Interaction of non-metallic inclusions, microstructure, and fatigue loading with small crack growth in high-strength steels

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School of Engineering | Doctoral thesis (article-based) | Defence date: 2017-10-27
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Date

2017

Department

Konetekniikan laitos
Department of Mechanical Engineering

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Mcode

Degree programme

Language

en

Pages

90 + app. 68

Series

Aalto University publication series DOCTORAL DISSERTATIONS, 191/2017, VTT Science, 166

Abstract

The increased demand and requirements for high-strength steels drives the need to better understand and predict the fatigue endurance and crack growth challenges related to their use in critical machine components. Non-metallic inclusions or defects in the steel become increasingly important as the hardness or strength of the steel increases. The distribution and the ability to predict the largest inclusion that causes failure is crucial for the proper and successful design and production of the components.  The extreme value distribution is effective in predicting the maximum inclusion in a volume of steel. The proper prediction and use of inclusion data gathered from polished specimen as well as differences in anisotropy are important to consider when gathering data for use in design and prediction of fatigue life or failure. The difference in non-metallic inclusions and the forging direction affects the distribution of the size of the inclusions as well as the fatigue endurance limit and its scatter of the steel. The extreme value distributions combined with the Murakami-Endo model are used as a design approach for fatigue failure for components with ultra-long fatigue lives and step loading. This design approach uses the master curve for Optically Dark Area (ODA) growth obtained by Murakami et al. and combines it with the prediction of the largest non-metallic inclusion along with the estimate of the fatigue life of the component.  The initiation and growth of small cracks from inclusions as well as small Focused Ion Beam (FIB) notches behave in a similar manner and show a strong tendency to follow the local microstructure. The effect of the local microstructure on the small fatigue crack growth is studied using FIB milling to create cross-sections of the microstructure. This showed that the microstructure is also linked to the formation of ODA around non-metallic inclusions in ultra-long fatigue. The behaviour of a small crack growing from notches in high cycle fatigue is studied by using high-speed microscopy and Rumul fatigue testing machines. The test results show that small cracks initiate and grow quickly in the beginning of the fatigue life after which they propagate slowly at a stress intensity range lower than the large crack growth threshold until it is reached.  Comparing different data results for different R-ratios shows that the parameter ΔK+ works well to compare the crack growth rate of small cracks in the studied quenched and tempered steel. For crack arrest the ΔK+ or Kmax thresholds are lower for cracks with higher compressive loading. Also test results showed that increasing only the compressive portion of loading can reinitiate arrested small cracks. Finally, the ΔK+ or Kmax values for small cracks are lower for similar crack growth rates under larger compressive loads.

Description

Supervising professor

Hänninen, Hannu, Prof., Aalto University, Department of Mechanical Engineering, Finland

Thesis advisor

Marquis, Gary, Prof., Aalto University, Finland

Keywords

non-metallic inclusions, small crack growth, fatigue

Other note

Parts

  • [Publication 1]: A. Roiko, H. Hänninen, H. Vuorikari, Anisotropic distribution of non-metallic inclusions in a forged steel roll and its influence on fatigue limit, International Journal of Fatigue, Volume 41, August 2012, Pages 158-167, ISSN 0142-1123.
    DOI: 10.1016/j.ijfatigue.2011.12.023 View at publisher
  • [Publication 2]: A. Roiko, Y. Murakami, A design approach for components in ultralong fatigue life with step loading, International Journal of Fatigue, Volume 41, August 2012, Pages 140-149, ISSN 0142-1123.
    DOI: 10.1016/j.ijfatigue.2011.12.021 View at publisher
  • [Publication 3]: A. Roiko, J. Solin, Measurement of small cracks initiating from inclusions, Focused Ion Beam notches and drilled holes, International Journal of Fatigue, Volume 62, May 2014, Pages 154-158, ISSN 0142-1123.
    DOI: 10.1016/j.ijfatigue.2013.03.010 View at publisher
  • [Publication 4]: A. Roiko, J. Solin, T. Sarikka, H. Hänninen, The paths of small fatigue cracks in high-strength steels initiated from inclusions and small defects, Accepted at: Materials Performance and Characterization on 21.4.2017
  • [Publication 5]: A. Roiko, J. Solin, H. Hänninen, Behavior of small cracks under negative stress ratio fatigue loading, International Journal of Fatigue, Volume 104, 2017, Pages 379-388, ISSN 0142-1123.
    DOI: 10.1016/j.ijfatigue.2017.07.006 View at publisher
  • [Publication 6]: A. Cetin, A. Roiko, M. Lind, Towards proper sampling and statistical modelling of defects. Fatigue & Fracture of Engineering Materials & Structures, Volume 38, September 2015, Pages 1056-1065, ISSN 1460-2695.
    DOI: 10.1111/ffe.12317 View at publisher

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https://urn.fi/URN:ISBN:978-952-60-7648-5

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[diss] Insinööritieteiden korkeakoulu / ENG