Materials Congress 2006

Damage Nucleation in Nuclear Graphite1

J MARROW, A HODGKINS, M JOYCE and B.J. MARSDEN

Materials Performance Centre, University of Manchester, Oxford Rd, Manchester, UK

Tel. +44-161-306-3611, Fax. +44-161-306-3586, E-mail: james.marrow@manchester.ac.uk

ABSTRACT: This paper reports the use of advanced materials characterisation techniques, X-ray micro-tomography and surface strain mapping by electronic speckle pattern interferometry (ESPI), to study the mechanisms of damage nucleation in polygranular nuclear graphite.

It is found that strain localisation occurs due to the heterogeneous microstructure, giving rise to micro-crack nucleation and coalescence prior to tensile failure. This produces a substantial damage zone at stress concentrations, such as notches and crack tips, which can be observed directly and in-situ. Crack propagation occurs by the coalescence of micro-cracks in the damage zone. The measured R-curve is a consequence of both frictional bridging in the crack wake and energy dissipation in the micro-crack process zone.

KEYWORDS: Graphite, X-Ray Tomography, Optical Strain Mapping, Crack Propagation, Damage.

1 NOTICE: this is the author’s version of a work that was accepted for publication in Energy Materials. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Energy Materials, 1, pp. 167-170 , (2006), DOI: 10.1179/174892406X160651

Materials Congress 2006

1. INTRODUCTION

Nuclear graphite is employed as a neutron moderator or reflector in high temperature reactors. Irradiation causes dimensional change (shrinkage and swelling), creep, variation in thermal expansion properties and thermal conductivity, and changes in strength and elastic modulus. In CO2 gas-cooled reactors, there is also an additional effect of radiolytic oxidation on these properties. The rates and magnitudes of these property changes with neutron fluence and temperature are sensitive to the microstructure of the nuclear graphite.

The safe design and operation of graphite moderated reactors is therefore dependent on mechanistic understanding of the processes of irradiation damage, materials test reactor (MTR) data and monitoring data obtained during the reactor operation. New build of high temperature reactors will require new nuclear graphites, with improved resistance to irradiation damage and oxidation.

This paper summarises results from a programme of research into nuclear graphite structure-property relationships. This is part of a larger activity which ranges from modelling stress development in irradiated components to the study of new disposal methods for irradiated graphites.

Recent observations, which are described in this paper include

• Optical strain mapping methods for the non-contact measurement of elastic properties and damage nucleation.

• Characterisation of strain and damage development at cracks and notches using image correlation and X-ray tomography to map the development of cracks in three-dimensions.

II. STRUCTURE AND DAMAGE CHARACTERISATION

1. Optical Strain Measurement for Damage Observation

Electronic Speckle Pattern Interferometry (ESPI) [1] is an optical strain mapping technique in which two beams of coherent light with a known phase shift are used to illuminate the sample. This produces speckles on the optically rough sample surface, which are observed using a CCD camera. Recording a series of speckle pattern images at increasing load levels allows deformation and hence strain fields to be determined with high resolution during post-test-analysis.

Electronic Speckle Pattern Interferometry has been applied to nuclear graphite to observe the development of strain in diametrically compressed discs of isotropic nuclear graphite [2]. Flattened surfaces, of different size, at the loading points were used to spread the compressive load and to minimise the likelihood of shear failure at these locations.

Analysis of the data, by optimization of the fit between the measured strains in two dimensions compared to an analytical solution, has been used to determine the elastic modulus and Poisson’s ratio. Typical data are given in Figure 2, which show the average modulus and average Poisson’s ratio as a function of load. These values are consistent with the expected Young’s modulus of isotropic graphite at high strain [3]. It is possible that the variation of Poisson’s ratio with load is an artefact of the test caused by microcracking at high strain, and this is under investigation. Work is in progress to refine the analysis and to determine whether the technique is suitable for the monitoring of irradiation and oxidation effects on elastic properties of nuclear graphites.

