) of − 1 (fully reversed tension-compression). The occurrence of crack initiation in the VHCF regime was observed at interior non-metallic inclusions. The microstructural changes following VHCF failure underneath the fracture surfaces near the crack initiation sites were further investigated through transmission electron microscopy. Formation of a clear nanograined subsurface layer adjacent to the crack initiating site at the interior inclusion was noticeably evident. The variation of average grain size (60 − 140 nm) and thickness (300 − 1200 nm) of nanograined layers at different number of cycles to failure were measured in detail. Apparently, localized plastic deformation caused the fragmentation of martensitic laths and hence, the formation of aforesaid ultrafine-grained layers in the microstructures. Ξ ϮϬϮϬ dŚĞ ƵƚŚŽƌƐ͘ WƵďůŝƐŚĞĚ ďLJ ůƐĞǀŝĞƌ ͘s͘ dŚŝƐ ŝƐ ĂŶ ŽƉĞŶ ĂĐĐĞƐƐ ĂƌƚŝĐůĞ ƵŶĚĞƌ ƚŚĞ zͲE ͲE ůŝĐĞŶƐĞ ;ŚƚƚƉ͗ͬͬĐƌĞĂƚŝǀĞĐŽŵŵŽŶƐ͘ŽƌŐͬůŝĐĞŶƐĞƐͬďLJͲŶĐͲŶĚͬϰ͘ϬͬͿ WĞĞƌͲƌĞǀŝĞǁ ƵŶĚĞƌ ƌĞƐƉŽŶƐŝďŝůŝƚLJ ŽĨ Ϯϯ ƵƌŽƉĞĂŶ ŽŶĨĞƌĞŶĐĞ ŽŶ &ƌĂĐƚƵƌĞ Ͳ &Ϯϯ 23 European Conference on Fracture - ECF23 Micro-mechanism associated with very high cycle fatigue crack initiation of advanc d DQ&P processed ste l Sumit Ghosh a *, Bernd M. Schönbauer a,b , Sakari Pallaspuro a , Mahesh Somani a , Herwig Mayer b , Jukka Kömi a a Materials and Mechanical Engineering, Centre for Advanced Steels Research, University of Oulu, Oulu 90014, Finland. b Department of Material Sciences and Process Engineering, Institute of Physics and Materials Science, University of Natural Resources and Life Sciences, Vienna (BOKU, Vienna 1190, Austria. Abstract In the present study, very high cycle fatigue (VHCF) characteristics and properti s of a direct quenched and partitioned (DQ&P) mediu carb (0.4 wt.% C) steel having ultrahigh-strength (UTS > 2 GPa) as investigated using an ultrasonic-fatigue testing technique (~19 kHz frequency) up to ~10 10 cycles at the load ratio ( R ) of − 1 (fully reversed tension-compression). The occurrence of crack initiati n in the VHCF regime was observed at interior non-metallic inclusio s. The microstructural changes following VHCF failure underneath the fracture surfaces near the crack initi ti sites were further investigated through transmission electron microscopy. Formation of clear nanograined su surface layer adjacent to the cr ck initiating site at the interior inclusion was noticeably evident. The variation of average grain size (60 − 140 nm) and thickness (300 − 1200 nm) of nanograined layers at different number of cycles to failure were measured in detail. Apparently, localized plastic deformation caused the fragmentation of martensitic laths and hence, the formation of aforesaid ultrafine-grained layers in the microstructures. Ξ ϮϬϮϬ dŚĞ ƵƚŚŽƌƐ͘ WƵďůŝƐŚĞĚ ďLJ ůƐĞǀŝĞƌ ͘s͘ dŚŝƐ ŝƐ ĂŶ ŽƉĞŶ ĂĐĐĞƐƐ ĂƌƚŝĐůĞ ƵŶĚĞƌ ƚŚĞ zͲE ͲE ůŝĐĞŶƐĞ ;ŚƚƚƉ͗ͬͬĐƌĞĂƚŝǀĞĐŽŵŵŽŶƐ͘ŽƌŐͬůŝĐĞŶƐĞƐͬďLJͲŶĐͲŶĚͬϰ͘ϬͬͿ WĞĞƌͲƌĞǀŝĞǁ ƵŶĚĞƌ ƌĞƐƉŽŶƐŝďŝůŝƚLJ ŽĨ Ϯϯ ƵƌŽƉĞĂŶ ŽŶĨĞƌĞŶĐĞ ŽŶ &ƌĂĐƚƵƌĞ Ͳ &Ϯϯ 23 European Conference on Fracture - ECF23 Micro-mechanism associated with very high cycle fatigue crack initiation of advanced DQ&P processed steel Sumit Ghosh a *, Bernd M. Schönbauer a,b , Sakari Pallaspuro a , Mahesh Somani a , Herwig Mayer b , Jukka Kömi a a Materials and Mechanical Engineering, Centre for Advanced Steels Research, University of Oulu, Oulu 90014, Finland. b Department of Material Sciences and Process Engineering, Institute of Physics and Materials Science, University of Natural Resources and Life Sciences, Vienna (BOKU, Vienna 1190, Austria. Keywords: Direct Quenched and partitioned steel; Very high cycle fatigue; ; Crack initiation; Optically dark area; Failure micromechanisms.

Keywords: Direct Quenched and partitioned steel; Very high cycle fatigue; ; Crack initiation; Optically dark area; Failure micromechanisms.

* Corresponding author. Tel: +358-294482149 E-mail address: sumit.ghosh@oulu.fi * Corresponding author. Tel: +358-294482149 E-mail address: sumit.ghosh@oulu.fi

2452-3216 © 2022 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 the scientific committee of the 23 European Conference on Fracture – ECF23 10.1016/j.prostr.2022.12.116 ϮϰϱϮͲϯϮϭϲ © 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND licens (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of 23 European Conference on Fracture - ECF23 ϮϰϱϮͲϯϮϭϲ © 2020 Th Authors. Published by Els vier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of 23 European Conference on Fracture - ECF23