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Joint ASTM-EPRI International Symposium on Creep-Fatigue Interactions: Test Methods and Models

Posted on 10. March, 2011.

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The Joint ASTM-EPRI International Symposium on Creep–Fatigue Interactions:
Test Methods and Models was sponsored jointly by ASTM Committee E08 on Fatigue and Fracture and EPRI was held at the Grand Hyatt Hotel at San Antonio from 17 to 19 November 2010.

The first International Workshop on creep-fatigue damage interaction was organized by EPRI and held in Amsterdam in July 2006. Recognizing that existing Design Codes for elevated temperature application require some re-assessment, the aim was to bring together key industry experts in order to evaluate the multitude of analytical methodologies with respect to crack initiation and growth, and their bearing on life prediction of components in power plant and aero engine applications Deficiencies in the field were to be addressed and future research and development requirements identified. Subsequent workshops were then held annually at Marco Island, Florida (2007), Kyoto (2008), Paris (2009), culminating in the San Antonio Symposium, This was organized by Ashok Saxena, University of Arkansas, USA and Bilal Dogan of EPRI Charlotte, USA. The creep-fatigue Symposium was organized instead of a specialist “5th Workshop” in order to establish a wider dissemination of the work. Peer-reviewed papers will be published in the ASTM online Journal of ASTM International (JAI). Selected papers will then be published as a Special Technical Publication (STP).

Working closely with the appropriate ASTM Standards Committees, the conference organizers have over the past 4 years, assisted in piloting the production of two new Standards which were presented at San Antonio. These are respectively, ‘Standard Test Method for Creep-Fatigue Testing’ (Designation E 2714-09) and ‘Standard Test method for Creep-Fatigue Crack Growth Testing’ (Designation E2760-10). These documents indicate precisely the difficulties faced by investigators when switching from room-temperature to elevated-temperature testing of appropriate representative specimens.

One of the major difficulties in the field of creep-fatigue is that fairly long-term (and thus expensive) tests are required to demonstrate degradation properties or to assist in life prediction of components at risk. Recourse must be had to extrapolation techniques coupled with physical mechanisms, for example transgranular cracking is associated with fast strain rates and intergranular cracking/cavitation damage with low strain rates. As to the conference itself, some 35 papers were presented. Major topics in creep-fatigue concerned the behaviour of 9Cr ferritic steels, environmental effects, modelling, and Ni-base alloys and test methods. In addition, several keynote lectures were presented concerning a comparative study of life prediction methods, data requirements when assessing components, environmental effects on creep-fatigue behaviour of Alloy 617, the fracture-mechanics approach to creep-fatigue crack growth, creep-fatigue behaviour of strength-enhanced ferritic steels and the influence of stress relaxation on the creep-fatigue behaviour of Ni-based superalloys. Other topics concerned the fracture-life law for directionally-solidified alloys, modelling deformation behaviour using crystal viscoplasticity, models for small crack growth under creep-fatigue and creep crack growth under complex loading.

It would of course be impossible to do justice to all the presentations here; we each come away with our personal impressions. The full technical programme (paper titles and presenters' names) may be found on:

http://myastm.astm.org/MEETINGS/SYMPOSIAPROGRAMS/E08ID1700.pdf

This reporter, however, noted the following:

  • Energy-based methods of life assessment appear to offer the most consistent predictions
  • Some creep-fatigue tests in Japan have been running for 5 years
  • In some alloys a 60 minute dwell is sufficient to reach saturation conditions (and hence enough for lifetime assessment purposes?)
  • In 9Cr steels a compression dwell is more damaging than for one in tension
  • This can be associated either with oxidation behaviour or an earlier nucleation of cracks
  • Many alloys display discontinuous yielding (dynamic strain ageing) during hysteresis loop deformation, presenting severe challenges for conventional strain-control systems
  • Tests in air and vacuum in Ni-based superalloys demonstrate that an environmentally brittle zone within the creep zone is responsible for growth rates i.e., there appears to be no unique parameter for a given crack size
  • A new issue (Issue 3, Revision 1) of Vols 2/3 of the British Energy (EDF) R5 High temperature Assessment procedure is planned
  • An EPRI/ORNL exercise in data retrieval (from archived hysteresis loops, stress-time plots etc.) is supplying valuable data for an up-to-date EPRI creep-fatigue digital data base
  • Preliminary results have been obtained on an ASTM round robin exercise entailing low-cycle-fatigue endurance of a 9Cr steel and Alloy 617 with dwell. The aim is to provide a bias statement and more precision on Standard E 2714-09 mentioned above, especially with regard to the ‘failure’ criterion i.e., the percentage tensile load drop at which tests should be stopped. There was much debate on this aspect.

The general consensus was that good progress had been made over the past four years, work on newer alloys had been introduced, and, having regard to alternative approaches with their underlying physical mechanisms, a way forward for changes and improvements in life assessment procedures had been established.

R.P. Skelton

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