The elastic strains in coarse grain size graphite are inhomgeneously distributed in the microstructure. This is demonstrated by the distribution of strain observed (Figure 1). This is further illustrated in Figure 3, which shows that the degree of strain inhomogeneity increases prior to failure of the sample. Outliers of strain (i.e. localised strains significantly higher than the homogenous strain expected at that location) were identified as the sample load was increased. The majority of these strains increase linearly with applied load. This is indicative of local strain concentrations, probably due to porosity. As the failure load was approached, more significant outliers developed. Their strain was shown by unloading experiments to be irreversible, and is evidence for damage nucleation in the form of stable cracks [2]. These cracks appear to coalesce before sample failure. Work is in progress to improve the sensitivity and resolution of the strain mapping analysis and to monitor the development of damage under different stress states, in order to refine models for crack nucleation and failure. An objective is to determine whether the cracks initiate at some critical load or strain, or whether they are present in the virgin microstructure.

2. Three-Dimensional X-Ray Tomography for Damage Characterisation

Coarse grain nuclear graphites exhibit a rise in fracture toughness with crack extension. This is known as R-curve behaviour [4]. Knowledge of the R-curve and its sensitivity to irradiation and oxidation are required if fracture mechanics is to be applied to the structural integrity assessment of graphite components. However, traditional techniques for monitoring crack propagation (e.g. compliance and electrical direct current potential drop, DCPD) for toughness measurement are not easily applied to graphitic materials due to the tendency for crack bridging [5]. In this work, optical strain mapping by ESPI has been used to observe the crack tip strain field, and hence to monitor crack propagation. However, ESPI provides no information on the crack shape within the specimen. This has been obtained using X-ray tomography.

Materials Congress 2006

X-ray tomography is a useful means of studying changes in local density, allowing calibrated 3D density maps to be obtained from the attenuation of the X-rays [6, 7]. In particular, tomography measurements allow density and structural variations to be evaluated in a sample at a range of important length scales. Quantitative analysis of the data can be used to measure the three-dimensional variation of density in the structure, and to test models for the effects of thermal oxidation on properties, such as thermal diffusivity [8].

X-ray tomography illustrates the three-dimensional development of cracks within test specimens (Figure 5a), and has been used to show that side-grooved specimens may be required to ensure the planar, straight-fronted crack propagation necessary for valid toughness measurements [4, 9]. X-ray tomography also shows that crack bridging occurs due to frictional contact between fracture surface asperities. This is observed as transparent regions in the crack plane where the crack opening displacement is smaller than the tomography resolution (Figure 5a) [9, 10].

X-ray tomography also provides evidence for microstructural damage (Figure 5b). Regions of low X-ray attenuation develop in the crack wake and ahead of the crack tip. Subsequent metallographic sectioning shows microcracking in this region. Optical strain mapping observations also reveal a residual tensile strain field ahead of a sharp notch in a specimen (Figure ). This was observed in specimens after loading in tension to 80% of its failure stress, followed by unloading. The residual tensile strain is attributed to damage nucleation in the microstructure by microcracking, such as that observed in the disc compression tests. The effect is analogous to the development of a plastic zone by yielding in metals. The observation of damage implies that crack propagation in nuclear graphite occurs by the coalescence of microcracks, which develop in a damage zone that is the result of the stress concentration from the notch or crack. This may provide a mechanism for the development of a curved crack front, in terms of an effect of constraint (i.e plane stress or plane strain deformation) on microcrack development and coalescence.

Both crack bridging and microstructural damage ahead of the crack tip are R-curve mechanisms in brittle materials [11]. Energy dissipation or relief of tensile elastic strains may occur from micro-cracking, and from frictional stresses in the crack wake. The resultant R-curve is thus expected to be sensitive to the specimen geometry and mode of loading. Work is in progress to assess the relative contributions of bridging and micro-cracking on the R-curve, the effects of mixed-mode loading and their sensitivity to microstructure degradation by fast neutron irradiation and oxidation. An objective is to derive a microstructure-based model which correlates tensile strength and fracture toughness.

