The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
Aalto

Engineering Tools for Robust Creep Modeling

Stefan Holmström

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Faculty of Engineering and Architecture for public examination and debate in Auditorium K216 at the Aalto University School of Science and Technology (Espoo, Finland) on the 5th of February, 2010, at 12 noon.

Overview in PDF format (ISBN 978-951-38-7379-0)   [3421 KB]
VTT Publications 728, ISSN 1455-0849

Dissertation is also available in print (ISBN 978-951-38-7378-3)
Copyright © 2010 VTT Technical Research Centre of Finland
VTT Publications 728, ISSN 1235-0621
VTT-PUBS-728
TKK-DISS-2729

Abstract

High temperature creep is often dealt with simplified models to assess and predict the future behavior of materials and components. Also, for most applications the creep properties of interest require costly long-term testing that limits the available data to support design and life assessment. Such test data sets are even smaller for welded joints that are often the weakest links of structures. It is of considerable interest to be able to reliably predict and extrapolate long term creep behavior from relatively small sets of supporting creep data.

For creep strain, the current tools for model verification and quality assurance are very limited. The ECCC PATs can be adapted to some degree but the uncertainty and applicability of many models are still questionable outside the range of data. In this thesis tools for improving the model robustness have been developed. The toolkit includes creep rupture, weld strength and creep strain modeling improvements for uniaxial prediction. The applicability is shown on data set consisting of a selection of common high temperature steels and the oxygen-free phosphorous doped (OFP) copper. The steels assessed are 10CrMo9-10 (P22), 7CrWVMoNb9-6 (P23), 7CrMoVTiB10-10 (P24), 14MoV6-3 (0.5CMV), X20CrMoV11-1 (X20), X10CrMoVNb9-1 (P91) and X11CrMoWVNb9-1-1 (E911).

The work described in this thesis has provided simple yet well performing tools to predict creep strain and life for material evaluation, component design and life assessment purposes. The uncertainty related to selecting the type of material model or determining weld strength factors has been reduced by the selection procedures and by linking the weld behavior to the base material master equation. Much of the resulting improvements and benefits are related to the reduced requirements for supporting creep data. The simplicity and robustness of the new tools also makes them easy to implement for both analytical and numerical solutions.

This thesis consists of an overview and of the following 7 publications:

  1. S. Holmström and P. Auerkari. 2008. Effect of short-term data on predicted creep rupture life – pivoting effect and optimized censoring. Materials at High Temperatures, volume 25, number 3, pages 103-109. © 2008 Science Reviews 2000. By permission.
  2. Stefan Holmström and Pertti Auerkari. 2006. Predicting weld creep strength reduction for 9% Cr steels. International Journal of Pressure Vessels and Piping, volume 83, numbers 11-12, pages 803-808.
  3. S. R. Holdsworth, M. Askins, A. Baker, E. Gariboldi, S. Holmström, A. Klenk, M. Ringel, G. Merckling, R. Sandstrom, M. Schwienheer, and S. Spigarelli, on behalf of Working Group 1 of the European Creep Collaborative Committee. 2008. Factors influencing creep model equation selection. International Journal of Pressure Vessels and Piping, volume 85, numbers 1-2, pages 80-88.
  4. S. Holmström and P. Auerkari. 2006. Robust prediction of full creep curves from minimal data and time to rupture model. Energy Materials: Materials Science and Engineering for Energy Systems, volume 1, number 4, pages 249-255. © 2006 Institute of Materials, Minerals and Mining. By permission.
  5. Stefan Holmström and Pertti Auerkari. 2009. Predicting creep rupture from early strain data. Materials Science and Engineering A, volumes 510-511, pages 25-28.
  6. Stefan Holmström, Anssi Laukkanen, and Kim Calonius. 2009. Finding critical damage locations by Λ-filtering in finite-element modelling of a girth weld. Materials Science and Engineering A, volumes 510-511, pages 224-228.
  7. Stefan Holmström, Juhani Rantala, Anssi Laukkanen, Kari Kolari, Heikki Keinänen, and Olli Lehtinen. 2009. Modeling and verification of creep strain and exhaustion in a welded steam mixer. Journal of Pressure Vessel Technology, volume 131, number 6, 061405, 5 pages. © 2009 by authors and © 2009 American Society of Mechanical Engineers (ASME). By permission.

Keywords: creep, strain, damage, modeling, steel, OFP copper

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© 2010 Aalto University School of Science and Technology

Last update 2011-05-26