PSI Scientific Report 2008
Research focus and highlights – Nuclear energy and safety 81
change of mechanical properties as a result of damage. Figure 3 shows results from compression tests of the ferritic 1000
ODS steel before and after helium implantation. The sample material was the same as that for which the nano-indenter
800
20% was found for the indenter tests as well as for the micropillar tests. Important additions to the micro-mechanical investigations are micro-characterization with electron microscope and advanced beamline techniques such as extended X-ray absorption fine structure (EXAFS). These techniques allow quantitative assessments of damage to be made, such as the analysis
Stress (MPa)
results were shown in Figure 1. Irradiation hardening of about
600 400 200 0
0
2
4
of point defect clusters or coordination analysis.
6 8 Strain (%)
Unirradiated
Another important issue concerns the quantitative understand-
10
12
14
Irradiated
ing of damage with respect to component life. Constitutive equations and other parameterizations of material properties
Figure 3: Stress-strain curve of a ferritic ODS steel before
are usually applied with time-independent coefficients and
and after helium implantation determined by micro-pillar
exponents using the properties of virgin material. These can
compression.
change as microstructure changes. Conversion of these changes into mechanical response could provide a possibility for
more fundamental understanding of the mechanisms causing
better assessments of the development of mechanical proper-
material aging. Information from such methods of condition
ties with time. The inclusion of multiscale modelling tools for
monitoring goes far beyond the possibilities of current NDE.
describing materials through several length (and time) scales,
Micro-sample/micro-scale modelling for condition monitoring
starting at the atomic level up to the level of finite element
should be used complementarily to conventional non-destruc-
analysis, is expected to enhance the current modelling
tive methods, to provide a sound picture of the status of a
schemes used. A detailed discussion of these methods is
component, which can be used for safety considerations and
given in [3].
reliable risk assessment. This work was essentially supported by the Swiss Competence Center Energy and Mobility (CCEM).
References: [1] C. A.Volkert, E. T. Lilleodden, Size effects in the d eformation of sub-micron Au columns, Phil. Mag. 86 5567–5579 (2006). Figure 2: Micro-pillar of a ferritic ODS steel before and after a
[2] M.A. Pouchon, J. Chen, R. Ghisleni, J. Michler, W. Hoffelner, Characterization of Irradiation Damage
compression test. The slip plane
of Ferritic ODS Alloys with Advanced Micro-Sample
is clearly visible [2].
Methods, Experimental Mechanics, DOI:10.1007/ s11340–008–9214–5 (2009).
Mechanical testing of small samples, together with advanced
[3] M. Samaras, M. Victoria, Modelling in nuclear energy
analytical methods and materials modelling, provide a very
environments, Materials Today 11 (12), 54–62 (2008).
promising option for the determination of damage in nuclear plants. It is proposed to use these combined tools for the assessment of the residual life of components with an expected lifetime of 60 years or more. Even very small samples (not affecting the integrity of a component) could be investigated. Taking such a “fingerprint” of the condition at scheduled time intervals would provide an improvement in relevant material parameters and design rules. Using these fingerprints in synergy with a multiscale modelling scheme would bring a