III. CONCLUSIONS

• Damage nucleation occurs in polygranular nuclear graphite by cracking associated with strain localisation in the heterogeneous microstructure.

• Crack propagation occurs by coalescence of microcracks, and is associated with a microcrack process zone. Both microcracking and crack bridging contribute to the observed R-curve behaviour (i.e. rising crack propagation resistance with crack extension).

IV. ACKNOWLEDGMENTS

The authors are grateful for the support of the Health and Safety Executive (Nuclear Installations Inspectorate) and British Energy. The opinions expressed are those of the authors, and not necessarily those of the research sponsors. Figure 1: Strain contour map of the horizontal strain distribution in a diametrically compressed disc of isotropic nuclear graphite (20 mm diameter). Crack nucleation has occurred. The compressive load axis is vertical, the horizontal tensile strains are shown.

Figure 2: The effect of applied load on a) the elastic modulus and b) Poisson’s ratio for 20 mm discs of isotropic graphite under diametral compression. Data is given for the variation of the width of the flattened contact area between 1 and 6 mm. Comparison of the Young’s modulus is made with literature observations by Taylor et al [3] at equivalent maximum compressive stress (!max) and mean compressive stress (!mean).]

Figure 3: Evolution of strain within regions of strain localisation (outliers) with the application of load. The dashed line marks the load at which the sample failed. (Data for a 3 mm flattened width disc). Similar data were obtained for all other analysed samples.

Figure 4: Crack propagation resistance, with increasing crack length, obtained for compact tension of various thicknesses and different crack monitoring methods [9]. The data are for specimens with side-grooves (one face only for ESPI and Vernier data, two faces for Compliance).

Figure 5: X-ray tomography of a crack, propagating from the notch in a compact tension specimen of nuclear graphite (modified shape to improve tomography resolution). The arrows indicate the viewing direction. a) A bowed crack, with discontinuities which are due to local reductions in the crack opening, which leads to frictional contact between the crack faces, b) damage zone of reduced attenuation at the crack tip and in the crack wake. Optical microscopy shows that the low attenuation region around the crack may be caused by extensive micro cracking (A). Outside the region there was no evidence of micro cracking (B). The arrow indicates the direction of observation

Figure 6: Optical strain map (uncalibrated, contours for the vertical strain) showing residual tensile strain ahead of a sharp notch in nuclear graphite, observed unloaded after loading to 80% of the failure stress.

V. REFERENCES

1. J.N. Petzing and J.R. Tyrer, Recent developments and applications in electronic speckle pattern interferometry Journal of

Materials Congress 2006

Strain Analysis,.33. 153-169, (1998).

2. M.R. Joyce* and T.J. Marrow, Observations of Strain Localisation and Failure in Nuclear Graphite, Aging Graphite Management, Cardiff, (2005).

3. R Taylor, R G. Brown, K. Gilchrist, E. Hall, A. T. Hodds, B. T. Kelly and F. Morris, The mechanical properties of reactor graphite, Carbon, 5, 519-531, (1967).

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7. L. Babout, PM Mummery, TJ Marrow, A Tzelepi, PJ Withers The effect of thermal oxidation on polycrystalline graphite studied by X-ray tomography, Carbon, 43, 765-774, (2005).

8. L J Babout, A Hodgkins, R Dewhurst, T J Marrow, A Fok, B J Marsden and P M Mummery, The Use of X-Ray Microtomography to Determine Relationships Between Microstructure and Mechanical Behaviour of Nuclear Graphite, HTR-2004, International Conference on High Temperature Reactors, Beijing, China, (2004).

9. A Hodgkins, T.J. Marrow, P. Mummery, B. Marsden and A. Fok, Crack propagation resistance and damage mechanisms in nuclear graphite, Aging Graphite Management, Cardiff, (2005).

10. A Hodgkins, T.J. Marrow, P. Mummery, B. Marsden and A. Fok, X-Ray Tomography Observation of Crack Propagation in Nuclear Graphite, accepted for publication, Materials Science and Technology (2006).

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